IMPROVED METHOD OF THE FIRE REFINING OF SECONDARY COPPER-CONTAINING MATERIALS

ABSTRACT

The object of this study was secondary solid copper-containing materials from a newly established private enterprise in the Tashkent region, which is aimed at the commercial production of 12,000 tons of cathode copper per year. Charcoal was used as the reducing agent in the processing of solid raw materials, and two types of slags—phosphate slag and copper phosphide (Cu₃P)—were applied as deoxidizers. The main thermodynamic principles of the chemical reactions occurring during the process were thoroughly investigated, along with the factors contributing to the acceleration of fire refining of blister copper and the influence of macroscopic parameters. The positive impact on the quality of the anode castings obtained during electrolytic refining was also confirmed by X-ray spectral analysis. The application of a charcoal and copper phosphide mixture as a reducing agent in the fire refining process positively influenced the quality of anode casting: improvements included smoother anode surface, a 15–20% reduction in natural gas consumption, and lower oxygen content in the anodes, which in turn enhanced the efficiency of the subsequent electrolysis process.

АННОТАЦИЯ

Объектом исследования стали вторичные твёрдые медьсодержащие материалы нового частного предприятия, созданного в Ташкентской области, которое ориентировано на коммерческое производство 12 000 тонн катодной меди в год. В качестве восстановителя при переработке твёрдого сырья использовался древесный уголь, а в качестве раскислителей применялись два типа шлаков — фосфатный и фосфид меди (Cu₃P). Были тщательно изучены основные термодинамические закономерности химических реакций, происходящих в процессе, факторы, способствующие ускорению огневого рафинирования черновой меди, а также влияние макроскопических параметров. Положительное влияние качества анодных отливок, полученных в процессе электролитического рафинирования, также было подтверждено результатами рентгеноспектрального анализа. Применение смеси древесного угля и фосфида меди в качестве восстановителя в процессе огневого рафинирования положительно сказалось на качестве анодных отливок: отмечено улучшение гладкости поверхности анодов, снижение расхода природного газа на 15–20 % и уменьшение содержания кислорода в анодах, что, в свою очередь, повысило эффективность последующего электролиза.

Keywords: Fire refining, Secondary copper materials, Anode furnace, Copper blister, Charcoal reducing agent, Copper phosphide (Cu₃P), Phosphorite slag.

Ключевые слова: Огневое рафинирование, вторичное медьсодержащее сырье, анодная печь, черновая медь, восстановитель – древесный уголь, фосфид меди (Cu₃P), фосфоритный шлак.

Introduction. During the transition from the age of technology to the age of information technologies, it is no secret that metal forms the basis of newly invented equipment and devices used in the national economy and other branches of industry, such as military technology, transport, cosmonautics, medicine, electrical engineering, and electronics [1]. Especially after silver in terms of electrical conductivity, the role of copper metal as an electrolytic conductor in electrical engineering is incomparable. It is known that household appliances, vehicles, plumbing fixtures and equipment, and gadgets are made of fine-fiber copper wires [2]. Copper smelter of JSC “Almalyk MMC” had been the only copper-producing enterprise in Uzbekistan, till a new private copper plant was launched [3]. At the copper smelter of JSC “Almalyk MMC” during the fire cleaning process of copper blister, materials containing liquid untreated copper and solid copper in a ratio of 70/30 or 50/50 are loaded [4]. In this case, hot and liquid copper blister with an initial temperature carries heat into the furnace and then heat is absorbed by solid copper scrap. At the “Uralelektromed” plant of the Ural Mining Metallurgical Company in the city of Verkhnyaya Pishma (Russia), solid cast copper brought from the company's other local plants is processed. Ensuring the temperature regime in anodic, electric arc, induction, and other types of furnaces designed only for melting solid copper containing materials, which do not have the possibility of inpouring liquid copper blister is a complex process. At the cathode copper plant, secondary solid materials containing copper is periodically loaded into a cylindrical, horizontal, movable smelting furnace with a capacity of 60 tons. At this local private copper cathode plant, secondary materials containing copper from 70 to 95% are imported and after two-stage fire and electrolytic refining, a pure commodity copper cathode is produced. The composition of this raw material consists of copper-containing waste electrical conductors, wires, high-voltage conducting cables, bushings made of bronze and brass alloys, nozzles, plumbing equipment, fittings, (faucet, bracket, pipes, tubes, holders).

