Method for separating zinc and cadmium. Method for extracting zinc and cadmium from aqueous solutions of electrolytes Finding in nature
| Parameter name | Meaning |
| Article subject: | Zinc and cadmium. |
| Rubric (thematic category) | Metals and Welding |
Since both metals have a relatively low boiling point, special care should be taken when introducing them into silver melts. These metals are the most important alloying components in the production of solders, and in this regard, their influence on the properties of alloys should be considered in more detail.
Ag-Zn. Up to 20% zinc dissolves in silver in the solid state, but practically the zinc content in the alloy should not exceed 14%. Such alloys do not tarnish, are well polished and have good ductility.
Ag-CD. The solubility limit of cadmium in silver is about 30%. These alloys are ductile and resistant to corrosion in air.
Ag-Zn-Cd. The alloys have a low melting point and in some cases are used as solders. The alloys have a wide crystallization area, and the brazed seam has low mechanical properties, which leads to the limited use of solders based on this system.
Ag-Cu-Cd. Copper does not dissolve cadmium at all, but forms a brittle Cu 2 Cd compound with it. With a sufficiently high content of silver in the alloy, cadmium, dissolving in silver, makes the alloy viscous, ductile and very resistant to tarnishing.
Silver-copper alloys with small additions of cadmium are especially well suited for deep drawing and coinage.
Ag-Cu-Zn. A few hundredths of a percent of zinc, introduced into the melt before casting, significantly increase the fluidity of silver-copper alloys. However, small additions of zinc make the alloys more resistant to tarnishing and more ductile. Copper dissolves up to 39% zinc. With a higher zinc content in silver-copper alloys, ternary alloys with a low melting point are formed. Such alloys are widely used as solders.
To obtain solders, an alloy of silver - copper of a eutectic composition with zinc additives is used, which lowers the melting point of the alloy.
Ag-Cu-Zn-Cd. The alloys of this four-component system have a low melting point and, as a result, have found wide application as solders. The large decrease in the melting point of these alloys is explained by the fact that zinc and cadmium form a low-melting eutectic.
Lead. Silver and lead form a eutectic with a melting point of 304°C. Located along the grain boundaries, these eutectic compounds make the alloy red-brittle. According to GOST 6836-72, the lead content in silver alloys should not exceed 0.005%.
Tin. The presence of tin in small amounts significantly reduces the melting point of alloys of the silver-copper system. In pure silver, up to 19% of tin dissolves. This produces alloys that are softer and more ductile than silver-copper alloys, but these alloys have a dull color. When the tin content in silver-copper alloys is more than 9% and at a temperature of 520 ° C, a brittle Cu 4 Sn compound is formed. However, due to the formation of tin oxide SnO 2 during melting, brittleness increases.
Aluminum. In silver-copper alloys in the solid state, aluminum dissolves up to 5%, while the structure and properties of the alloy remain almost unchanged. At a higher aluminum content in the alloy, a brittle Ag 3 Al 2 compound is formed. During melting and annealing, aluminum oxide A1 2 O 3 is also formed, which is located along the grain boundaries. These compounds make the alloy cold brittle and unsuitable for processing.
Iron. It does not dissolve in silver and is always a harmful impurity in silver alloys. Getting into the alloy, iron particles remain in it in the form of foreign solid inclusions. At the same time, iron interacts with the crucible material, coal particles, emery, salts used in smelting, and forms hard and brittle compounds. Getting on the surface of an ingot or product, these compounds break out of the metal during grinding, and leave characteristic elongated marks on the surface of the product.
Silicon. Silicon does not dissolve in silver, and at a content of 4.5% in the alloy, silicon-silver eutectic is formed with a melting point of 830°C. Located along the grain boundaries, these eutectic precipitates significantly reduce the ductility of the alloy, and in most cases make the alloy completely unsuitable for plastic deformation processing. Silicon can get into the alloy from quartz, which serves as a material for the manufacture of crucibles.
Sulfur. With the main components of the alloys, sulfur forms hard and brittle compounds Ag 2 S and Cu 2 S, which, located between the crystals and inside the grains, cause brittleness of the alloys. For the appearance of brittleness of the alloy, the presence of 0.05% sulfur in it is sufficient. Sulfur often contains charcoal, under which annealing is carried out, combustible materials, gases, etchants, etc.
The presence of sulfur or sulfur compounds in the alloy leads to its darkening due to the formation of silver sulfide.
Phosphorus. Before casting, silver alloys are deoxidized in most cases with phosphorous copper containing 10 to 15% phosphorus. Phosphorus quickly reacts with alloy oxides, adding oxygen in them, and forms a gaseous compound, ĸᴏᴛᴏᴩᴏᴇ either volatilizes or reacts with other particles of copper oxides, forming slag compounds of copper metaphosphate. Due to the fact that phosphorous copper is added, as a rule, in excess, since the content of oxides in the metal is unknown, phosphorus enters the metal. A small amount of phosphorus is sufficient to form brittle intermetallic compounds AgP 2 and Ag 3 P, which are located along the grain boundaries in the form of eutectics. The melting point of the ternary eutectic Ag - Cu - P is 641°C. As a result of the formation of phosphides, the alloys become red-brittle, quickly tarnish, and galvanic coatings do not adhere well to them.
Carbon. Carbon does not react with silver and does not dissolve in it. Getting into the melt, carbon particles remain in it in the form of foreign inclusions.
Below are the composition, properties and grades of some silver-based solders.
Zinc and cadmium. - concept and types. Classification and features of the category "Zinc and cadmium." 2017, 2018.
In 1968, an article appeared in a well-known magazine, which was called "Cadmium and the Heart." It said that Dr. Carroll, a US public health officer, had discovered a relationship between atmospheric cadmium levels and the incidence of deaths from cardiovascular disease. If, say, in city A the content of cadmium in the air is higher than in city B, then the cores of city A die earlier than if they lived in city B. Carroll made this conclusion after analyzing data for 28 cities. By the way, in group A there were such centers as New York, Chicago, Philadelphia...
So once again they were charged with poisoning an element opened in a pharmacy bottle!
Pharmaceutical bottle element
It is unlikely that any of the Magdeburg pharmacists uttered the famous phrase of the mayor: “I invited you, gentlemen, in order to tell you unpleasant news,” but they had a common feature with him: they were afraid of the auditor.
The district doctor Rolov was distinguished by a sharp temper. So, in 1817, he ordered all preparations with zinc oxide produced at Herman's Shenebek factory to be withdrawn from sale. By the appearance of the preparations, he suspected that there was arsenic in zinc oxide! (Zinc oxide is still used for skin diseases; ointments, powders, emulsions are made from it.)
To prove his case, the strict auditor dissolved the suspected oxide in acid and passed hydrogen sulfide through this solution: a yellow precipitate fell out. Arsenic sulfides are just yellow!
The owner of the factory began to challenge Rolov's decision. He himself was a chemist and, having personally analyzed the Product Samples, he did not find any arsenic in them. He reported the results of the analysis to Rolov, and at the same time to the authorities of the land of Hanover. The authorities, of course, requested samples in order to send them for analysis to one of the reputable chemists. It was decided that the judge in the dispute between Rolov and Herman should be Professor Friedrich Stromeyer, who since 1802 had been the chair of chemistry at the University of Göttingen and the position of inspector general of all Hanoverian pharmacies.
Stromeyer was sent not only zinc oxide, but also other zinc preparations from the Hermann factory, including ZnC0 3 , from which this oxide was obtained. Having calcined zinc carbonate, Strohmeyer obtained oxide, but not white, as it should have been, but yellowish. The owner of the factory explained the coloration with an impurity of iron, but Stromeyer was not satisfied with this explanation. Having bought more zinc preparations, he made a complete analysis of them and without much difficulty isolated the element that caused yellowing. The analysis said that it was not arsenic (as Rolov claimed), but not iron (as Herman claimed).
Friedrich Stromeyer (1776-1835) It was a new, previously unknown metal, chemically very similar to zinc. Only its hydroxide, unlike Zn(OH) 2 , was not amphoteric, but had pronounced basic properties.
In free form new element It was a white metal, soft and not very strong, covered with a brownish oxide film on top. Stromeyer called this metal cadmium, clearly alluding to its “zinc” origin: the Greek word has long denoted zinc ores and zinc oxide.
In 1818, Stromeyer published detailed information about the new chemical element, and almost immediately its priority began to be encroached upon. The first to speak was the same Rolov, who previously believed that there was arsenic in the preparations from the German factory. Shortly after Stromeyer, another German chemist, Kersten, discovered a new element in Silesian zinc ore and named it mellin (from the Latin mellinus, "yellow like quince") because of the color of the precipitate formed by the action of hydrogen sulfide. But it was already discovered by Strohmeyer cadmium. Later, two more names were proposed for this element: klaprotium - in honor of the famous chemist Martin Klaproth and junonium - after the asteroid Juno discovered in 1804. But the name given to the element by its discoverer was nevertheless established. True, in Russian chemical literature of the first half of the 19th century. cadmium was often called cadmium.
