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Process for refining copper in solid state
MineralsEngineering,Vol. 12, No. 4, pp. 405-414, 1999 © 1999Publishedby ElsevierScienceLtd All rightsreserved
Pergamon
0892-6875/99/5 - - see front matter
0892-6875(99)00020-5
PROCESS
FOR
REFINING
COPPER
IN SOLID
STATE
D. VILLARROEL Departamento de Ffsica, Facultad de Ciencias Ffsicas y Matemfiticas, Universidad de Chile, Blanco 2008, Santiago, Chile
(Received 10 August 1998; accepted 28 December 1998)
ABSTRACT
A process for removing impurities from copper concentrate prior to smelting is presented here. The concentrate is subjected to thermal decomposition in a vacuum chamber at temperatures of about 950°C. This pretreatment allows the complete removal of arsenic, antimony, bismuth, lead, zinc, as well as of other impurities from the copper concentrate. © 1999 Published by Elsevier Science Ltd. All rights reserved
Keywords Industrial minerals, Sulphide ores, Pyrometallurgy, Mineral processing, Pollution
INTRODUCTION In copper metallurgy, the removal of impurities is critical to the production of high-quality copper. In the copper pyrometallurgy technique, which accounts for nearly 80% of the world's production of copper, the removal of impurities is attained through a tortuous process which is significant in the final cost of production. Given the constant increment in impurities in copper ore and the decrease in the content of copper as existing mines are exploited, it can be reasonably expected to have an even more critical situation in the future. Currently, a large number of mines around the world - - such as the Chuquicamata mine in Chile - - face this problem. The removal of impurities is carried out by volatilization and slagging during the smelting, conversion, and fire-refining stages, a process that ends with electrorefining. Despite the significant improvements introduced in the past decades to the smelting and conversion stages, especially due to the invention of the flash furnace, the methods of removing impurities have not changed, and have become a serious limitation to the modern processes of fusion and conversion. The main advantage of flash fusion lies in the generation of high-grade mattes, which can reach levels higher than 70%, but this requires the removal of impurities such as arsenic, antimony and bismuth, which is very difficult and costly. In particular, for concentrates with high levels of impurity it is more convenient to limit the grade of the matte [1,2,3]. The difficulty in removing impurities has thwarted the metallurgist's dream of obtaining blister copper in a single step. The difficulty in removing impurities from blister copper was an important factor working against the Noranda process of direct coppermaking [4], which was finally transformed into a rather traditional process in which the concentrate is smelted in one unit and then treated in the standard Peirce-Smith converter.
405
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D. Villarroel
A new process for the removal of impurities in the context of pyrometallurgy is presented in this paper. This process presents dramatic differences from the current process in that impurities such as arsenic, antimony, bismuth, lead, zinc, and others are removed prior to the smelting of the concentrate. The process consists of the gasifidation and removal of the impurities by subjecting the concentrate to thermal decomposition in a vacuum chamber at a temperature of about 950°C. [5] The pressure gradient induced by the vacuum pump forces the multicomponent gaseous mass through a tunnel with a decreasing temperature profile, thus allowing a stepwise condensation and recovery of the different substances. This paper deals mainly with the practical aspects of the process, especially those concerned with its effectiveness in removing impurities such as arsenic, antimony, bismuth, lead, selenium, and zinc. The advantages of the process over the current technology are also emphasized. The design of the furnace to carry out the process at an industrial scale is not discussed here. Also not treated in this paper are other fundamental aspects which will require a more detailed study.
THE REMOVAL
OF IMPURITIES
A simplified layout of the equipment used in the laboratory for the removal of impurities is shown in Figure 1. The electric furnace 1 is of the tubular type with three sections, each of which can be independently controlled by means of thermocouples and microprocessors. The concentrate in the form of dust is deposited in a quartz crucible located in the left-hand side of quartz tube 2, the zone with the highest temperature. A decreasing temperature profile is generated along the quartz tube with the help of thermal barriers. The samples can be extracted from the quartz tube by means of spherical joint 3, where the seal is obtained simply with silicone, given that the vacuum - measured with vacuum gauge 4 - is not very high, around lo-* millimeters of mercury. In order to extract the sample at the end of the process, the pressure inside of tube 2 is set at the atmospheric pressure by means of vacuum valve 5. The substances that remain in a gas state at room temperature are condensed in liquid nitrogen condenser 6. These gases are mainly SO,, H,S, and steam, all of which are harmful to mechanical vacuum pump 7. The toxic gases are freed to the atmosphere by means of vacuum valve 8 in condenser 6, which can be physically isolated from quartz tube 2 and vacuum pump 7 by means of valves 9 and 10, respectively.
,,‘_“_~_~_“~‘__‘~“.~ .._._ _._ .__.._ _____._ ..._._.._....__.-.-...,-_._._.__.. ___._.____.__._..___.---.-..--.-_-.--...---...-.--. -.
)
1
tD Fig.1 Experimental
layout for the thermal decomposition
I
in vacuum of the concentrates.
This paper deals with two types of concentrates coming from two different Chilean mines. These concentrates will be referred to as concentrates 1 and 2 respectively. Concentrate 1 is relatively clean, with concentration of arsenic of about 3,000 ppm, and will be discussed in first place.
