A method of ore processing based on chemical reactions. Pyrometallurgy - ore processing methods based on chemical
The vast majority of metals are found in nature in the form of compounds with other elements. Only a few metals are found in a free state, and then they are called native. Gold and platinum are found almost exclusively in native form, and silver and copper - partly; Sometimes native mercury and some other metals are also found.
Gold and platinum are extracted either by mechanically separating them from the rock in which they are contained, for example by washing with water, or by extracting them from the rock using various reagents and then separating them from solution. All other metals are extracted by chemical processing of their natural compounds.
Minerals and rocks containing metal compounds and suitable for producing these metals in a factory are called. The main ores contain metal oxides, sulfides and carbonates. The extraction of metals from ores is the task of metallurgy - one of the most ancient branches of the chemical industry. Metallurgical processes occurring at high temperatures are called pyrometallurgical. For example, cast iron and steel are produced by pyrometallurgical processes.
The most important method for obtaining metals from metals is based on the reduction of their oxides with coal or CO. If, for example, you mix red copper ore with coal and heat it, then the coal, reducing copper, turns into carbon monoxide (II), and copper is released in a molten state:
Pig iron is smelted by reducing iron ores with carbon monoxide.
When processing sulfide ores, the sulfides are first converted into oxides by firing in special furnaces, and then the resulting oxides are reduced with coal. For example:
In addition to pyrometallurgical methods, hydrometallurgical methods are used in the extraction of metals. They represent the extraction of metals from ores in the form of their compounds with aqueous solutions of various reagents, followed by the isolation of the metal from the solution. For example, gold is obtained hydrometallurgically (see § 202).
Ores are usually a collection of minerals. Minerals containing extractable metal are called ore minerals, all others are called gangue rock. The latter most often consists of sand, clay, limestone, which are difficult to melt. To facilitate metal smelting, special substances - fluxes - are added to the ore. Fluxes form fusible compounds with waste rock substances - slags, which usually collect on the surface of the molten metal and are removed. If the waste rock consists of limestone, then sand is used as a flux. For ores containing large amounts of sand, limestone is used as a flux. In both cases, calcium silicate is formed as a slag, since the sand consists mainly of silica.
In many ores, the amount of waste rock is so large that direct smelting of metal from such ores is not economically viable. Such ores are pre-enriched - part of the waste rock is separated from them. In the remaining concentrate, the ore mineral content increases.
There are various methods for beneficiating ores. The most commonly used methods are flotation, gravity and magnetic methods.
The flotation method is based on the different wettability of the surface of minerals with water. Finely ground ore is treated with water to which a small amount of flotation reagent has been added, which enhances the difference in wettability between the particles of the ore mineral and the waste rock. Air is vigorously blown through the resulting mixture; at the same time, its bubbles stick to the grains of those minerals that are less wetted. These minerals are carried along with air bubbles to the surface and are thus separated from the gangue.
Gravity enrichment is based on the difference in density and, as a result, the speed of falling of mineral grains in the liquid.
The magnetic method is based on the separation of minerals according to their magnetic properties.
Not all metals can be obtained by reducing their oxides with carbon or CO. Let us calculate, for example, the standard Gibbs energy of the chromium reduction reaction:
Using the table. 7 (p. 194), we find where . The resulting value is positive. This shows that at standard concentrations of reactants, the reaction does not proceed in the direction of interest to us. A positive and large absolute value indicates that the reaction does not proceed in the direction of metal reduction not only under standard conditions, but also at temperatures and concentrations noticeably different from standard ones.
For metals that are not reduced by either coal or carbon (II) monoxide, stronger reducing agents are used: hydrogen, magnesium, aluminum, silicon. The reduction of a metal from its oxide using another metal is called metallothermy. If, in particular, aluminum is used as a reducing agent, the process is called aluminothermy. Metals such as chromium and manganese are produced mainly by aluminothermy, as well as by reduction with silicon. If we count the reactions
then we get a negative value. This suggests that the reduction of chromium by aluminum can occur spontaneously.
