Study of the Effect of Fluxing Ability of Flux Ores on Minimizing of Copper Losses with Slags during Copper Concentrate Smelting

Abstract

The article describes the effect of the fluxing ability (FA) of ores used as a flux on slag formation and copper losses. In Kazakhstan, at the Balkhash copper smelting plant (BCSP), currently used fluxes have a very low SiO₂ content—about 62%—whereas the Al₂O₃ content is 12%. Its fluxing ability (FA) was estimated, and it was shown that currently used silica fluxes have an exceedingly low FA. Only half of the fluxes participate in the slag formation. To obtain slags with a low magnetite concentration, a considerable surplus of flux must be added, which will result in a sharp reduction in its melt temperature, increased slag output, and therefore copper losses. The slag structure was studied by means of scanning electron microscopy and electron probe microanalysis (EPMA). To determine one of the primary causes of flux ores’ low FA, it is necessary to use the microstructural pattern of experimental samples.

1. Introduction

At the moment, a huge amount of copper-containing waste of man-made origin has accumulated on the surface of the Earth. This is because the pyrometallurgical production of non-ferrous metals is characterized by a high yield of slag in relation to the smelted metal. About 2.2 tons of slag are produced per each ton of copper produced [1].

Pyrometallurgical slags play an important role in the smelting process, acting as collectors for certain impurities that accompany copper at all stages of its production. Slag from copper smelting is a solid waste from copper pyrometallurgy processes, containing about 30–40% Fe and many valuable and, at the same time, environmentally hazardous metals, for example, Cu, As, Co, Ni, Pb, Zn, Al, and Ca [2,3,4,5].

Often, due to the lack of effective approaches to slag processing, more than 80% of copper smelting slags in many countries are simply stored, which not only involves significant environmental risks but also leads to the waste of a large number of valuable resources [6,7]. Therefore, it is necessary to search for new and effective methods for processing copper smelting slags to reduce the level of risk to the environment and increase economic benefits.

A number of technologies [8,9,10,11,12,13,14,15] can be used to extract copper from slags, remove iron from slag into commercial products and use of the silicate part of slags for the production of building materials [16,17,18,19]. However, they have several drawbacks, including limited technological and financial indexes, complex equipment design, and labor requirements, and they require large and extensive areas. Therefore, the more economically feasible alternative is to optimize the content of the feed to reduce copper losses with slags.

Since the implementation of the Vanyukov furnace in 1985, the processed copper-containing charge was fusible, enabling the furnace to ensure stable operation at a relatively low temperature of 1523–1543 K. For processing today, a charge with a low copper content is supplied, which requires an increase in its oxidation state, which subsequently leads to a rise in the Fe₃O₄ content and, accordingly, to the losses of copper with slag [20,21]. Currently, concentrates with a high zinc content are involved in copper production, which in turn, entailed a change in the physical and chemical properties of the slag. The melting point, viscosity, and specific gravity increased, and the interfacial tension between the slag–matte phase decreased; this resulted in an increase in copper losses during charge smelting.

Therefore, following the concept of environmentally friendly production, to reduce material and energy costs, and reduce the amount of waste before it leaves the production process, it is necessary to optimize the smelting parameters and assess the impact of the main components entering the smelting, to reduce the loss of valuable metals with slags and reduce the amount of waste copper slag [22].

2. Materials and Methods

Slag samples to assess the effect of the amount of silica on the copper content in the slag were taken from the Vanyukov furnace (Kazakhmys Corporation LLC, Balkhash, Kazakhstan, the melt temperature during sampling was 1513–1548 K.

Chemical analysis of slag samples was performed by titrimetric method of analysis. Chemical composition of flux ores currently used in copper plant was taken from BCSP chemical control cards.

An X-ray phase analysis was performed on a D8 ADVANCE (Bruker Elemental GmbH, Kalkar, Germany) diffractometer with Cu-Kα radiation. Tests were conducted in the temperature range of 1483–1493 K.