The technological process includes the following technological stages:

1. Initially, secondary copper containing materials, wastes, scraps are compacted in a pressing machine;
2. Portional loading of the pressed solid materials.
3. Heating and melting of the materials inside of the furnace. At the edge of the furnace, the blowing source heats the furnace through the main gas-air mixture burner.
4. Oxidation of unwanted elements. Compressed air is sprayed through Ø - 20 mm with its immersed pipeline ends into the surface of the molten layer.

This process is described by the following reaction equation below:

MeS + O₂ = MeO + SO₂

In this reaction, elements Zn, Fe, S, Pb, Sn, As, Sb, Ni, Bi, Ca, Mg, C, Cr, Mo, Co, C, Al and others expressed as, Me oxidized [5]. A high content of unnecessary impurities negatively affects the course of the process. In fact, it is assumed that with a raw material load of copper content at least 95%, only 5% of unnecessary additives will not have a significant impact on the process. If this indicator exceeds the permissible amount and reaches 20-30%, a heat deficit happens inside the furnace. Meanwhile, other copper enterprises with the complete cycles receive liquid copper blister with a purity of Cu 97 – 99.5%. A large part of the heat is absorbed by the slag phase, which divided separately by liquation floating into the upper part of the furnace bath. The slag spreading across the entire surface prevents heat spreading in the upper atmosphere inside the furnace from seeping through the slag phase, absorbing heat into its body. When the molten slag is periodically poured out through the anode furnace neck, its physical volume decreases, but this definitely causes the heat removal. To raise the temperature using natural gas and air spraying possibly, but it is necessary to take into account factors that have a negative impact. Firstly, this accelerates the thermal deformation of the furnace refractory especially, firebricks collapses may occur around throat area, subsequently reduces the time between repairs. Secondly, when intensifying the process at the expense of fuel, an increase in the melt temperature and the gaseous mixture released from it above the norm is observed. The hot temperature of the exhaust off-gas diffusing along the gas path and its initial laminar flow causes to heating at some points of the water-cooled element based on the heat exchange mechanism. Necessary impurities, such noble metals as Au, Ag, Pd, Pt and also Se, Te falls to the bottom of the bath, which are extracted as slime in the next stage.

Reduction-deoxidation process. Traditionally, natural gas is used as a reducing agent. Organic matters entering with solids (isolation of cables, wire) also burn completely and decompose in the gas phase. The significant increase in the cost of hydrocarbon resources in recent years and its negative impact on the cost of metal production require an increase in the efficiency of the fire refining process and energy saving. Thus, this study proposes improved method of the fire refining of secondary copper-containing materials.

Materials and methods. Carbon containing coal is loaded as an additive because of a lack of temperature. At the same time, it is known that the coefficient of effective influence on the process is low due to the high degree of ash content in the coals of the local Angren, Shargun and Baysun deposits up to 50 %. An industrial experiment has been carried out. A petroleum coke of Fergana oil refinery is much more effective in the process of sulfide copper concentrate smelting in Vanyukov furnace of the Copper smelter for the purpose of keeping temperature balance and also zinc cake roasting with subsequent obtaining of copper clinker in the Zinc Plant of JSC “Almalyk MMC”. Initially, petroleum coke was tested. Due to the particle size, most of it escaped with the gases. Combustion occurred in the upper layer of the liquid bath and its penetration into the bath didn’t. subsequently, the temperature increased along the gas flow. Then, charcoal with a high heat capacity - 30,000-35,000 kJ/kg, a density of charcoal - 380 kg/m3, and a size of charcoal pieces of 5-50 mm was applied.

The natural gas consumption for processing 60 tons of secondary material after adding the charcoal was minimized for 15 %. Hourly consumption of the natural gas was 550 m³. The amount of reducing agent per each ton of copper was 0,045 tons.

