Copper production by pyrometallurgical method

Copper is produced mainly by pyrometallurgical method, the essence of which is the production of copper from copper ores, including its enrichment, roasting, smelting on a semi-product – matte, smelting from matte blister copper and its purification from impurities (refining).

Copper ores containing 1-6% Cu, as well as copper and its alloys waste, are used for copper production. In ores, copper is usually in the form of sulfur compounds, oxides or bicarbonates. Before melting copper ore is enriched and concentrate is obtained. To reduce the sulfur content in the concentrate, it is subjected to oxidative firing. The obtained concentrate is melted in reflecting or electric furnaces. Copper oxide (CuO) and higher iron oxides are recovered.

Copper and iron sulphides fused and form matte, and molten iron silicates dissolve other oxides and form a slag. After that, the melted copper matte is poured into converters and blown with air to oxidize copper and iron sulfides and produce blister copper. Rough copper contains 98.4-99.4% Cu and a small amount of impurities. This copper is poured into ingots. Rough copper is refined to remove harmful impurities and gases. First, fire refining is carried out in the reflecting furnaces. Impurities S, Fe, Ni, As, Sb and others are oxidized by air oxygen supplied through steel pipes immersed in molten black copper. Then remove the gases, for which they remove the slag and immerse the raw wood in copper. Water vapor mixes copper and helps to remove other gases. The bath of liquid copper is covered with charcoal and immersed in it wooden poles. When dry distillation of wood, immersed in copper, hydrocarbons are formed.

After fire refining, copper is obtained with a purity of 99-99.5%. It casts ingots for melting alloys of copper (bronze and brass) or plates for electrolytic refining.

Electrolytic refining is carried out to obtain a pure copper impurity (99.5% Cu). Electrolysis is carried out in baths covered from the inside with viniplast or lead. Anodes make copper fire refining, and cathodes – from sheets of pure copper. When a direct current is passed, the anode dissolves, copper passes into solution, and copper ions are discharged at the cathodes.

Impurities (arsenic, antimony, bismuth, etc.) are deposited on the bottom of the bath, they are removed and processed to recover these metals. Cathodes are unloaded, washed and melted in electric furnaces.

The implementation of this scheme at various stages, especially at the initial stages before obtaining copper matte, can be carried out in different furnaces and in various technological options. In the scheme under consideration, the first redistribution of copper ore is enrichment. However, there are cases when ores enriched with sulfur (over 35%) are melted without enrichment to extract from them not only copper but also sulfur. However, the bulk of the sulphide copper ore extracted from the earth’s interior is subjected to flotation enrichment.

Methods for processing copper ores

Copper is the oldest metal. It began to be used by man for a very long time: in nature there were nuggets of copper, from which it was possible to make jewelry or the simplest weapon by blowing stone tools. At present, native copper is rare and the bulk of the metal is extracted from ores containing only 1-2% copper.

World copper production in the 1970s in the capitalist countries exceeded 5 million tons per year.
The melting point of copper is 1083 ° C, and the boiling point is 2369 ° C. The strength of pure copper is not very high and is 220 MPa (22 kgf / mm2). Its crystal lattice is a cubic face-centered lattice parameter a = 0.361 Nm (3.61 A). The density is 8.93 g / cm3, and the hardness of copper is half that of iron HB = 35.

Methods for processing copper ores

Copper is extracted from ores in various ways and in various apparatuses. For the production of copper, pyrometallurgical methods (smelting on matte, reduction melting) can be used, but some ore is successfully processed and hydrometallurgical methods, for example, • leached with sulfuric acid or ammonia.
Let us consider one of the methods of copper extraction, which has received the greatest distribution – smelting matte. Varieties of this method are used in many countries, and in our country this method produces almost all primary copper. The general schematic diagram of this method is presented in Fig. The implementation of this scheme at various redistribution, especially in its upper part, before obtaining copper matte, can be carried out in different furnaces and in various technological options. In the scheme under consideration, the first redistribution of copper ore is enrichment. However, there are occasions when ores rich in sulfur (over 35% S) are melted without enriching to extract not only copper but also sulfur (for example, copper-sulfur smelting) from them. However, the bulk of the sulphide copper ore extracted from the earth’s interior is subjected to flotation enrichment.

