STUDY OF THE EFFECT OF TEMPERATURE AND PROCESS TIME ON THE PREPARATION OF THE LEACHING PROCESS OF CRUDE WAELZ OXIDE

АННОТАЦИЯ

Цинковый завод АО «Алмалыкский ГМК» — единственное предприятие по производству цинка в нашей республике. Предприятие производит металлические цинк и кадмий пиро- и гидрометаллургическими процессами. В частности, сульфидные цинковые концентраты обжигают в печах с кипящим слоем, огарок растворяют в растворе серной кислоты. Цинковый кек, образующийся в процессе выщелачивание, перерабатывается в вельц печах для получения медного клинкера и сырого вельц оксида. В данной работе исследуются изменения химического состава и размера частиц вельц оксида при прокальки различных температурах и времени перед селективным выщелачиванием. Для экспериментальной работы использовалась муфельная печь типа СНОЛ 8,2/1100, предназначенная для лабораторных работ. Нагретые образцы механически перемешивали каждые 15 минут. По результатам научных исследований приемлемыми для вращающихся трубчатых печей являются температуры 800-1000°С, для многоподовых печей - 800-850°С. Было также подтверждено, что размер частиц сырой вельц оксида увеличивается с температурой.

ABSTRACT

Zinc plant of Almalyk MMC JSC is the only zinc production enterprise in our republic. The enterprise produces metallic zinc and and cadmium by pyro- and hydrometallurgical processes. In particular, sulfide zinc concentrates are roasted in fluidized bed furnaces, the cinders are dissolved in sulfuric acid solution. The zinc cake generated during the leaching process is processed in velz furnaces to produce copper clinker and crude velz oxide. This paper investigates the changes in the chemical composition and particle size of the velc oxide at different temperatures and times prior to selective leaching. In the experimental work, a SNOL 8.2/1100 laboratory type muffle furnace was utilized. Mechanical stirring of the heated samples occurred every 15 minutes. Based on the results of scientific research тemperatures ranging from 800-1000 °C are deemed suitable when employing rotary tube furnaces, whereas temperatures of 800-850 °C are recommended for multiple hearth furnaces. Furthermore, it was affirmed that particle size of crude waelz oxide increases with temperature.

Ключевые слова: цинковый кек, сырой вельц оксид, шихта, прокалка, прокаленная вельц оксид, химический состав, фракционный состав, температура, время процесса,  извлечение.

Keywords: zinc cake, crude waelz oxide, charge, glowing, calcined waelz oxide, chemical composition, fractional composition, temperature, process time, extraction.

Introduction. Zinc production primarily relies on processing sulfide concentrates. The conventional method involves fluidized bed roasting, followed by sulfuric acid leaching of the calcine, purification, and electrolytic zinc extraction. Moreover, leaching process generates a zinc-containing residue known as "zinc cake."

The composition of zinc cake in most cases is as follows, %: Zn – 18.0-21.0; Pb - 1.5-5.0; Cd - 0.3-0.8; Fe – 13.0-30.0; Cu – 1.0-3.0; CaO - 2.0-5.0; SiO₂ – 5.0-8.0; Total - 4.0-8.0; S_SO4 – 3.0-6.0; S_S – 0.8-2.0; MgO - 0.5-1.5; Ag – 250.0-400.0 g/t; Au – 1.0-5.0 g/t.

Zinc cake processing utilizes pyrometallurgical (e.g., the Waelz process) and hydrometallurgical methods. Pyrometallurgical methods play a pivotal role in the extraction of non-ferrous metals, with the Waelz process standing as a prominent example. Additionally, zinc cake has been explored as a viable feed for agglomeration roasting in lead factories.

Recent advancements have seen research directed towards the hydrometallurgical processing of zinc cake utilizing ozone [1].

The historical evolution of the Waelz process, patented in Canada in 1909 and implemented industrially in Germany in 1925, underscores its enduring significance in metal extraction. Operating at temperatures between 1200-1300 °C, the Waelz process yields clinker containing copper, gold, and silver, along with crude waelz oxide. Further refinement involves purifying the crude waelz oxide from impurities such as halogens and reductants, rendering it suitable for selective dissolution in sulfuric acid solutions. Subsequently, the purified oxide is processed alongside zinc-containing calcine in zinc electrolysis.

About 6,750,000 tons of electric arc furnace dust are produced worldwide each year. The dust contains 1,600,000 tons of zinc per year. About 35 waelz klins with an average capacity of 75,000 tons per year are installed worldwide. They process 3,400,000 tons of electric arc furnace dust annually [2].

Given the diminishing sulfide deposits globally, there is an increasing imperative to extract zinc from metallurgical industry waste, a task often accomplished through the Waelz process. Notably, at the zinc plant of JSC "Almalyk Mining and Metallurgical Company" (JSC AMMC), around 20-25% of the zinc deposited on the cathodes is originated from the waelz oxide and 75-80% from calcine after roasting. This ratio is subject to modification based on the degree of waelz oxide purification, indicating the importance of impurity reduction for optimized electrolysis.

The purpose of the research. This study aims to investigate methods for enhancing the purity of waelz oxides to facilitate zinc electrolysis in high-purity zinc sulfate solutions.

Research methods and results. To reduce metals such as zinc, cadmium, lead, and indium in the form of waelz oxides through the Waelz process, carbon-containing reducing agents are added to the zinc cake (0.5 tons of reducing agent per 1 ton of cake) and roasted in a waelz furnace. In the Waelz process, the main part of lead, zinc, cadmium, indium, and a small amount of copper, gold, and silver are separated into waelz oxide [3].