Seven colors of the rainbow
Cadmium sulfide CdS was probably the first compound of element #48 that the industry was interested in. CdS are cubic or hexagonal crystals with a density of 4.8 g/cm 3 . Their color is from light yellow to orange-red (depending on the method of preparation). This sulfide is practically insoluble in water; it is also resistant to the action of alkali solutions and most acids. And getting CdS is quite simple: it is enough to pass, as Stromeyer and Rolov did, hydrogen sulfide through an acidified solution containing Cd 2+ ions. It can also be obtained in an exchange reaction between a soluble cadmium salt, such as CdSO 4 , and any soluble sulfide.
CdS is an important mineral dye. It used to be called cadmium yellow. Here is what they wrote about cadmium yellow in the first Russian "Technical Encyclopedia", published at the beginning of the 20th century.
“Light yellow tones, starting from lemon yellow, are obtained from pure weakly acidic and neutral solutions of cadmium sulfate, and when cadmium sulfide is precipitated with a solution of sodium sulfide, darker yellow tones are obtained. A significant role in the production of cadmium yellow is played by the presence of other metal impurities in the solution, such as zinc. If the latter is present together with cadmium in solution, then during precipitation, a cloudy yellow color with a whitish tint is obtained ... In one way or another, six shades of cadmium yellow can be obtained, ranging from lemon yellow to orange ... This paint in finished form has a very beautiful shiny yellow color. It is quite constant to weak alkalis and acids, and is completely insensitive to hydrogen sulfide; therefore it is dry mixed with ultramarine and produces a fine green dye, which is called cadmium green in the trade.
Being mixed with drying oil, it goes like oil paint in painting; very opaque, but due to the high market price it is mainly used in painting as oil or watercolor paint, but also for printing. Due to its great fire resistance, it is used for painting on porcelain.
It only remains to add that subsequently cadmium yellow became more widely used "in the painting business." In particular, passenger cars were painted with it, because, among other advantages, this paint resisted locomotive smoke well. As a dye, cadmium sulfide was also used in the textile and soap industries.
But in recent years, industry has been using pure cadmium sulfide less and less - it is still expensive. It is replaced by cheaper substances - cadmopon and zinc-cadmium lithopone.
The reaction for obtaining cadmopone is a classic example of the formation of two precipitates at the same time, when practically nothing remains in the solution except water:
CdSO 4 4- BaS (both salts are soluble in water) _ * CdS J + BaS04 J.
Kadmopon is a mixture of cadmium sulfide and barium sulfate. The quantitative composition of this mixture depends on the concentration of the solutions. It is easy to vary the composition, and therefore the shade of the dye.
Cadmium zinc lithopone also contains zinc sulfide. In the manufacture of this dye, three salts precipitate simultaneously. The color of lithopon is cream or ivory.
As we have already seen, tangible things can be colored with cadmium sulfide in three colors: orange, green (cadmium green) and all shades of yellow, but cadmium sulfide gives the flame a different color - blue. This property is used in pyrotechnics.
So, with just one combination of element 48, you can get four of the seven colors of the rainbow. Only red, blue and purple remain. The blue or violet color of the flame can be achieved by supplementing the glow of cadmium sulfide with certain pyrotechnic additives - this will not be difficult for an experienced pyrotechnician.
And the red color can be obtained using another compound of element No. 48 - its selenide. CdSe is used as an artistic paint, by the way, very valuable. Ruby glass is stained with cadmium selenide; and not chromium oxide, as in the ruby itself, but cadmium selenide made the stars of the Moscow Kremlin ruby red.
However, the value of cadmium salts is much less than the value of the metal itself.
Exaggerations ruin reputation
If you build a chart with dates on the horizontal axis and demand for cadmium on the vertical axis, you get an ascending curve. The production of this element is growing, and the sharpest "jump" falls on the 40s of our century. It was at this time that cadmium turned into a strategic material - they began to make control and emergency rods of nuclear reactors from it.
In popular literature, one can come across the assertion that if it were not for these rods that absorb excess neutrons, then the reactor would go "peddling" and turn into an atomic bomb. This is not entirely true. In order for an atomic explosion to occur, many conditions must be met (this is not the place to talk about them in detail, but you can’t explain ET0 briefly). A reactor in which a chain reaction has become uncontrollable does not necessarily explode, but in any case a serious accident occurs, fraught with huge material costs. And sometimes not only material ... So the role of regulating and;
The statement is equally inaccurate (see, for example, the well-known book II. R. Taube and E. I. Rudenko “From hydrogen to ...”. M., 1970) that cadmium is the most suitable material. If there were also “thermal” before the word “neutrons”, then this statement would become really accurate.
Neutrons, as is known, can differ greatly in energy. There are low-energy neutrons - their energy does not exceed 10 kiloelectronvolts (keV). There are fast neutrons - with an energy of more than 100 keV. And there are, on the contrary, low-energy - thermal and "cold" neutrons. The energy of the former is measured in hundredths of an electron volt, for the latter it is less than 0.005 eV.
Cadmium at first turned out to be the main "core" material, primarily because it absorbs thermal neutrons well. All reactors at the beginning of the "atomic age" (and the first of them was built by Enrnco Fermi in 1942) worked on thermal neutrons. Only many years later it became clear that fast neutron reactors are more promising both for energy and for obtaining nuclear fuel - plutonium-239. And cadmium is powerless against fast neutrons, it does not delay them.
Therefore, the role of cadmium in reactor construction should not be exaggerated. And also because the physico-chemical properties of this metal (strength, hardness, heat resistance - its melting point is only 321 ° C) leave much to be desired. And also because, without exaggeration, the role that cadmium has played and is playing in nuclear technology is quite significant.
Cadmium was the first core material. Then boron and its compounds began to play the leading roles. But cadmium is easier to obtain in large quantities than boron: cadmium was obtained and obtained as a by-product of the production of zinc and lead. In the processing of polymetallic ores, it, an analogue of zinc, invariably turns out to be mainly in zinc concentrate. And cadmium is reduced even more easily than zinc, and has a lower boiling point (767 and 906°C, respectively). Therefore, at a temperature of about 800 ° C, it is not difficult to separate zinc and cadmium.
Cadmium is soft, malleable, easily machinable. This also facilitated and accelerated his path to atomic technology. The high selectivity of kad-)1IA, its sensitivity specifically to thermal neutrons, also played into the hands of physicists. And according to the main performance characteristic - the capture cross section of thermal neutrons - cadmium occupies one of the first places among all elements of the periodic system - 2400 barn. (Recall that the capture cross section is the ability to “take in” neutrons, measured in conventional units of barns.)
Natural cadmium consists of eight isotopes (with mass numbers 106, 108, 110, 111, 112, IS, 114 and 116), and the capture cross section is a characteristic in which the isotopes of one element can differ very much. In a natural mixture of cadmium isotopes, the main "neutron-eater" is an isotope with a mass number of IZ. Its individual capture cross section is huge - 25 thousand barns!
By attaching a neutron, cadmium-113 turns into the most common (28.86% of the natural mixture) isotope of element No. 48 - cadmium-114. The share of cadmium-113 itself is only 12.26%.
Control rods of a nuclear reactor.
Unfortunately, separating eight isotopes of cadmium is much more difficult than separating two isotopes of boron.
Control and emergency rods are not the only place of "atomic service" of element No. 48. Its ability to absorb neutrons of strictly defined energies helps to study the energy spectra of the resulting neutron beams. With the help of a cadmium plate, which is placed in the path of the neutron beam, it is determined how homogeneous this beam is (in terms of energy values), what is the proportion of thermal neutrons in it, etc.
Not many, but there
And finally - about the resources of cadmium. His own minerals, as they say, one or two and miscalculated. Only one of them has been sufficiently fully studied - a rare CdS greenockite that does not form clusters. Two more minerals of element No. 48 - otavite CdCO 3 and monteponite CdO - are very rare. But cadmium is not "alive" with its own minerals. Zinc minerals and polymetallic ores are a fairly reliable raw material base for its production.
Cadmium plating
Everyone knows galvanized tin, but not everyone knows that not only galvanizing, but also cadmium plating is used to protect yagelez from corrosion. Cadmium coating is now applied only electrolytically, most often in industrial conditions cyanide baths are used. Previously, iron and other metals were cadmium-plated by immersing products in molten cadmium.