Ore dressing by flotation

Flotation is rarely used for iron ore; Usually it is used for the enrichment of poor ores of non-ferrous metals and, necessarily, when enriching complex ores containing several non-ferrous metals, as well as sulphide or mixed copper ores containing about one percent of copper, which directly smelt very expensive.

The essence of flotation consists in the selective adherence of certain mineral particles suspended in an aqueous medium to the surface of air bubbles by means of which these mineral particles rise to the surface. Pass through the pulp (a mixture of liquid and fine solids) air bubbles. Due to the different wettability of the particles of some minerals, poorly wetted by water (or other liquid in which enrichment occurs), attached to air bubbles and, rising with them to the surface, form mineralized foam and thereby separate from other, well-wetted minerals remaining in the pulp .

To successfully implement this method of enrichment it is necessary:
A) finely grind the ore to a particle size of less than 0.1 mm, which makes it possible to obtain pieces of ore consisting of one mineral, rather than from splices of several, and facilitates small air bubbles to lift heavy minerals;
B) get a lot of small air bubbles in the pulp and create conditions for the formation of a stable foam on the pulp surface.

Distinguish the following flotation reagents:

Foaming agents that make foam bubbles resistant, not bursting, interfering with their co-licenses (pine oil and other substances obtained during distillation of wood and coal);
Collectors (collectors), reducing the wettability of a certain group of minerals with water and facilitating their adhesion to air bubbles.

During flotation, depressants (suppresses) are often used, which prevent the collector from affecting certain minerals. Suppressors are inorganic electrolytes, for example sodium cyanide NaCN, CaO lime, which is used in the flotation of copper-zinc pyrite ores, since lime does not act on chalcopyrite CuFeS2, but it suppresses flotation of zinc blende ZnS and pyrite FeS2. In so-called selective flotation, when concentrates of several metals are to be extracted from the ore, many other chemicals are used. The total consumption of flotation reagents is low, it is 50-300 g per 1 ton of ore.

For the mechanization of certain labor-intensive preparatory and auxiliary stages of flotation enrichment, various machines are used to facilitate these operations, for example, for grinding ore (crushers and mills), to separate it into small and large fractions (screens and classifiers), pulp separation apparatus for liquid and solid Particles (thickeners and filters), actually flotation machines and many others. Let’s briefly consider only one of the types of machines: a pulp consisting of water, small ore particles and the necessary flotation reagents already introduced into the pulp is continuously fed into the machines through the side pipe. Above the pipe, air drawn into the machine by a rapidly rotating impeller (300-600 rpm) is sucked in. The pulp circulating in the machine, mixed with air bubbles in the upper right side of the machine, collects the foam, which is continuously removed from the machine by a slowly turning foam mixer. The remaining pulp is drained through the threshold in the side wall of the machine (on the scheme in the rear wall) and falls into its adjacent section, since the flotation machine consists of 4-20 chambers (sections).
The copper powder concentrate obtained after flotation enrichment, containing 11-35% of copper, 15-35% of sulfur, 15-37% of iron, and also a little silica, alumina, calcium oxide, small admixtures of zinc, nickel and some other compounds, is directed to Further processing.

Reception and processing of copper matte

The most important operation of processing copper ore is smelting on matte. Matte is a sulphide alloy formed during the smelting of copper ore, mainly copper and iron (usually 80-90%), the rest being sulphides of zinc, lead, nickel, as well as oxides of iron, silicon, aluminum and calcium, concentrating mainly in slag, But partially soluble in matte. Liquid mattes dissolve gold and silver well, and if these precious metals are in the ore, they almost completely concentrate in matte.

The purpose of smelting on matte is the separation of sulfur compounds of copper and iron from the impurities present in the ore, which are present in it in the form of oxidized compounds. The resulting matte should not contain too little copper, since this makes the subsequent redistribution unproductive, but also very copper-rich mattes can not be obtained, since a significant amount of copper is lost in the slags.