To study the microstructure of slag samples, scanning electron microscopy with X-ray spectral microanalysis JXA-8230 (JEOL, Tokyo, Japan) was used. Observations and the corresponding microanalysis were carried out, as a rule, at ×350 and ×1000 magnifications. The choice of areas of polished sections of the slag phase was carried out by avoiding drops of matte, metallic copper, or magnetite.

To evaluate the influence of the composition of silicate flux ores on the rate of slag formation in the system FeO-SiO₂ number of experiments were done. A sample of FeC₂O₄×2H₂O (oxalic iron) and ore were taken to completely flux iron oxides. After that the sample was mixed and placed in the crucible, which was further put into a pre-heated furnace. The sampling interval was 1 min, that is, 1, 2… 10 min after putting the crucible in the furnace.

3. Results and Discussion

The autogenicity of the Vanyukov process is achieved because of metal sulfide exothermic oxidation reactions. Figure 1 shows a scheme of the Vanyukov furnace.

In the above tuyere zone, reactions of dissociation of higher sulfides (1–3) proceed with the formation of stable compounds and with their further oxidation (4):

2CuFeS₂ = Cu₂S + 2FeS +1/2S₂

(1)

2CuS = Cu₂S + 1/2S₂

(2)

FeS₂ = FeS + 1/2S₂

(3)

FeS + 3/2O₂ = FeO + SO₂

(4)

As a result of the interaction of oxides and sulfides of copper and iron, copper is redistributed between the smelting products and concentrated in the matte:

Cu₂O + FeS = Cu₂S + FeO

(5)

and the formed iron oxide is slagged by quartz flux and passes into slag:

2FeO + SiO₂ = Fe₂SiO₄

(6)

The flux (quartz) added to the charge, in an amount sufficient to flux iron oxides, reacts with FeO to form a stable compound fayalite 2FeO×SiO₂ (6), which prevents further oxidation of iron with the formation of magnetite. Therefore, the amount of SiO₂ in the slag is an important parameter influencing the formation of magnetite and the loss of copper.

Figure 2a,b show the dependence of the content of copper and magnetite in the molten slags of the Vanyukov furnace (Kazakhmys Corporation LLC, Balkhash, Kazakhstan) on the content of silica in slags, according to the data obtained from the chemical analysis of the samples.

As Figure 2 shows, the higher the SiO₂ content in the slags, the lower the amount of magnetite and copper in slags. It was found that when the SiO₂ content was in the range of 30–35%, the mechanical losses of copper were minimal, which is explained by the predominance of interfacial tension over the viscosity of the melt [23]. Moreover, along with temperature, the SiO₂ content is an important technological parameter of the smelting process in the Vanyukov furnace. Many indicators of charge smelting, the stability of the process, and, as mentioned earlier, the losses of copper with slag depend on its value [24].

To evaluate the influence of the composition of silicate flux ores Zhezkazgan (1), Taskara (2), and Konyrat (3) (

Table 1. Composition of flux ores used in copper plant (mas. %).

№ of Flux	Content of Components (mas. %) *
	SiO2	Fe	CaO	Al2O3	MgO	Rest
1 (Zhezkazgan)	65.2	3.3	3.45	11.30	0.43	16.32
2 (Taskara)	65.5	3.3	1.70	13.95	1.30	14.25
3 (Konyrat)	65.7	3.4	0.70	17.50	0.18	12.52

* The contents of the main components are taken from the chemical control card of the BCSP.

) on the slag formation in the system FeO-SiO₂ number of experiments were done. The experiment is described in detail in [24].

Phase semi-quantitative analysis carried out on a D8 ADVANCE diffractometer (Bruker Elemental GmbH, Kalkar, Germany) with Kα-Cu radiation showed that in silicate ores Zhezkazgan (1) and Taskara (2) (Figure 3): quartz, albite Na(AlSi₃O₈), muscovite KAl₂(AlSiO₃)O₁₀(OH)₂, calcite CaCO₃ and orthoclase KAlSi₂O₈ are determined. In the Konyrat (3) ore, in addition to quartz, albite, muscovite, and calcite, Ca₃Al₂Si₃O₁₂ grossular and (Na,K)(Si₃Al)O₈ anorthoclase were found.