Despite the similar properties of cadmium and zinc, cadmium coating has several advantages: it is more resistant to corrosion, it is easier to make it even and smooth. In addition, cadmium, unlike zinc, is stable in an alkaline environment. Cadmium tin is used quite widely; it is denied access only to the production of food containers, because cadmium is toxic. Cadmium coatings have another interesting feature: in the atmosphere of rural areas, they are much more resistant to corrosion than in the atmosphere of industrial areas. Such a coating fails especially quickly if the content of sulfurous or sulfuric anhydrides is increased in the air.
Cadmium in alloys
About a tenth of the world's cadmium production is spent on the production of alloys. Cadmium alloys are mainly used as antifriction materials and solders. A well-known alloy composition of 99% Cd and 1% No is used for the manufacture of bearings operating in automobile, aircraft and marine engines at high temperatures. Insofar as cadmium is not sufficiently resistant to acids, including organic acids contained in lubricants, sometimes bearing alloys based on cadmium are coated with indium.
Solders containing element No. 48 are quite resistant to temperature fluctuations.
Alloying copper with small additions of cadmium makes it possible to make more wear-resistant wires on electric transport lines. Copper with the addition of cadmium almost does not differ in electrical conductivity from pure copper, but it noticeably surpasses it in strength and hardness.
ACCUMULATOR AKN AND NORMAL WESTON ELEMENT.
Among the chemical current sources used in industry, a prominent place belongs to nickel-cadmium batteries (NAC). The negative plates of such batteries are made of iron meshes with cadmium sponge as the active agent. The positive plates are coated with nickel oxide. The electrolyte is a potassium hydroxide solution. Nickel-cadmium alkaline batteries differ from lead (acid) batteries in greater reliability. Based on this, the couples make very compact batteries for guided missiles. Only in this case, not iron, but nickel grids are installed as the basis.
Element No. 48 and its compounds were used in yet another chemical current source. In the construction of a normal Weston element, both cadmium amalgam, and cadmium sulfate crystals, and a solution of this salt work.
Toxicity of cadmium
Information about the toxicity of cadmium is rather contradictory. Rather, the fact that cadmium is poisonous is undeniable: scientists argue about the degree of danger of cadmium. Cases of fatal poisoning with vapors of this metal and its compounds are known - so such vapors pose a serious danger. If it enters the stomach, cadmium is also harmful, but cases of fatal poisoning with cadmium compounds that have entered the body with food are unknown to science. Apparently, this is due to the immediate removal of poison from the stomach, undertaken by the body itself. ] Nevertheless, in many countries the use of cadmium coatings for the manufacture of food containers is prohibited by law.
PROPERTIES OF METALS
The next element of the fourth period after copper, its outer shell of the atom is filled with d-electrons and has the structure 3d 10 4s 2, therefore the only stable oxidation state of the element is two.
It is very similar to it, is located in the same subgroup, and as an element of the fourth period, has a similar atomic structure, completed according to the scheme 4d 10 5s 2. In simple compounds, it is also divalent, however, having a larger atom, it also exhibits the oxidation state (I), known in a number of unstable salts.
Rice. Temperature dependence of metal vapor pressure
Some authors, referring to the filling of the d-layer, do not classify them as transition metals, arguing that the temperatures of aggregate transformations are also unexpectedly low for periods IV and V. The melting and boiling points, respectively, are 419.5 and 907 ° C (Zn); 321 and 767°C (Cd). Indeed, in terms of melting, boiling, and vapor pressures, and differ significantly from other heavy metals (Table 2, Fig.).
The redox potentials of both elements are more negative than hydrogen and increase with the complication of the atom (see Table 4); however, zinc and cadmium are easily reduced at the cathode from moderately acidic solutions due toa high overvoltage of hydrogen, especially on cadmium. Zinc cements cadmium from solutions:
Cadmium hydroxide exhibits pronounced basic properties, while in zinc it is amphoteric and dissolves not only in acids, but also in alkalis:
Zn + 2OH - \u003d ZnO 2 2 - + H 2
It is more correct to represent the zincate ion in solid salts as Zn (OH) - 3 or Zn (OH) 2 4 -, and in solutions, without having more accurateinformation, it is conditionally accepted by reaction.
Simple, obtained by the oxidation of metals in air, not melting. Brown CdO is stable up to 900°C, and white ZnO sublimates at temperatures above 1800°C. The former is poisonous, and the toxicity of ZnO is relatively low.
Both metals are insoluble in water. ZnS crystals can be obtained and are found in nature in two modifications: (α) and wurtzite (β), the structure of the latter is false, less stable and less common. Solubility product of sphalerite 1.6 10 -24 , wurtzite - 2.5 10 -22 . Cadmium sulfide is isomorphic with sphalerite, and as a separate mineral - howlite it is very rare.
Of the complex compounds for metallurgy, mine ones are of interest: Zn (NH 3) 2 n + (n from 1 to 4) and Cd(NH 4) 2 + ( t from 1 to 6). the most stable are Zn(NH 3) 2 4 +; pK=SJ0 and Cd(NH 3) 2+ ; p K=6.56.
Article on the topic Metallurgy of zinc and cadmium
- Specialty HAC RF05.17.02
- Number of pages 86
2. LITERATURE REVIEW
2.1. Physical and chemical properties of cadmium and zinc
2.2. Methods for the separation of zinc and cadmium
2.3. Extraction with organic solvents
2.4. Effect of Hydration of Organic Phase Components on the Extraction Equilibrium
2.5. Tributyl phosphate as an extractant for acids and metal salts
2.6. Extraction of zinc and cadmium chlorides with tri-n-butyl phosphate
Recommended list of dissertations
Phase and extraction equilibria in systems water - synthamide-5 - salting out agent - water 2011, candidate of chemical sciences Golovkina, Anna Vladimirovna
Thermodynamic description of the extraction of hydrochloric and hydrobromic acids and uranyl chloride with tri-n-butyl phosphate 2000 PhD Bacaro Fernandez Eduardo
Extraction recovery and separation of lanthanides in the processing of poor non-traditional raw materials 2007, candidate of technical sciences Starshinova, Veronika Stepanovna
Dipyrazolonylmethanes as Extraction Reagents of Elements from Ammonia, Alkaline and Acid Solutions 1984, candidate of chemical sciences Khorkova, Margarita Aleksandrovna
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Introduction to the thesis (part of the abstract) on the topic "Extraction of cadmium and zinc chlorides with tri-n-butyl phosphate"
Relevance of the topic.
Extraction with organic solvents is increasingly used in hydrometallurgy. Especially promising is the use of extraction methods in the separation of metals with similar properties, when it is necessary to organize a multistage process. In industry, extraction processes are used to separate zirconium and hafnium, rare earth elements, platinoids, cobalt and nickel, etc. In the nuclear industry, the so-called "Purex process" is widely used to separate uranium, plutonium and neptunium from fission products.
For a wider application of extraction, it is necessary to advance in the development of the theory of extraction and the creation on its basis of methods for describing the extraction equilibrium. To the greatest extent, this concerns extraction by the hydrate-solvate mechanism, where a significant amount of water passes into the organic phase, as a result of which the description of the extraction equilibrium is a very difficult problem. Many strong acids and metal acid complexes, including halide complexes, are extracted by this mechanism, and neutral organic substances are used as extractants, among which the most popular of them is tri-n-butyl phosphate (TBP).
Cadmium occupies an important place among the metals used in nuclear power engineering. Along with boron, cadmium is the main element for the absorption and registration of thermal neutrons. In nature, cadmium occurs in lead-zinc sulfide ores and the main problem in its extraction is its separation from zinc. Note that the special
The application of zinc, especially for household purposes, requires its good purification from cadmium due to the high toxicity of the latter. For this purpose, the different stability of the halide complexes of cadmium and zinc is often used, and extraction with an organic solvent can be used to separate the complexes. Thus, the extraction of chloride complexes of cadmium and zinc with TBP has been repeatedly described in the literature, but attempts to quantitatively describe this system, which is necessary to find optimal separation conditions, have not led to the desired result.
The aim of this work was to develop, using the example of the HpC12-CdC12-H20-TBP-salting out system, methods for describing the extraction equilibrium, taking into account the activity coefficients in the organic and aqueous phases and the complexation constants applicable for extraction by the hydrate-solvate mechanism, and predicting the optimal separation conditions. .
To achieve this goal, it was necessary to solve the following tasks:
1. To study the extraction of zinc and cadmium chlorides separately with TBP in the presence of a salting out agent and develop a method for describing the equilibrium in these systems.
2. To study the extraction of zinc and cadmium chlorides together with TBP.
3. To develop a method for calculating the equilibrium in the joint extraction of metal chlorides with TBP based on data for individual chlorides.
Scientific novelty.
1. Data on the extraction of zinc and cadmium chlorides with TBP from 0.1 mol/l (individually or in total) aqueous solutions containing sodium chloride as a salting out agent.