Depending on the chemical composition of the ore and its physical state, matte is produced either in shaft furnaces, if raw materials are lumps of copper ore containing a lot of sulfur, or in reflective or arc-furnace electric furnaces, if powdered flotation concentrates are the starting material.
The reflecting furnace is built in length 35-40, width 7-10 and height 3.5-4.5 m. The walls and the vault are made of dinas or magnesite bricks. Refractories are chosen depending on the prevalence of basic or acid oxides in the charge, since the composition of the charge and refractory materials is longer than their service life. Under the stove is made in several layers, and the surface is covered with quartz sand, which before melting the furnace is melted, turning into a dense mass.

Reflective furnaces are heated with fuel oil, coal dust or gas, injecting fuel with injectors (4-10 pcs.) Through the windows available at the end of the furnace. The maximum temperature at the head of the furnace is 1550 ° C and, gradually decreasing, to the tail part is usually 1250-1300 ° C. The charge in these furnaces is fed through the vaults located along the furnace at the side walls. When charging, the burden falls along the walls along slopes, preventing the clutch from direct exposure to slags and gases. As the charge is heated, partial reduction reactions of higher oxides of iron and copper, sulfur oxidation and slag formation begin.

Smelting of copper concentrates in electric furnaces due to power shortage and the possibility of using low-grade sulfur fuels in this operation has not yet found wide application. But for the smelting of lumps of copper ore, mine-based water-jet furnaces are still widely used. There are often cases when even sulfur-rich concentrates are pre-agglomerated in order to subject them to smelting in shaft furnaces. This furnace has in the plan a rectangular section with a width of about 1 m and several meters in length.

The main working walls of the furnace are made of hollow steel boxes, cooled from the inside by water, called caissons, since known technical refractories are not sufficiently stable under these conditions. During melting on the cold walls, the melted charge stiffens, protecting the caisson from destruction. The charge is loaded from a platform located at the level of the upper edge of the caisson, the combustion air is fed through tuyeres along the longitudinal walls in the lower part of the caissons.

The release of matte and slag from the furnace is performed jointly and continuously through a drain chute having a hydraulic shutter. The liquid viscous mixture flows into a large oval sump, called the front rock, lined with chromium-magnesite brick. It is a slow stratification of matte and slag. Excess slag is drained along the gutter in the opposite end of the front bugle, and the matte is discharged as needed through the tapholes located at the berm of the hearth. Over the stove, a so-called tent for collecting and discharging off-gases and lining them for dust collection and gas cleaning are made with fireproof materials lined.

Copper matte processing

The most common are now cylindrical barrel-shaped converters. The outer diameter of the converter is usually 2.3-4 m, the length is 4.3-10 m. The largest converters produce up to 100 tons of copper per process cycle. Air to the converter is fed through a series of tuyeres located along the generatrix of the cylinder. The cylinder is supported by two strong bandages for four pairs of rollers. Turn the converter on the rollers to the required angle for pouring the matte into the neck and casting out the smelting products by a gear transmission and a toothed rim fixed to the steel casing. Inside the converter is lined with magnesite and chromium-magnesite brick.

Matte processing in the converter takes place in two periods. The converter is loaded with lump quartz, poured the molten matte and blows it with air,).
The resulting slag is periodically drained and fresh portions of copper matte and lump quartz are added to the converter. The temperature of the matte to be filled is usually about 1200 ° C, but during the purging time, due to a greater heat release during the oxidation of sulphides, the temperature rises to 1350 ° C. The duration of the first period depends on the amount of copper in matte and is 6-20 hours.

The introduction of an oxygen blast in the air blast increases the temperature in the converter and allows the cold concentrate to be loaded into it, replacing some of the molten matte.

The first period ends when sulphurous iron is oxidized in the blown matte. After that, the slag is carefully removed and blowing continues without the addition of matte and quartz. The second period begins when only the Cu2S, called the white matte, remains in the converter, and in some factories “white matte”.