Finally, metals whose oxides are the most durable (aluminum, magnesium and others) are obtained by electrolysis (see § 103).
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Methods for obtaining metals are usually divided into three types:
- pyrometallurgical (reduction at high temperatures);
- hydrometallurgical (reduction from salts in solutions);
- electrometallurgical (electrolysis of solution or melt).
Pyrometallurgically obtained(methods for extracting metals from ores under high temperatures. Oxide ores and oxides are reduced with coal, carbon monoxide (II), more active metals (aluminum, magnesium)): cast iron, steel, copper, lead, nickel, chrome and other metals.
FeO + C –> Fe + CO
Fe2O3 + 2Al –> 2Fe + Al2O3
Hydrometallurgically obtained(methods for obtaining metals based on chemical reactions occurring in solutions ) : gold, zinc, nickel and some other metals.
CuSO4 + Fe –> FeSO4 + Cu
Electrometallurgically obtained(separation of metals from their salts and oxides under the influence of electric current ) : alkali and alkaline earth metals, aluminum, magnesium and other metals.
When developing technology for the production of chemicals, the laws of thermodynamics, kinetics, heat engineering, physical and chemical analysis, etc. are used. Naturally, economic conditions are also taken into account. If the reaction is reversible, apply Le Chatelier's principle:
If a system in equilibrium is influenced from the outside, then the equilibrium in the system will shift towards the reaction (direct or reverse) that leads to partial compensation of this influence.
Chemical methods are also used in the purification of emissions and wastewater from chemical industries.
General methods for obtaining metals
1. Reduction of metals from oxides with coal or carbon monoxide
Me x O y + C = CO 2 + Me,
Me x O y + C = CO + Me,
Me x O y + CO = CO 2 + Me
For example,
ZnO y + C t = CO + Zn
Fe 3 O 4 + 4CO t = 4CO 2 + 3Fe
MgO + C t= Mg + CO
2. Roasting of sulfides followed by reduction (if the metal is in the ore in the form of a salt or base, then the latter are first converted into oxide)
1 stage– Me x S y +O 2 =Me x O y +SO 2
2 stage- Me x O y + C = CO 2 + MeorMe x O y + CO = CO 2 + Me
For example,
2 ZnS + 3 O 2 t= 2 ZnO + 2 SO 2
MgCO 3 t = MgO + CO 2
3 Aluminothermy (in cases where it is impossible to reduce with coal or carbon monoxide due to the formation of carbide or hydride)
Me x O y + Al = Al 2 O 3 + Me
For example,
4SrO + 2Al t = Sr(AlO 2) 2 + 3Sr
3MnO 2 + 4Al t = 3Mn + 2Al 2 O 3
2 Al + 3 BaO t= 3 Ba + Al 2 O 3 (obtain high purity barium)
4. Hydrothermy - for the production of high purity metals
Me x O y + H 2 = H 2 O + Me
For example,
WO 3 + 3H 2 t = W + 3H 2 O
MoO 3 + 3H 2 t = Mo + 3H 2 O
5. Reduction of metals by electric current (electrolysis)
A) Alkali and alkaline earth metals obtained in industry by electrolysis molten salts (chlorides):
2 NaCl – melt, electric current. → 2 Na+Cl2
CaCl2 – melt, electric current. → Ca+Cl2
hydroxide melts:
4 NaOH – melt, electric current. → 4 Na + O 2 + 2 H 2 O (!!! used occasionally for Na)
B) Aluminum in industry it is obtained by electrolysis aluminum oxide melt in cryolite Na 3 AlF 6 (from bauxite):
2 Al 2 O 3 – melt in cryolite, electr. current. → 4 Al + 3 O 2
IN) Electrolysis of aqueous salt solutions use to obtain metals of intermediate activity and inactive:
2 CuSO 4 +2 H 2 O – solution, electric current. → 2 Cu + O 2 + 2 H 2 SO 4 3