2. Method for calculating the activity of TBP in the organic phase during the extraction of metal salts.
3. A model for the extraction of zinc chloride with TBP in the presence of sodium chloride and a set of constants that makes it possible to describe the extraction equilibrium taking into account the formation of acid complexes in one phase.
4. A model for the extraction of cadmium chloride with TBP in the presence of sodium chloride and a set of constants that makes it possible to describe the extraction equilibrium.
5. Calculation of the extraction equilibrium in the joint extraction of zinc and cadmium chlorides with TBP using a previously determined set of constants and the adequacy of the calculated distribution coefficients to the experimental ones.
Practical significance.
The developed method for calculating the equilibrium in the extraction of zinc and cadmium chlorides with TBP in the presence of a salting out agent is of a general nature and can be used to describe the equilibrium in the extraction of metal salts, including those with similar properties.
Approbation of work. The results of the work were discussed at the XI Russian Conference on Extraction (Moscow, 1998), the XI International Conference of Young Scientists in Chemistry and Chemical Technology "MKHT-97" (Moscow, 1997), the Moscow Seminar on Extraction (1999).
2. LITERARY REVIEW.
Similar theses in the specialty "Technology of rare, scattered and radioactive elements", 05.17.02 VAK code
Study of regularities of liquid-phase and extraction equilibria in systems water - antipyrine derivative - benzoic acid 2006, candidate of chemical sciences Poroshina, Natalya Vitalievna
Extraction of metals with tetraoctylalkylenediamines from sulfate chloride solutions 1999, candidate of chemical sciences Zheleznov, Veniamin Viktorovich
2005, candidate of chemical sciences Koroleva, Marina Valerievna
Regularities of extraction of metal ions by melts in separating systems diantipyrilalkane - benzoic acid - inorganic acid - ammonium thiocyanate - water 2009, Candidate of Chemical Sciences Alikina, Ekaterina Nikolaevna
Obtaining compounds of individual REMs and by-products in the processing of low-quality rare-metal raw materials 2014, Doctor of Technical Sciences Litvinova, Tatyana Evgenievna
Dissertation conclusion on the topic "Technology of rare, scattered and radioactive elements", Nekhaevsky, Sergey Yurievich
5. CONCLUSIONS.
1. The extraction of zinc and cadmium chlorides by TBP, separately and together, from aqueous solutions containing chlorides of these metals with a total initial concentration of 0.1 mol/l and various concentrations of zinc chloride as a salting out agent, as well as the extraction of trace amounts of cadmium chloride by TBP from aqueous sodium chloride solutions. It has been found that the distribution coefficient of zinc passes through a maximum with an increase in the salting-out agent concentration, while no extremum is observed for cadmium.
2. A method has been developed for calculating the activity of TBP in the organic phase during the extraction of metal salts, based on determining its mole fraction and calculating the activity coefficient according to the empirical equation found for the binary system TBP-H-O.
3. The value (11.5) of the extraction constant of zinc chloride with TBP with the formation of an rpCb*2TBP complex hydrated with two water molecules was calculated taking into account the activity coefficients in the organic and water* phases and the formation of zinc acid complexes in the aqueous phase. A method is proposed for taking into account the formation of acid complexes of zinc hpCl3~ in the aqueous phase, taking into account the activity coefficients of the components.
4. The value (15.0) of the extraction constant of cadmium chloride with TBP was calculated to form the complex Сс1С12 *2TBP, hydrated with two water molecules, taking into account the activity coefficients in the organic and aqueous phases.
5. The found extraction constants of zinc and cadmium chlorides with TBP and the zinc acid complex formation constant 2nC13" were used to calculate by iteration the distribution coefficients of zinc and cadmium chlorides during their joint extraction with TBP, taking into account the activity coefficients in the organic and aqueous phases. The calculated and experimental distribution coefficients are consistent between with an accuracy of about 10%, which corresponds to the accuracy of the experimental data.
6. The decrease in the separation coefficients of zinc and cadmium chlorides described in the literature with an increase in the salting-out agent concentration and the formation of zinc acid complexes is explained.
2.7. Conclusion.
The considered literature materials show that the description of the equilibrium in the extraction of acids and metals by the hydrate-solvate mechanism is a difficult task. Previous attempts
ODSY*S"DYA
44 UT! ■ "■ ■ ■ , ■ ■■. G" had certain shortcomings. It is also likely that the calculations of the extraction constants should be carried out on the scale of mole fractions, since it is difficult to recalculate the activity coefficients in other scales. However, in this case, it is necessary to take into account the distribution of water, which, as shown in the literature, is practically not related to TBP. It is also reasonable to take into account the effect of hydration on the extraction equilibrium through the activity coefficients of hydrated components within the concept of nonstoichiometric hydration.
3. EXPERIMENTAL AND CALCULATION TECHNIQUES.
3.1. Reagents.
We used sodium chloride of chemical pure grade, zinc and cadmium chlorides, sodium carbonate, and hydrochloric acid of analytical grade without additional purification.
The concentration of the initial solutions of sodium, cadmium, and zinc chlorides was determined by the ion exchange method. To do this, a column was prepared with a KU-2 cationite in the H-form, washing it with a 2 mol/l solution of hydrochloric acid. After that, the column was washed with distilled water to pH=5.0. Then, 1 ml of the initial solution was added to the column and it was washed with distilled water until pH=5.0. Wash water containing the acid released during ion exchange was collected and titrated with an alkali solution prepared from fixanal.
Tri-n-butyl phosphate "tech." was purified by the usual method | I! 1 I 1 1 | |
For zinc-65, the positron yield is only 1.41%, and its activity was determined by gamma radiation (including annihilation radiation with an energy of 511 keV) on a MaLT1 scintillation counter using liquid samples. Determination of the activity of cadmium-109 was carried out on end counters SBT-7. Since the soft photon radiation of this radionuclide is noticeably absorbed in the preparation layer, cadmium was first isolated by precipitation of its sulfide, the procedure of which will be described below, and then preparations of cadmium-109 were prepared, the layer thickness of which could be neglected.
The count rate of the drug 1R is the difference between the count rates of the drug with the background 1P + F and the background 1f: 1p + f "Ch ^
The error in determining the count rate 1st is calculated by the formula:
A1P \u003d [(1u + fAl + f) + (1fAf)] 0 "5. (3.2) where C + f and tf are the time for measuring the count rates of the drug with the background and the background. The time for the measurements of the drugs and the background was chosen so that the error for determining the counting rate D1P did not exceed 3%, and was calculated using the formulas:
- (yuo / g) 2 * / [(1p + f- 1f) 2] (s.z)
C = (100/r)2 * /[(1p + f - 1f)2] (3.4) where r is the specified relative error. When using formulas (3.3) and (3.4), we first determined the count rates 1p + f and 1F for 30 s, and then calculated the required measurement time and rounded it up to a multiple of 100 s.
3. 3. Extraction technique.
The extraction of zinc and cadmium was carried out in separatory funnels at room temperature within 296±2K. The aqueous and organic phases were equilibrated using a shaker for 10 min. Preliminary experiments showed that equilibrium was reached in 3-5 minutes. After shaking, settling was carried out for 30–40 min. After that, the organic and aqueous phases were separated and centrifuged for 10 min. on laboratory centrifuge LC-425.
When extracting zinc-65 from solutions of hydrochloric acid, 5 ml of TBP was equilibrated twice with equal volumes of fresh acid solutions of a given molarity, each time after equilibration, the aqueous phase drained as completely as possible. Next, 5 ml of hydrochloric acid of the same concentration containing the radionuclide was added. After shaking, the phases were separated and the resulting solutions were centrifuged. Next, 4 ml of each phase was taken and a radiometric determination of zinc-65 was performed on a scintillation counter, as described above. Experimental data, averaged over 5 parallel experiments, are presented in Table. 3.1.
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Cadmium - uncommon toxic and unknown
a wide range of silver hazardous metal
Toxic and poisonous stones and minerals
Cadmium(Latin Cadmium, denoted by the symbol Cd) is an element with atomic number 48 and atomic mass 112.411. It is an element of a secondary subgroup of the second group, the fifth period of the periodic system of chemical elements of D.I. Mendeleev. Under normal conditions, a simple substance cadmium is a heavy (density 8.65 g / cm3 - lighter than uranium) soft malleable ductile transition metal silver white colors (does not devour the flesh, like the "Kerbersky stone" of the Zhytomyr region of Ukraine - not uranium oxide pitchblende, brown dangerous stone). On the picture - cadmium sulfide, greenockite(earthy crusts yellow colors).
Natural cadmium consists of eight isotopes, six of which are stable: 106Cd (isotope abundance 1.22%), 108Cd (0.88%), 110Cd (12.39%), 111Cd (12.75%), 112Cd (24, 07%), 114Cd (28.85%). Radioactivity was detected for two other natural isotopes: 113Cd (isotopic abundance 12.22%, β-decay with a half-life of 7.7∙1015 years) and 116Cd (isotopic abundance 7.49%, double β-decay with a half-life of 3.0 ∙1019 years).