Phosphorite slags used in exactly for the following purposes:

Ca₃(PO₄)₂ (calcium phosphate) – helps bind oxygen, other gases and impurities;

SiO₂ (silicon dioxide) – participates in the formation of low-melting silicate slags.

CaO (calcium oxide, lime) – reduces the viscosity of slag and helps remove sulfur.

In practice, granulated phosphorite slag containing phosphorus, calcium is added for deoxygenation during the reducing stage of the melt. This additive is imported from abroad. In factories producing copper pipes and tubes, copper phosphide is added as a deoxidizer for removing gas impurities from saturated melts. About 900-1000 kg of MF-9 grade copper phosphide (in two parts) is used to deoxidate 60 tons of copper in an anode furnace.

Results and discussions. The chemical compositions and spectral analysis of the anodic slag are presented in Table 1 and Figure 1. The use of a mixture of charcoal and copper phosphide as a reducing-oxidizing agent in the fire refining process has positively impacted the quality of the cast anode copper. During the reducing-deoxidation phase, natural gas consumption decreased by 15-20%. Additionally, the smoothness of the cast anode in the molds improved significantly, resulting in a 2.2% reduction in defects.

Table 1.

Chemical composition of copper containing anodic slag

Figure 1. The spectral analysis of anodic slag containing copper

The molten copper is poured onto molds using a rotary anode casting machine moving in a horizontal cross-section. In cases where the liquid solution temperature is low, it should be evenly poured onto the extreme points of the ear-forming mold. Chemical analysis results of anodic cast is demonstrated in Table 2.

Table 2.

Cast anode copper chemical composition

During the casting of finished anode copper, improvements were made in its fluidity and temperature retention capacity, resulting in a more even and high-quality pour into molds. The chemical composition of the main product, cathodic copper, after electrolytic refining, is provided in Table 3. The oxygen content was reduced to a minimal 0.006%, which, in turn, accelerated the duration of one complete electrolysis cycle. This enhancement increased the solubility of crystalline copper particles in the sulfuric acid electrolyte, where the anode casting, weighing 250 kg, was influenced by electrical voltage in the form of copper ions. Additionally, the number of short circuits was significantly reduced, and current-related parameters were stabilized. As a result, the copper transfer efficiency from anode to cathode improved from 80.8% to 86.2%, allowing for an additional 515 kg of copper to be extracted from a single bath.

Table 3

Chemical composition of cathode copper

Table 4.

Chemical composition of copper-electrolyte slime

Above, in table 4, it’s indicated the slime deposited on the bottom of the bathtub. The smoothness of anode cast and reducing of internal defects ensured a uniform distribution of electrical voltage during the subsequent electrolytic cleaning of the anode castings and improved the process chemistry, ultimately removing unnecessary impurities, and ensured maximum copper transfer from the anode castings to the cathode over a full cycle. In Figure 2, it’s demonstrated spectral analysis of a slime sample.

Figure 2. Spectral analysis of sample of the slime

Table 5 shows chemical composition of waste electrolytic solution.

Table 5.

Chemical compostion of waste electrolytic solution

Spectral analysis of waste electrolytic solution is demonstrated in Figure 3.

Figure 3. Spectral analysis of waste electrolytic solution

Conclusion. The use of a mixture of charcoal and copper phosphide as a reducing-oxidizing agent in the fire refining process has a positive effect on the quality of the cast anode copper. During the reducing-deoxidation phase, natural gas consumption decreased by 15-20%. The smoothness of the cast anode in the molds increased, defectiveness decreased by 2.2%. Existing defects inside the anode were also reduced. Reducing the oxygen content to 0.006% in the subsequent electrolysis process accelerated the duration of one complete cycle, increasing the solubility of crystalline copper particles with an anode casting weighing 250 kg in the sulfuric acid electrolyte and under the influence of electrical voltage in the state of copper ions. The number of short circuits significantly decreased and the stabilization of current-related parameters was ensured. The degree of copper transfer from anode to cathode was improved from 80.8 % to 86.2 % and an additional 515 kg of copper was obtained from one bath.

References:

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