When heated, muscovite decomposes and mullitization of the ore occurs: at 673–873 K, moisture is removed, metakaolinite 3(Al₂O₃×2SiO₂) is formed, which at a temperature of 1273–1673 K decomposes into mullite 3Al₂O₃×2SiO₂ and β cristobalite. The melting point of mullite is 2183 K.

The experiments performed showed that when heated, a rather viscous slag melt (albite, anorthoclase, mullite) was formed with inclusions of unmelted individual aluminosilicates and quartz. These stable compounds of silica with Al₂O₃ are refractory and contribute to an increase in viscosity. This significantly reduces the fluxing ability of the ore, i.e., the content of “free” silica in it, which, in turn, will reduce the rate of fluxing of iron oxide and increase the content of magnetite in the slag.

Along with this the FA of currently used fluxing ores were calculated using the data from Table 1. The following formula is used to calculate the fluxing ability of various ores:

FA = C_SiO2 + C_CaO – 0.54C_Fe − 0.46C_Zn − 0.145C_Pb − 1.73C’_Al2O3 − 0.39C”_Al2O3 − 0.75C_MgO

(7)

where: Cn-content of ore components, in% mass; C’_Al2O3-the amount of alumina ore used for the formation of mullite, and C”_Al2O3-the amount of alumina ore used for the formation of an anorthite.

Then, the fluxing ability of Zhezkazgan (1), Taskara (2) and Konyrat (3) ores will be equal to: 57.5%, 47.21%, 37.83%, respectively. The data obtained show that the fluxing ability of flux ores used in Kazakhstan nowadays is exceptionally low, whereas all over the world, flux ore with a SiO₂ content of at least 80% is used. Almost half of the silica fluxes do not flux iron oxides, and in order to obtain homogeneous fluid slags with a low content of magnetite, a large excess of flux must be supplied, which will significantly lower the melt’s temperature and increase the slag output, and thus the losses of copper.

In Figure 4, the element mapping of the slag sample could be observed.

In the general view of the micrograph obtained in back-scattered electrons (COMPO), light-contrasted conglomerates are a mechanical suspension of sulfide or metal compounds. Dark-contrasted areas are related to substances having smaller atomic numbers. The window of iron shows that these conglomerates from the general micrograph are mainly iron compounds. In the micrograph of the distribution of silicon, these conglomerates are represented by dark spots. It indicates that silicon is not in compounds with metals, in particular, with iron.

It is clearly seen from Figure 4 that areas rich in silicon do not match with the areas rich in iron, but match with the rich areas of aluminum and calcium, which means that silicon is in strong bonds with aluminum and calcium and could not slag the FeO in order to prevent magnetite formation.

As can be seen from the presented data the results of elemental microanalysis confirm the results of X-ray phase analysis.

4. Conclusions

The use of concentrates with a high zinc content and a low copper content in copper production leads to an increase in their melting point, viscosity, specific gravity, and a decrease in interfacial tension between the slag and the matte melt. This also leads to an increase in copper losses during charge smelting and an increase in the degree of charge oxidation, which subsequently leads to an increase in the content of Fe₃O₄ and, accordingly, to the copper loss with slag.

In the course of the experiments, it was found that the flux ores currently used in copper production have a low fluxing ability (Konyrat (3)—37.83%, Taskara (2)—47.21%, and Zhezkazgan (1)—57.5%), and it becomes necessary to use ore with a high silicon content for complete slagging of oxidizing iron. In addition, flux ores should be modified to prevent the formation of refractory aluminosilicates, which interfere with the complete slagging of iron oxide by the flux. It could be accomplished by improving ore quality, i.e., using ore with high fluxing ability: SiO₂ should be 75% and above; the content of Al₂O₃ should not exceed 8%. That is why further study is required to find the optimum flux ores with high fluxing ability.