Cadmium of the periodic system was partially described by the German professor Friedrich Stromeyer in 1817 (distinguished from zinc). Magdeburg pharmacists, when studying preparations containing zinc oxide ZnO, suspected the presence of arsenic (oxidation catalyst from sulfide) in them. Since zinc oxide is part of many ointments, powders and emulsions used for various skin diseases inspectors categorically forbade the sale of suspicious drugs.
Naturally, the manufacturer of medicines, defending his personal interests, demanded an examination. Stromeyer acted as an expert. He isolated a brown-brown oxide from ZnO, reduced it with hydrogen and obtained a silver-white metal, which he called "cadmium" (from the Greek kadmeia - zinc oxide, also zinc ore). Regardless of Professor Stromeyer, cadmium was discovered in Silesian zinc ores (satellite) by a group of scientists - K. Hermann, K. Carsten and W. Meisner in 1818.
Cadmium absorbs slow neutrons, for this reason cadmium rods are used in nuclear reactors to control the rate of a chain reaction (ChNPP). Cadmium is used in alkaline batteries and is included as a component in some alloys. So, for example, copper alloys containing about 1% Cd (cadmium bronze) are used for the manufacture of telegraph, telephone, trolleybus and tram wires, subway cables, since these alloys have greater strength and wear resistance than copper.
Greenockite (yellow dope) on calcite. Yunnan, China. 7x5 cm. Photo: A.A. Evseev.
A number of fusible alloys, such as those used in fire extinguishers, contain cadmium. In addition, cadmium is a part of substandard jewelry alloys (soldering after evaporation of the amalgam component from amalgam alloys that have burst due to temperature and are prohibited in the open sale - amalgams of gold, silver and platinum with toxic mercury).
This metal is used for cadmium plating of steel products, because it carries an oxide film on its surface, which has a protective effect. The fact is that in sea water and in a number of other media, cadmium plating is more effective than galvanizing. Cadmium has a long history of use in homeopathic (basic treatment with herbs and microdoses - the so-called "Biologically Active Supplements in Food" - dietary supplements and animal feed) medicine. Cadmium compounds have also found wide application - cadmium sulfide is used to make yellow paint and colored glasses, and cadmium fluoroborate is a flux used to solder aluminum and other metals.
Cadmium is found in the body of vertebrates (bones, ligaments, tendons and muscles), it is established that it affects carbon metabolism, the activity of a number of enzymes and the synthesis of hippuric acid in the liver. However, cadmium compounds are poisonous, and the metal itself is a carcinogen. Especially dangerous is the inhalation of vapors of cadmium oxide CdO, fatal cases are not uncommon. The penetration of cadmium into the gastrointestinal tract is also harmful, but no cases of fatal poisoning have been recorded, most likely this is due to the fact that the body seeks to get rid of the toxin (vomiting).
Biological properties
It turns out that cadmium is present in almost all living organisms - in terrestrial cadmium content is approximately equal to 0.5 mg per 1 kg of mass, in marine organisms (sponges, coelenterates, echinoderms, worms of the Pacific Ocean) - from 0.15 to 3 mg / kg, the content of cadmium in plants is about 10-4% (on dry matter). Despite the presence of cadmium in most living organisms, its specific physiological significance has not been fully established (growth hormone). Scientists managed to find out that this element affects carbohydrate metabolism, the synthesis of hippuric acid in the liver, the activity of a number of enzymes, as well as the metabolism of zinc, copper, iron and calcium in the body (a favorite stone of bodybuilders who increase muscle mass and strengthen their bones in sports - in microdoses).
Greenockite (yellow). Curly volcano, about. Iturup, Kuril Islands, Russia. Photo: A.A. Evseev.
Can be issued for talc, sulfur and other greenoctite-like minerals
There is a suggestion, supported by research, that microscopic amounts of cadmium in food can stimulate body growth in mammals. For this reason, scientists have long ranked cadmium as a conditionally essential trace element, that is, vital, but toxic in certain doses. In the body healthy person contains no a large number of cadmium. Sung in ancient Greek and Roman epic - Cadmeus(place poison trade in the south-east of Europe ("Shield on the gates of Tsaregrad", Istanbul), in Greece (porticos and amphitheaters) and in the Mediterranean near Turkey - a drug). On the slang miners and stone miners cadmium called " snake poison" (jargon).
Cadmium is one of the most toxic heavy metals- in Russia (metrology) it is assigned to the 2nd hazard class - highly hazardous substances - which include antimony, strontium, phenol and other toxic substances (equivalent to ADR dangerous goods N 6 - poison, skull and crossbones in a rhombus). In the Bulletin of the Russian Federation on Environmental Safety and Poison Transportation Technologies "Problems of Chemical Safety" dated April 29, 1999, cadmium appears as "the most dangerous ecotoxicant at the turn of the millennium"!
Like other heavy metals, cadmium is a cumulative poison, that is, it can accumulate in the body - its half-life is from 10 to 35 years. By the age of fifty, the human body is able to accumulate from 30 to 50 mg of cadmium. The main "depots" of cadmium in the human body are the kidneys, which contain from 30 to 60% of the total amount of this metal in the body, and the liver (20-25%). The following are able to accumulate cadmium to a lesser extent: the pancreas, spleen, tubular bones, and other organs and tissues. Small amounts of cadmium are present even in the blood. However, unlike lead or mercury, cadmium does not enter the brain.
For the most part, cadmium in the body is in a bound state - in combination with the protein metallothionein - this is a kind of protective mechanism, the body's reaction to the presence of a heavy metal. In this form, cadmium is less toxic, however, even when bound, it does not become harmless - accumulating over the years, this metal can lead to disruption of the kidneys and an increased likelihood of kidney stones. Much more dangerous is cadmium, which is in ionic form, because it is chemically very close to zinc and is able to replace it in biochemical reactions, acting as a pseudo-activator or, conversely, an inhibitor of zinc-containing proteins and enzymes.
Cadmium binds to the cytoplasmic and nuclear material of cells of a living organism and damages them, changes the activity of many hormones and enzymes, which is explained by its ability to bind sulfhydryl (-SH) groups. In addition, cadmium, due to the proximity of the ionic radii of calcium and cadmium, is able to replace calcium in bone tissue. The same situation is with iron, which cadmium is also able to replace. For this reason, the lack of calcium, zinc and iron in the body can lead to an increase in the absorption of cadmium from the gastrointestinal tract up to 15-20%. It is believed that a harmless daily dose of cadmium for an adult is 1 μg of cadmium per 1 kg of body weight, large amounts of cadmium are extremely hazardous to health.
What are the mechanisms of entry of cadmium and its compounds into the body? Poisoning occurs when drinking water (maximum concentration limit for drinking water is 0.01 mg/l) contaminated with cadmium-containing waste, as well as when eating vegetables and grains growing on lands located near oil refineries and metallurgical enterprises. The use of mushrooms from such territories is especially dangerous, since, according to some information, they are able to accumulate more than 100 mg of cadmium per kg of their own weight. Smoking is another source of cadmium intake into the body, both of the smoker himself and of the people around him, because the metal is found in tobacco smoke.
The characteristic signs of chronic cadmium poisoning are, as mentioned earlier, kidney damage, muscle pain, bone tissue destruction, and anemia. Acute food poisoning with cadmium occurs when large single doses are taken with food (15-30 mg) or with water (13-15 mg). At the same time, signs of acute gastroenteritis are observed - vomiting, pain and convulsions in the epigastric region, however, cases of fatal poisoning with cadmium compounds that have entered the body with food are unknown to science, but according to WHO estimates, a lethal single dose can be 350-3500 mg.
Much more dangerous is cadmium poisoning by inhalation of its vapors (CdO) or cadmium-containing dust (as a rule, this occurs in industries related to the use of cadmium) - similar to liquid mercury and red cinnabar (by toxicity). Symptoms of such poisoning are pulmonary edema, headache, nausea or vomiting, chills, weakness, and diarrhea (diarrhea). As a result of such poisoning, deaths have been recorded.
The antidote for cadmium poisoning is selenium, which helps to reduce the absorption of cadmium (they work on copiers and printers in modern data centers and refill cartridges for office equipment). However, a balanced intake of selenium is still required, this is due to the fact that its excess in the body leads to a decrease in the sulfur content (forms sulfur sulfide - binds it), and this will certainly lead to the fact that cadmium will again be absorbed by the body.