Waelz oxide is formed as a result of reduction-oxidation processes in furnaces and cannot be selectively dissolved directly in sulfuric acid solutions due to the presence of halogens, arsenic, antimony, and reductant in the crude waelz oxides collected in special devices. Maximum purification of the above elements is required before leaching. Elemental purification can be done using pyrometallurgical or hydrometallurgical methods. In some zinc factories, chlorine and fluorine contained in crude waelz oxide were also washed with water.

At Chelyabinsk zinc plant, arsenic and antimony are converted into pentavalent oxides due to the fact that waelz oxide is treated in 40-meter-long rotary tube furnaces at temperatures of 900-1100 °C. Therefore, it is easy to purify solutions from arsenic and antimony by hydrolysis method in the next hydrometallurgical stages [4].

A sample of crude waelz oxide belonging to the zinc plant of JSC AMMC was used in this experimental work. Below are the chemical (Table 1) and mineralogical (Table 2) compositions of the sample.

Table 1

The chemical

Table 2

The chemical

The halogens that need to be cleaned in raw waelz oxide are mainly in the form of the following compounds: NaCl, PbCl₂, KCl, ZnCl₂, CaCl₂, FeCl₂, NaF, PbF₂, KF, ZnF₂, CaF₂, FeF₂.

Мaximum concentration of chlorine and fluorine in zinc electrolyte must be below 100 mg/l and 50 mg/l respectively for zinc production High Quality [5].

For this reason, the oxide is pre-treated to remove impurities and then to extract the zinc and lead. [6].

In the experimental work, a SNOL 8.2/1100 laboratory type muffle furnace was utilized. Samples weighing 100 grams were placed in a special crucible and positioned within the furnace chamber, heated to the specified temperature. Initially, the experiment was conducted within temperature intervals of 600-1000 °C for 2 hours, followed by a subsequent period at 800 °C for durations ranging from 0.5 to 3.5 hours. Mechanical stirring of the heated samples occurred every 15 minutes. The quantities of chlorine and fluorine present in both crude waelz oxide and samples of waelz oxide subjected to high-temperature combustion were determined using the potentiometric method, employing the I-160MI type ionomer.

The results are shown below.

Figure 1. Results

The reduction of fluorine was observed to be significant within the temperature range of 600-800 °C, while chlorine reduction occurred prominently within the range of 600-900 °C. Beyond these thresholds, the reduction of fluorine became negligible after reaching 800 °C, and chlorine reduction similarly diminished after surpassing 950 °C. Additionally, at elevated temperatures ranging from 900-1000 °C, melting phenomena may occur within certain regions of the lead and silicon dioxide mixture. It is noteworthy that mechanical components within multiple hearth furnaces utilized for crude waelz oxide production are prone to rapid deterioration, whereas any detrimental effects on rotary tube furnaces may be comparatively less pronounced.

Figure 2. Results

The process of dechlorination and defluorination was observed to be time-dependent, with significant changes occurring within the initial 30 minutes to 120 minutes. Notably, a purification rate of 11.7% for chlorine and 8.0% for fluorine was achieved within the intervals of 150-210 minutes. Additionally, carbon content in the resultant sample decreased to 65%, while reductions in arsenic and antimony content to 18% and 6.5% respectively were observed. According to literature findings, the presence of pentavalent arsenic and antimony in the recovered waelz oxides facilitates hydrolytic treatment when incorporated into the leaching process of calcine following roasting in a fluidized bed furnace.

Under the influence of high temperatures, the reducibility of waelz oxide decreased to 0.1%, accompanied by a notable increase in the solubility of cadmium compounds in sulfuric acid solutions to 90-97%. Concurrently, density rose from 1.1 g/cm³ to 1.55 g/cm³.

For precise measurement of the dimensions of crude and heated waelz oxide particles, the Zetasizer Ver. 8.02 by Malvern Panalytical was used. Experimental assessments of crude waelz oxide samples were conducted at two different temperatures (700 and 800 °C). Samples weighing 50 grams were heated for 120 minutes in a porcelain crucible within a SNOL 8.2/1100 laboratory muffle furnace. Results revealed that particle sizes of crude waelz oxide samples ranged from 0.6-0.7 μm, whereas those of the obtained waelz oxide measured between 1.9-2.5 μm. This indicates a size increase attributed to particle rounding under the influence of temperature.

Conclusion. Prior to leaching crude waelz oxide derived from the Waelz process of zinc cakes in weak solutions of sulfuric acid, appropriate temperatures for removing halogens, arsenic, antimony, and reducing agents (carbon) should be selected based on furnace type. Temperatures ranging from 800-1000 °C are deemed suitable when employing rotary tube furnaces, whereas temperatures of 800-850 °C are recommended for multiple hearth furnaces. Furthermore, it was affirmed that particle size of crude waelz oxide increases with temperature.

References:

1. Kholikulov D.B., Ruzikulov K.M. Hydrometallurgical processing of zinc cakes in the conditions of the zinc plant of Almalyk MMC JSC. Problems of complex and environmentally safe processing of natural and technogenic mineral raw materials. Proceedings of the international conference. Vladikavkaz. 2021, 505-509.
2. Ноэль Массон, Паскаль Бриоль, PBSIM & BFS Consulting Engineering; Zinc One Resources Inc.: Brief overview of zinc oxide processing methods available for the Bongara mine. 2017.
3. D.B. Кholiqulov, A.U. Samadov, O.N. Boltaev, F.E. Akhtamov. The results of laboratory research processing of zinc cake zinc plant JSC "Almalyk MMC".  European science review, Premier Publishing s.r.o. 2018, Pp. 96-99.
4. Degtyarev A.M., Ivakin D.A. Mastering the technology of calcining Waelz oxide in a tubular rotary kiln. “Non-ferrous metals” 2015. No. 5. 31-35.
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