The second period ends when in the converter the entire white matte turns into copper, which usually takes 2-3 hours. In the converter and in the second period, a small amount of copper-rich slag is formed, which remains in it after the casting of the blister copper and is processed in the next cycle. Convertor slags of the first period are sent for processing to reflective furnaces.

When the process is finished, the blister copper is tilted into a bucket and poured into molds. The copper obtained in the converter is called rough, that is, not yet finished copper, since it contains 1.0-2.0% of iron, zinc, nickel, arsenic, antimony, oxygen, sulfur and other impurities, and precious metals are dissolved, Previously in matte.

Copper refining

Rough copper is always refined to remove impurities that impair its properties, as well as extract valuable metals such as gold, silver, and others. In practice, refining is carried out sequentially in two fundamentally different ways: sintered metallurgical and electrolytic.

Fire pyrometallurgical refining of copper is carried out in reflective furnaces, the sketch of which is shown in Fig. 145.

The whole cycle of fire refining consists of the following operations: loading and melting, oxidation of impurities, removal of dissolved gases, copper deoxidation and casting; It usually takes 12-16 hours.

The removal of dissolved gases from copper is commonly referred to as “teasing by density”. In the metal baths immerse raw wooden poles, the wood of which emits gaseous hydrocarbons, vigorously stirring copper and removing nametal la sulphide and other gases. After the removal of gases, for the production of plastic copper, deoxidation begins, or, as they say in factories, “mockery”.

Electrolytic refining of copper is carried out in baths filled with a solution of copper sulphate, acidified with sulfuric acid.
To produce oxygen-free copper and grades of copper with a reduced oxygen content, cathode remelting is carried out in channel induction electric furnaces with a steel core, and casting is continuous in a protective medium. Copper grades with the letter p are deoxidized with phosphorous copper.

Copper alloys

Technical copper may contain impurities Bi, Sb, As, Pb, Sn, Fe, Ni, S, O, accompanying it when it is produced from ores and when refining or got into it during waste processing. The total permissible amount of these impurities is given in Table. 17. More than 50% of pure copper consumes the electrical industry and energy as conductors of electric current. Therefore, a large amount of copper is subjected to rolling and drawing.

Copper has good ductility both in cold and hot state. But not all of these impurities have the same effect on the ductility and other properties of copper. Bismuth and lead, which do not dissolve in copper in the solid state, which form low-melting eutectics (bismuth with a melting point of 270 ° C, and lead with a melting point of 326 ° C), most complicate the hot rolling of copper. Therefore, their content in the higher grades of copper is limited by thousandths of a percent.

Negatively affects hot rolling and oxygen, but at high concentrations (0.1-0.2%). Other impurities (tin, zinc, nickel, silver) do not degrade the ductility of copper and other mechanical properties, since, when present in small amounts, they enter a solid solution.
The most common and known copper alloys are brass and bronze.

Brass is called a group of copper alloys with zinc, which received the most wide application in technology. The group of brasses includes tombak (90% or more of copper, the rest is zinc, if these alloys contain from 79 to 86% of copper, they are called a half-tamp) and many others, not only double but also more complex alloys.
The mechanical strength of brass is higher than copper, and they are well processed by cutting. Their great advantage is their reduced cost, because the zinc included in them is much cheaper than copper. Brass is widely used in instrument making, in general and chemical engineering.

Differences between copper and bronze

With a properly selected composition, bronzes have significantly higher mechanical properties than pure copper (bronze strength values ​​can reach 800-1200 MPa 1 more). Bronze has a small volumetric shrinkage (0.6-0.8%) compared with iron and steel, in which the shrinkage reaches 1.5-2.5%. Therefore, the most complex parts are cast from bronze.

Bronze letters are marked with letters Br (bronze), after which they put letters denoting the appearance and quantity of alloying additives. For example, beryllium bronze (2% beryllium Be, the rest copper Cu); Phosphor bronze 6.5-0.15 (6.5% tin, 0.15 phosphorus P, the rest copper Cu).

The introduction of cadmium into copper gives a significant increase in mechanical strength and hardness with a relatively small decrease in the specific electrical conductivity.
Cadmium bronze MK (0.9% cadmium Cd, the rest Cu) is used for contact wires and collector plates of a particularly important use, as well as welding electrodes for contact welding methods.