Interesting Facts
It has been established that one cigarette contains from 1 to 2 micrograms of cadmium. It turns out that a person who smokes a pack of cigarettes per day (20 pcs.) Gets about 20 micrograms of cadmium! The danger lies in the fact that the absorption of cadmium through the lungs maximum- from 10 to 20%, thus, in the body of a smoker, from 2 to 4 micrograms of cadmium is absorbed with each pack of cigarettes! The carcinogenic effect of nicotine contained in tobacco smoke is usually associated with the presence of cadmium, and it is not retained even by carbon filters - lung cancer.
An example of chronic cadmium poisoning with numerous fatal outcomes was described in the late 1950s. On the territory of Japan, there have been cases of a disease that the locals called "itai-itai" ("Italian disease"), which can also be translated into the local dialect as "oh, how it hurts!" (poisoning). Symptoms of the disease were severe lumbar pain, which, as it turned out later, was caused by irreversible kidney damage; severe muscle pain. The widespread spread of the disease and its severe course were caused by high environmental pollution in Japan at that time and the specific diet of the Japanese (rice and seafood accumulate large amounts of cadmium). It was found that those who fell ill with this disease consumed about 600 micrograms of cadmium daily!
Despite the fact that cadmium is recognized as one of the most toxic substances, it has also found application in medicine! Thus, inserted into the chest of a patient suffering from heart failure, a nickel-cadmium battery provides energy to a mechanical stimulator of the heart. The convenience of such a battery is that the patient does not have to lie down on the operating table to recharge or replace it. For uninterrupted battery life, it is enough to wear a special magnetized jacket once a week for just an hour and a half.
Cadmium is used in homeopathy, experimental medicine, and more recently it has been used to create new anticancer drugs.
Wood's metal alloy, containing 50% bismuth, 12.5% tin, 25% lead, 12.5% cadmium, can be melted in boiling water. The alloy was invented in 1860 by engineer B. Wood ) Several curious facts are associated with this low-melting alloy: firstly, the first letters of the components of Wood's alloy form the abbreviation "WAX", and secondly, the invention is also attributed to B. Wood's namesake - American physicist Robert Williams Wood, who was born eight years later ( peers fought at VAK).
Not so long ago, cadmium of the periodic system entered the “armament” of the police and forensic specialists: with the help of the thinnest layer of cadmium deposited on the surface being examined, it is possible to identify human fingerprints.
Scientists have established interesting fact: cadmium tin in the atmosphere of rural areas has a much greater resistance to corrosion than in the atmosphere of industrial areas. Such a coating fails especially quickly if the content of sulfurous or sulfuric anhydrides is increased in the air.
In 1968, one of the US health officials (Dr. Carroll) discovered a direct relationship between mortality from cardiovascular diseases and the content of cadmium in the atmosphere. He came to such conclusions by analyzing the data of 28 cities. In four of them - New York, Chicago, Philadelphia and Indianapolis - the content of cadmium in the air was higher than in other cities; the proportion of deaths due to heart disease was also higher.
In addition to "standard" measures to limit cadmium emissions into the atmosphere, water and soil (filters and cleaners at enterprises, removal of housing and crop fields from such enterprises), scientists are also developing new - promising ones. So scientists in the bay of the Mississippi River planted water hyacinths, believing that with their help it would be possible to clean the water from elements such as cadmium and mercury.
Story
History knows many "discoveries" that were made during fictitious checks, reviews and revisions. However, such finds are more criminal in nature than scientific. And yet there was such a case when the revision that had begun eventually led to the discovery of a new chemical element. It happened in Germany at the beginning of the 19th century. The district doctor R. Rolov checked the pharmacies of his district, during the audit - in a number of pharmacies near Magdeburg - he found zinc oxide, appearance which aroused suspicion and suggested the content of arsenic in it (pharmacolyte). To confirm the assumptions, Rolov dissolved the seized drug in acid and passed it through a solution of hydrogen sulfide, which led to the precipitation of a yellow precipitate, similar to arsenic sulfide. All suspicious medicines - ointments, powders, emulsions, powders - were immediately withdrawn from sale.
Such a move outraged the owner of the factory in Schenebek, which produced all the drugs rejected by Rolov. This businessman - Herman, being a chemist by profession, conducted his own examination of the goods. Having tried all the arsenal of experiments known at that time for the detection of arsenic, he was convinced that his products were pure in this respect, and iron, which confused the auditor, gave the yellow color of zinc oxide.
Having reported the results of his experiments to Rolov and the authorities of the land of Hanover, Herman demanded an independent examination and complete "rehabilitation" of his product. As a result, it was decided to find out the opinion of Professor Stromeyer, who headed the Department of Chemistry at the University of Göttingen, and concurrently held the post of Inspector General of all Hanoverian pharmacies. Naturally, Stromeyer was sent for verification not only zinc oxide, but also other zinc preparations from the Shenebek factory, including zinc carbonate, from which this oxide was obtained.
By calcining zinc carbonate ZnCO3, Friedrich Stromeyer obtained oxide, but not white, as it should have been, but yellowish. As a result of further research, it turned out that the drugs do not contain either arsenic, as Rolov suggested, or iron, as German thought. The reason for the unusual color was a completely different metal - previously unknown and very similar in properties to zinc. The only difference was that its hydroxide, unlike Zn (OH) 2, was not amphoteric, but had pronounced basic properties.
Stromeyer named the new metal cadmium, hinting at the strong similarity of the new element with zinc - the Greek word καδμεια (kadmeia) has long denoted zinc ores (for example, smithsonite ZnCO3) and zinc oxide. In turn, this word comes from the name of the Phoenician Cadmus, who, according to legend, was the first to find a zinc stone and discovered its ability to impart copper (when smelting it from ore) golden color. According to ancient Greek myths, there was another Cadmus - a hero who defeated the Dragon and built the fortress of Cadmeus on the lands of the enemy defeated by him, around which the great seven-gate city of Thebes subsequently grew. In the Semitic languages, "kadmos" means "eastern" or "serpentine" (Fergana, Kyrgyzstan, Central Asia - there are places where snakes accumulate), which, perhaps, builds the name of the mineral from the places of its extraction or export from any eastern country or province .
In 1818, Friedrich Stromeyer published a detailed description of the metal, the properties of which he had already studied well. In its free form, the new element was a white metal, soft and not very strong, covered with a brownish oxide film on top. Pretty soon, as often happens, Strohmeyer's priority in the discovery of cadmium began to be challenged, but all claims were rejected. Somewhat later, another chemist, Kersten, found a new element in Silesian zinc ore and named it mellin (from the Latin mellinus, "yellow like quince"). The reason for this name was the color of the precipitate formed under the action of hydrogen sulfide.
To Kersten's chagrin, the "mellin" turned out to be Stromeyer's "cadmium". Even later, other names were proposed for the forty-eighth element: in 1821, John proposed calling the new element "klaprotium" - in honor of the famous chemist Martin Klaproth - the discoverer of uranium, zirconium and titanium, and Gilbert "junonium" - after the asteroid discovered in 1804 Juno. But no matter how great Klaproth's merits to science, his name was not destined to gain a foothold in the list of chemical elements: cadmium remained cadmium. True, in the Russian chemical literature of the first half of the 19th century, cadmium was often called cadmium.
Being in nature
Cadmium is a typically rare and rather scattered element, the average content of this metal in the earth's crust (clarke) is estimated at about 1.3 * 10–5% or 1.6 * 10–5% by weight, it turns out that cadmium in the lithosphere is approximately 130 mg / t. There is so little cadmium in the bowels of our planet that even germanium, which is considered rare, is 25 times more! Approximately the same ratios for cadmium and other rare metals: beryllium, cesium, scandium and indium. Cadmium is close in abundance to antimony (2 * 10–5%) and twice as common as mercury (8 * 10–6%).
Cadmium is characterized by migration in hot groundwater along with zinc (cadmium is found as an isomorphic impurity in many minerals and always in zinc minerals) and other chalcophilic elements, that is, chemical elements prone to the formation of natural sulfides, selenides, tellurides, sulfosalts and sometimes found in the native state. In addition, cadmium is concentrated in hydrothermal deposits. Volcanic rocks are quite rich in cadmium, containing up to 0.2 mg of cadmium per kg; among sedimentary rocks, clay is the richest in the forty-eighth element - up to 0.3 mg / kg (for comparison, limestones contain cadmium 0.035 mg / kg, sandstones - 0.03 mg / kg). The average content of cadmium in the soil is 0.06 mg/kg.
Also, this rare metal is present in water - in dissolved form (sulfate, chloride, cadmium nitrate) and in suspension as part of organo-mineral complexes. Under natural conditions, cadmium enters groundwater as a result of leaching of non-ferrous metal ores, as well as as a result of the decomposition of aquatic plants and organisms capable of accumulating it. Since the beginning of the 20th century, anthropogenic contamination of natural waters with cadmium has become the predominant factor in the entry of cadmium into water and soil. The content of cadmium in water is significantly affected by the pH of the medium (in an alkaline medium, cadmium precipitates in the form of hydroxide), as well as sorption processes. For the same anthropogenic reason, cadmium is also present in the air.
