PROCESS
CZP uses the hydrometallurgical zinc recovery process at its facilities in Chelyabinsk to produce its main zinc products, together with the associated by-products.
The following diagram illustrates the principal steps in the process:
1. Zinc concentrate handling
Zinc concentrate with 9% moisture is delivered by rail and unloaded at the 25000 t capacity concentrate house equipped with a heated platform. When the concentrate house is full to capacity unloading takes place at the ore stock yard. The concentrates are stored separately depending on the ore deposits where the material is shipped from. Material blending is performed at the technological part of the concentrate house by means of grab cranes from where the blend is transported to roasting by a conveyor belt system.
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2. Zinc concentrate roasting
Roasting is performed in five fluid-bed roasters. The daily output of four roasters is 240-420 t. The daily output of the fifth roaster is 210-240 t. The process of oxidizing roasting of zinc concentrates is conducted at the range of temperatures 880 – 980 °C with the use of oxygen enriched air blow. The zinc concentrate roasting products – calcine and dust are passed over to further processing at the leaching plant. After cleaning sulfur dioxide gas goes to the sulphuric acid production plant. At present 2-3 roasters are engaged in operation, the rest of the roasters are either stand-by or are under maintenance. The roasting process was automated in 1982-1984. The zinc concentrate roasting process is exothermic. 80% of heat is captured and utilized for the Smelter technological requirements.
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3. Sulphuric acid production
The utilization of sulphur containing gases after zinc concentrate roasting is carried out at the sulphuric acid plant resulting in marketable sulphuric acid. The sulphuric acid plant has four sulphur containing gas treatment systems of total capacity 103 thousand Nm3/hour. At present the reconstruction of system No.5 of 40 thousand Nm3/hour design capacity is under way. SO2 concentration in the roasting off-gas is in the range of 8 – 9.5%. The process of oxidizing SO2 to SO3 is conducted in the double contacting scheme. The achieved contacting level is 99.8%. The use of "Begg Cousland", "KKK", "Alfa-Laval", "Haldor Topsoe" companies equipment allows to achieve SO2 content at the outlet to the atmosphere not more than 0.05%. Presently 1.5 t of sulphuric acid per 1 t of cathode zinc is produced.
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4. Zinc oxide calcine leaching
The roasting products (calcine and dust) undergo 2-stage (neutral and acidic) leaching with sulphuric acid at which 80-86% of zinc and a number of impurities including cadmium are recovered into the solution, said cadmium is recovered as metal to a by-product at a later stage. After settling in the thickeners the zinc solution flows to purification. The solid residue after leaching, so called "zinc cake", undergoes filtration on two belt 96 m2 filter-presses "Larox" and is transported to the Waelz process.
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5. Zinc electrolysis
After roasting products leaching the neutral zinc solutions are fed to the zinc electrolysis complex (ZEC). The complex comprises solutions purification section, cadmium section, cell-house, zinc spraying plant, smelting section, dross section. Solutions purification is carried out in two-stage process using zinc dust of different particles size. Industrial products after solutions purification (cakes) are fed for metal cadmium production. Purified neutral solution is fed to the cell-house. The cell-house is equipped with 204 cells in two series. Each cell contains 80 cathodes (1 cathode size is 3.4 m2) and 81 lead anodes. At present one series current capacity is 87-90 kA/h, current density is 3.3-3.4 V, current efficiency 90%. The process of cathode transportation, cathode zinc stripping, cathode and anode cleaning. Further cathode zinc melting makes it possible to produce ingot SHG zinc.
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6. Waelz processing
The zinc cake is processed in rotary horizontal cylindrical kilns (Waelz kilns) at 1200 °C. The process is designed for the maximally full recovery of zinc from the cakes produced in the course of leaching, as well as other valuable components contained in the cakes. The Waelz plant processes zinc and copper cakes containing 20-22% zinc, 30-35% iron, copper, lead, indium, cadmium and other zinc containing wastes. At the storage area zinc cake is mixed with coke fines and charged into the Waelz kilns. In the Waelz kilns there is fuming of the metals that are converted into oxides in the off-gas cooling system. The fumes (Waelz-oxides) are captured in bag filters and sent to hydrometallurgical processing. Clinker received in the Waelz process contains up to 1% zinc, 4-6% copper and precious metals and presents a marketable product sold to copper smelters.
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7. Hydro-metallurgical processing
Waelz-oxide hydrometallurgical treatment is carried out by means of its sulfuric acid leaching in order to extract other valuable products including zinc sulphate solution and industrial zinc sulphate, lead cake containing 35-40% of lead, In-000 and In-00 metal indium and indium-based alloys. Waelz-oxide treatment effluent waters containing chlorine are neutralized by soda ash with further filtering on filter-presses and then are fed to the industrial storm treatment plant.
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8. Melting and Casting Plant
Cathode metal zinc is melted in induction furnaces and 20-25 kg ingots and 0.5-1.0 t blocks are cast by means of carrousel foundry machines. More than 50% of the products are manufactured as 0.3-0.4% ZnAl alloys for galvanizing. The casting area has a special furnace and process line for the production and casting of ZAMAK alloy with prescribed properties, used by the die-casting industry. A special pant for the production of zinc dust of various size fractions is also available and operating, the value of said dust being considerably higher than the value of zinc.
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