Possessing even more mechanical strength, hardness and resistance to mechanical wear (tensile strength up to 1350 MPa), beryllium bronze does not change its properties to a temperature of about 250 ° C. It finds application in the manufacture of responsible current-carrying springs for electrical appliances, brush holders for current-carrying and sliding contacts.

Phosphorous bronze Br.OF 6.5-0.15 (6.5% Sn Sn, 0.1 phosphorus P, the rest copper Cu) is characterized by low electrical conductivity. From it, various low-equivalent current-carrying springs are manufactured in electrical appliances.

Brass

Brasses are copper alloys, in which the main alloying element is zinc (up to 43%). Brass is stronger, more plastic than copper, possesses a sufficiently high relative elongation at an increased tensile strength as compared to pure copper, they have a reduced cost, because the zinc entering them is much cheaper than copper. Sometimes, in order to increase the corrosion resistance, aluminum, nickel, and manganese are introduced into the alloy in a small amount.
Brass is well stamped and easily subjected to deep drawing (contacts of a thermobimetallic relay, contour screens, plates of air capacitors of variable capacity, caps of radio tubes).

Technology of copper production

Copper is obtained most often as a result of processing of sulfide ores. Impurities reduce the electrical conductivity of copper. The most harmful of them are phosphorus, iron, sulfur, arsenic. The phosphorus content of approximately 0.1% increases the copper resistance, by 55%. Admixtures of silver, zinc, cadmium give an increase in resistance by 1 … 5%. Therefore, copper, intended for electrical purposes, must be electrolytically cleaned. Copper cathode plates obtained as a result of electrolysis * are melted into 80 … 90 kg blanks, which are rolled and stretched, creating products of the required cross section.

To produce the wire, the blanks are first hot rolled into a rod with a diameter of 6.5 … 7.2 mm, which is then stretched without heating, to obtain the wire of the desired cross-sections.

As a conductor material, copper grades M1 and MO are used. Copper grade M1 contains 99.9% of copper, not more than 0.1% of impurities, in the total amount of which oxygen should not exceed 0.08%. Copper brand MO contains impurities not more than 0.05, including oxygen not more than 0.02%. Due to the smaller holding of oxygen, MoM grade copper has better mechanical properties than M1 copper. Even more pure conductive metal (not more than 0.01% impurities) is vacuum grade MB copper, smelted in vacuum induction furnaces.
When cold broaching, solid (hard) copper (MT) is obtained, which has a high tensile strength, hardness and elasticity (when bent, the solid copper wire somewhat springs).

Hard copper is used in cases where it is necessary to provide high mechanical strength, hardness and resistance to abrasion: for contact wires, buses of switchgears, for collector plates of electrical machines, for manufacturing waveguides, shields, conductors for cables and wires up to 0.2 mm in diameter.

After annealing up to several hundred degrees (copper recrystallizes at a temperature of about 270 ° C), followed by cooling gives a soft (annealed) copper (MM). Soft copper has a conductivity of 3 … 5% higher than that of solid copper.

Soft annealed copper serves as an electrical standard in relation to which the specific electric conductivity of metals and alloys is expressed at an ambient temperature of 20 ° C. The specific electrical conductivity of such copper is 58 μS / m.

Soft copper is widely used for the production of foil and conductors of round and rectangular cross-section in cables and winding wires, where flexibility and plasticity are important (absence of “bending” when bending), and strength does not matter much.
Of the special electrovacuum grades of copper, anodes of powerful generator lamps are produced, parts of microwave devices: magnetrons, klystrons, some types of waveguides, etc.

Copper is a relatively expensive and scarce material, so it should be used sparingly. Copper waste from electrical facilities must be collected without mixing with other metals and less pure copper, so that they can be melted down and re-used. In some cases, copper as a conductor material is replaced by other metals, most often aluminum.

In some cases, when conductor material requires not only high conductivity, but also increased mechanical strength, corrosion resistance and abrasion resistance, copper alloys with a low content of dopant impurities are used.