In rural areas, the content of cadmium in the air is 0.1-5.0 ng / m3 (1 ng or 1 nanogram = 10-9 grams), in cities - 2-15 ng / m3, in industrial areas - from 15 to 150 ng /m3. Cadmium is mainly released into the atmospheric air due to the fact that many coals burned at thermal power plants contain this element. Being deposited from the air, cadmium enters the water and soil. The increase in the content of cadmium in the soil is facilitated by the use of mineral fertilizers, because almost all of them contain minor impurities of this metal. From water and soil, cadmium enters plants and living organisms and further along the food chain can be "supplied" to humans.
Cadmium has its own minerals: howliite, otavite CdCO3, montemponite CdO (contains 87.5% Cd), greenockite CdS (77.8% Cd), xanthochroite CdS(H2O)x (77.2% Cd) cadmoselite CdSe (47% Cd ). However, they do not form their own deposits, but are present as impurities in zinc, copper, lead and polymetallic ores (more than 50), which are the main source of industrial production of cadmium. Moreover, the main role is played by zinc ores, where the concentration of cadmium ranges from 0.01 to 5% (in sphalerite ZnS). In most cases, the content of cadmium in sphalerite does not exceed 0.4 - 0.6%. Cadmium accumulates in galena (0.005 - 0.02%), stannite (0.003 - 0.2%), pyrite (up to 0.02%), chalcopyrite (0.006 - 0.12%), cadmium is extracted from these sulfides.
Cadmium is able to accumulate in plants (most of all in fungi) and living organisms (especially in water), for this reason, cadmium can be found in marine sedimentary rocks - shales (Mansfeld, Germany).
Application
The main consumer of cadmium is the production of chemical current sources: nickel-cadmium and silver-cadmium batteries, lead-cadmium and mercury-cadmium cells in backup batteries, normal Weston cells. Cadmium nickel batteries (AKN) used in industry are one of the most popular among other chemical current sources.
The negative plates of such batteries are made of iron meshes with sponge cadmium as an active agent, and the positive plates are coated with nickel oxide. The electrolyte is a solution of caustic potash (potassium hydroxide). Nickel-cadmium alkaline batteries are more reliable than lead acid batteries. Chemical current sources using cadmium are distinguished by a long service life, stable operation and high electrical characteristics. In addition, recharging these batteries takes less than one hour! However, AKN cannot be recharged without a complete preliminary discharge, and in this they, of course, are inferior to metal hydride batteries.
Another wide field of application of cadmium is the deposition of protective anticorrosive coatings on metals (cadmium plating). Cadmium coating reliably protects iron and steel products from atmospheric corrosion. In the past, cadmium plating was carried out by immersing the metal in molten cadmium, the modern process is carried out exclusively by electrolysis. Cadmium plating is applied to the most critical parts of aircraft, ships, as well as parts and mechanisms designed to operate in tropical climates.
It is known that some properties of zinc and cadmium are similar, but cadmium coating has certain advantages over galvanized coating: firstly, it is more resistant to corrosion, and secondly, it is easier to make it even and smooth. In addition, unlike zinc, cadmium is stable in an alkaline environment. Cadmium tin is used quite widely, however, there is an area in which the use of cadmium coating is strictly prohibited - this is the food industry. This is due to the high toxicity of cadmium.
Until a certain point, the spread of cadmium coatings was also limited for another reason - when cadmium is applied electrolytically to a steel part, hydrogen contained in the electrolyte can penetrate into the metal, and, as is known, this element causes hydrogen embrittlement in high-strength steels, leading to unexpected destruction of the metal under load . The problem was solved by Soviet scientists from the Institute of Physical Chemistry of the USSR Academy of Sciences. It turned out that a negligible addition of titanium (one atom of titanium per thousand atoms of cadmium) protects the cadmium-plated steel part from the occurrence of hydrogen embrittlement, since the titanium absorbs all the hydrogen from the steel during the coating process.
About a tenth of the world production of cadmium is spent on the production of alloys. The low melting point is one of the reasons for the widespread use of cadmium in low-melting alloys. Such, for example, is Wood's alloy containing 12.5% cadmium. Such alloys are used as solders, as a material for obtaining thin and complex castings, in automatic fire-fighting systems, for soldering glass with metal. Solders containing cadmium are quite resistant to temperature fluctuations.
Another distinguishing feature of cadmium alloys is their high antifriction properties. Thus, an alloy containing 99% cadmium and 1% nickel is used for the manufacture of bearings operating in automobile, aircraft and marine engines. Since cadmium is not sufficiently resistant to acids, including organic acids contained in lubricants, cadmium-based bearing alloys are coated with indium. Alloying copper with small additions of cadmium (less than 1%) makes it possible to make more wear-resistant wires on electric transport lines. Such negligible additions of cadmium can significantly increase the strength and hardness of copper, practically without worsening its electrical properties. Cadmium amalgam (a solution of cadmium in mercury) is used in dental technology for the manufacture of dental fillings.
In the forties of the XX century, cadmium acquired a new role - they began to make control and emergency rods of nuclear reactors from it. The reason why cadmium the shortest time became a strategic material was that it absorbs thermal neutrons very well. But the first reactors of the beginning of the "atomic age" worked exclusively on thermal neutrons. Later it turned out that fast neutron reactors are more promising both for energy and for obtaining nuclear fuel - 239Pu, and cadmium is powerless against fast neutrons, it does not delay them. In the days of thermal neutron reactors, cadmium lost its dominant role, giving way to boron and its compounds (actually, coal and graphite).
About 20% of cadmium (in the form of compounds) is used for the production of inorganic colorants. Cadmium sulfide CdS is an important mineral dye formerly called cadmium yellow. Already at the beginning of the 20th century, it was known that cadmium yellow could be obtained in six shades, ranging from lemon yellow to orange. The resulting paints are resistant to weak alkalis and acids, and are completely insensitive to hydrogen sulfide.
Paints based on CdS were used in many areas - painting, printing, porcelain painting, they covered passenger cars, protecting them from locomotive smoke. Dyes containing cadmium sulfide were used in the textile and soap industries. However, at present, rather expensive cadmium sulfide is often replaced with cheaper dyes - cadmopone (a mixture of cadmium sulfide and barium sulfate) and zinc-cadmium litopone (the composition, like that of cadmopone, plus zinc sulfide).
Another cadmium compound, cadmium selenide CdSe, is used as a red dye. However, cadmium compounds have found their application not only in the production of dyes - cadmium sulfide, for example, is also used for the production of film solar cells, the efficiency of which is about 10-16%. In addition, CdS is a fairly good thermoelectric material, which is used as a component of semiconductor materials and phosphors. Sometimes cadmium is used in cryogenic technology, which is associated with its maximum thermal conductivity (relative to other metals) near absolute zero (vacuum).
Production
The main "suppliers" of cadmium are by-products of processing zinc, copper-zinc and lead-zinc ores. As for the native minerals of cadmium, the only one of interest in obtaining cadmium is greenockite CdS, the so-called "cadmium blende". Greenockite is mined together with faerite during the development of zinc ores. During the recycling process, cadmium accumulates in the by-products of the process, from where it is then recovered.
In the processing of polymetallic ores, as mentioned earlier, cadmium is often a by-product of zinc production. These are either copper-cadmium cakes (metal precipitates obtained as a result of cleaning solutions of zinc sulfate ZnSO4 by the action of zinc dust), which contain from 2 to 12% Cd, or pussieres (volatile fractions formed during the distillation of zinc), containing from 0.7 to 1.1% cadmium.
The richest in the forty-eighth element are concentrates obtained during the rectification purification of zinc, they can contain up to 40% cadmium. From copper-cadmium cakes and other products with a high content of cadmium, it is usually leached with sulfuric acid H2SO4 with simultaneous air aeration. The process is carried out in the presence of an oxidizing agent - manganese ore or recycled manganese sludge from electrolysis baths.
In addition, cadmium is recovered from dust from lead and copper smelters (it can contain 0.5 to 5% and 0.2 to 0.5% cadmium, respectively). In such cases, the dust is usually treated with concentrated H2SO4 sulfuric acid, and then the resulting cadmium sulfate is leached with water. A cadmium sponge is precipitated from the resulting cadmium sulfate solution by the action of zinc dust, after which it is dissolved in sulfuric acid and the solution is purified from impurities by the action of sodium carbonate Na2CO3 or zinc oxide ZnO, it is also possible to use ion exchange methods.
Cadmium metal is isolated by electrolysis on aluminum cathodes or by reduction with zinc (displacement of cadmium oxide CdO from CdSO4 solutions by zinc) using centrifugal separator reactors. The refining of cadmium metal usually consists in melting the metal under a layer of alkali (to remove zinc and lead), while using Na2CO3 is possible; treatment of the melt with aluminum (to remove nickel) and ammonium chloride NH4Cl (to remove thallium).
Higher purity cadmium is obtained by electrolytic refining with intermediate purification of the electrolyte, which is carried out using ion exchange or extraction; rectification of the metal (usually under reduced pressure), zone melting or other crystallization methods. Combining the above purification methods, it is possible to obtain metallic cadmium with the content of the main impurities (zinc, copper and others) of only 10-5% by weight. In addition, the methods of electrotransfer in liquid cadmium, electrorefining in a melt of sodium hydroxide NaOH, and amalgam electrolysis can be used to purify cadmium. When zone melting is combined with electrotransfer, separation of cadmium isotopes can occur along with purification.
The world production of cadmium is largely related to the scale of zinc production and has increased significantly over the past decades - according to 2006 data, about 21 thousand tons of cadmium were produced in the world, while in 1980 this figure was only 15 thousand tons. The growth of cadmium consumption continues even now. The main producers of this metal are Asian countries: China, Japan, Korea, Kazakhstan. They account for 12 thousand tons of total production.
Russia, Canada and Mexico can also be considered major producers of cadmium. The shift of mass production of cadmium towards Asia is due to the fact that in Europe there has been a reduction in the use of cadmium, and in the Asian region, on the contrary, the demand for nickel-cadmium elements is growing, which forces many to transfer production to Asian countries.
Physical Properties
Cadmium is a silvery-white metal that shimmers blue when freshly cut, but tarnishes in air due to the formation of a protective oxide film. Cadmium is a rather soft metal - it is harder than tin, but softer than zinc, it is quite possible to cut it with a knife. In combination with softness, cadmium has such important qualities for industry as ductility and ductility - it is perfectly rolled into sheets and drawn into wire, and can be polished without any problems.
When heated above 80 o C, cadmium loses its elasticity, and so much so that it can easily be crushed into powder. The hardness of cadmium according to Mohs is equal to two, according to Brinell (for an annealed sample) 200-275 MPa. Tensile strength 64 MN/m2 or 6.4 kgf/mm2, relative elongation 50% (at 20 o C), yield strength 9.8 MPa.
Cadmium has a hexagonal close-packed crystal lattice with periods: a = 0.296 nm, c = 0.563 nm, c/a ratio = 1.882, z = 2, crystal lattice energy 116 μJ/kmol. Space group С6/mmm, atomic radius 0.156 nm, ionic radius Cd2+ 0.099 nm, atomic volume 13.01∙10-6 m3/mol.
A rod made of pure cadmium, when bent, emits a weak crack like tin ("tin scream") - this is metal microcrystals rubbing against each other, however, any impurities in the metal destroy this effect. In general, in terms of its physical, chemical and pharmacological properties, cadmium belongs to the group of heavy metals, having the most similarities with zinc and mercury.
The melting point of cadmium (321.1 o C) is quite low and can be compared with the melting points of lead (327.4 o C) or thallium (303.6 o C). However, it differs from the melting points of metals similar in a number of properties - lower than that of zinc (419.5 o C), but higher than that of tin (231.9 o C). The boiling point of cadmium is also low - only 770 o C, which is quite interesting - lead, like most other metals, has a large difference between melting and boiling points.
So lead has a boiling point (1745 o C) 5 times higher than the melting point, and tin, whose boiling point is 2620 o C, is 11 times higher than the melting point! At the same time, zinc, similar to cadmium, has a boiling point of only 960 o C at a melting point of 419.5 o C. The thermal expansion coefficient for cadmium is 29.8 * 10-6 (at a temperature of 25 o C). Below 0.519 K, cadmium becomes a superconductor. The thermal conductivity of cadmium at 0 o C is 97.55 W / (m * K) or 0.233 cal / (cm * sec * o C).
The specific heat capacity of cadmium (at a temperature of 25 o C) is 225.02 j/(kg * K) or 0.055 cal/(g * o C). The temperature coefficient of electrical resistance of cadmium in the temperature range from 0 o C to 100 o C is 4.3 * 10-3, the specific electrical resistance of cadmium (at a temperature of 20 o C) is 7.4 * 10-8 ohm * m (7.4 * 10-6 ohm*cm). Cadmium is diamagnetic, its magnetic susceptibility is -0.176.10-9 (at a temperature of 20 o C). The standard electrode potential is -0.403 V. The electronegativity of cadmium is 1.7. The effective cross section of capture of thermal neutrons is 2450-2900-10 ~ 28 m2. Work function of electrons = 4.1 eV.
The density (at room temperature) of cadmium is 8.65 g/cm3, which makes it possible to classify cadmium as a heavy metal. According to the classification of N. Reimers, metals with a density of more than 8 g/cm3 should be considered heavy. Thus, heavy metals include Pb, Cu, Zn, Ni, Cd, Co, Sb, Sn, Bi, Hg. And although cadmium is lighter than lead (density 11.34 g/cm3) or mercury (13.546 g/cm3), it is heavier than tin (7.31 g/cm3).
Chemical properties
In chemical compounds, cadmium always exhibits valency 2 (configuration of the outer electron layer 5s2) - the fact is that the atoms of the elements of the secondary subgroup of the second group (zinc, cadmium, mercury), like the atoms of the elements of the copper subgroup, have a d-sublevel of the second outside electronic layer is completely filled. However, for elements of the zinc subgroup, this sublevel is already quite stable, and the removal of electrons from it requires a very large expenditure of energy. Another characteristic feature of the elements of the zinc subgroup, which brings them closer to the elements of the copper subgroup, is their tendency to complex formation.
As already mentioned, cadmium is located in the same group of the periodic system with zinc and mercury, occupying an intermediate position between them, for this reason a number of chemical properties of all these elements are similar. For example, oxides and sulfides of these metals are practically insoluble in water.
In dry air, cadmium is stable, but in humid air, a thin film of CdO oxide slowly forms on the metal surface, protecting the metal from further oxidation. With strong incandescence, cadmium burns out, also turning into cadmium oxide - a crystalline powder from light brown to dark brown in color (the difference in color gamut is partly due to particle size, but to a greater extent is the result of crystal lattice defects), CdO density 8.15 g /cm3; above 900 o C cadmium oxide is volatile, and at 1570 o C it sublimates completely. Vapors of cadmium react with water vapor to release hydrogen.
Acids react with cadmium to form salts of this metal. Nitric acid HNO3 easily dissolves cadmium, while nitric oxide is released and nitrate is formed, which gives the hydrate Cd (NO3) 2 * 4H2O. From other acids - hydrochloric and dilute sulfuric - cadmium slowly displaces hydrogen, this is explained by the fact that in the series of voltages cadmium is further than zinc, but ahead of hydrogen. Unlike zinc, cadmium does not interact with alkali solutions. Cadmium reduces ammonium nitrate NH4NO3 in concentrated solutions to ammonium nitrite NH4NO2.
Above the melting point, cadmium combines directly with halogens, forming colorless compounds - cadmium halides. CdCl2, CdBr2 and CdI2 are very easily soluble in water (53.2% by mass at 20 o C), much more difficult to dissolve cadmium fluoride CdF2 (4.06% by mass at 20 o C), which is completely insoluble in ethanol. It can be obtained by the action of fluorine on a metal or hydrogen fluoride on cadmium carbonate. Cadmium chloride is obtained by reacting cadmium with concentrated hydrochloric acid or metal chlorination at 500 o C.
Cadmium bromide is obtained by metal bromination or by the action of hydrogen bromide on cadmium carbonate. When heated, cadmium reacts with sulfur to form CdS sulfide (lemon yellow to orange red), insoluble in water and dilute acids. When cadmium is fused with phosphorus and arsenic, phosphides and arsenides of compositions Cd3P2 and CdAs2 are formed, respectively, with antimony - cadmium antimonide. Cadmium does not react with hydrogen, nitrogen, carbon, silicon and boron. CdH2 hydride and Cd3N2 nitride, which readily decompose on heating, were obtained indirectly.
Solutions of cadmium salts are acidic due to hydrolysis, caustic alkalis precipitate white hydroxide Cd (OH) 2 from them. Under the action of very concentrated alkali solutions, it is converted into hydroxocadmates, such as Na2. Cadmium hydroxide reacts with ammonia to form soluble complexes:
Cd(OH)2 + 6NH3 * H2O → (OH)2 + 6H2O
In addition, Cd(OH)2 goes into solution under the action of alkali cyanides. Above 170 o With it decomposes to CdO. The interaction of cadmium hydroxide with hydrogen peroxide (peroxide) in an aqueous solution leads to the formation of peroxides (peroxides) of various compositions.
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