Process for refining copper in solid state
407
A typical treatment begins at room temperature with a vacuum between 10-2 and 10-3 mm of Hg. The temperature increases at a rate of 20°C per minute up to a final temperature of 850°C, while a simultaneous pressure increment is induced by volatilization. The pressure shows a temperature dependence with maximums at 350°C and 550°C, corresponding to maximum rates of gas generation. The main gasified substance in terms of mass is sulfur, most of which gasifies below 600°C. The pressure is generally below one millimeter of mercury during the heating period, and starts decreasing to 10-2 mm of Hg a few minutes after reaching the working temperature of 850°C. Due to the decreasing temperature profile, the gasified multicomponent gaseous mass begins to condense as it propagates along the quartz tube, thus forming several deposits. Given their different colors, each deposit is clearly distinguished from the rest. Although this paper does not deal with the physical and chemical compositions of the deposits, on general ground, it is easy to infer their main constituents. The deposits are all in solid state, and form a film that is uniformly distributed around the internal periphery of the quartz tube. Four deposits are clearly identified after the treatment when concentrate 1 is treated for one hour at 850°C in a vacuum of 10-2 m m of Hg. The sulfur condenses below 200°C, forming two deposits to the right end of the quartz tube. The first deposit from right to left forms at temperatures ranging between room temperature and about 100°C, presenting a milky-white color; it corresponds to amorphous sulfur. The second deposit forms at temperatures ranging between about 100°C and 200°C, presenting a bright yellow color. The arsenic deposit forms at temperatures ranging between a little over 200°C and 550°C. This makes it possible for the arsenic deposit and the sulfur deposits to occur practically without overlapping. Finally, there is a white deposit that forms at temperatures ranging between about 500°C and 800°C, consisting mainly of zinc and antimony. It should be pointed out that, strictly speaking, the ranges of temperatures quoted for the different deposits are not specific to the process, since they depend on factors such as the temperature of treatment, amount of concentrate, length of the quartz tube, temperature profile, capacity of the vacuum pump, etc. When concentrate 1 is treated for one hour at 850°C in a vacuum of 10 -2 m m of Hg, the level of arsenic goes from about 3000 ppm to less than 1 ppm; this makes this treatment very effective in removing the arsenic. This result is noticeable because, in general, the arsenic in the copper concentrate is present mainly as enargite and arsenopyrite, as happens with concentrate 1. The effectiveness of the treatment will therefore be high, regardless of the mine from which the concentrate is taken. Regarding the effectiveness of the removal of impurities, it is interesting to study the influence of the three fundamental variables: temperature, time of treatment, and degree of vacuum. Table 1 shows (in ppm) the effectiveness of the treatment in removing arsenic, antimony, lead, and zinc, when concentrate 1 is treated at 900°C for one hour with a vacuum of about one millimeter of mercury. This treatment proves to be quite effective for the removal of arsenic, but is of practically no effect in the removal of antimony, lead, and zinc. The levels of lead and zinc in sample 2 are higher than in the original sample due to the fact that the concentrate has lost nearly 12% of its mass. It has already been noted that when concentrate 1 is treated at 850°C during one hour in a vacuum of about 10-2 mm of Hg, it ends up completely free of arsenic. Thus, according to Table 1, the pressure plays a significant role in the effectiveness of the process. Consequently, a vacuum of nearly 10-2 m m of Hg will always be used in further tests, but with different temperatures and times of treatment.
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D. Villarroel
TABLE 1
Removal of impurities from concentrate 1 with a vacuum of a few millimeters of Hg for one hour at 900°C
Element
Arsenic
Antimony
Lead
Zinc
Original sample
2700
260
860
5600
Sample 2 (Treated)
370
220
880
6100
Table 2 shows values for concentrate 1, where the data for the original sample has been repeated in order to facilitate comparisons. Sample 2 corresponds to a treatment at 900°C for one hour, so the temperature and the time of treatment are the same as in sample 2 in Table 1, but as just noted the vacuum here is now nearly 10-2 mm of Hg. The effectiveness of the treatment increases strongly with respect to sample 2 in Table 1, given that the arsenic, the antimony, and the lead are now almost completely removed. However, the effectiveness in removing the zinc is rather poor. For this reason, the time of treatment is extended to two hours in sample 3, with the temperature and the degree of vacuum remaining the same as in sample 2. In this case, the removal of zinc reaches nearly 70%. To better understand the removal of zinc, the temperature in sample 4 was raised to 950°C, with the same time of treatment and vacuum degree as in sample 2 in Table 2. Under these new conditions, it is possible to remove more than 95% of the original content of zinc from the concentrate. TABLE 2
Removal of impurities from concentrate 1 with a vacuum of different temperatures and times of treatment
Element Original sample Sample 2 (T = 900°C t = 1 hr) Sample 3 (T = 900°C t = 2 hr) Sample 4 (T = 950°C t = 1 hr)
II
U Arsenic
1 0 -2 m m
Antimony
Lead
Zinc
2700
260
860
5600
< 1
< 5
< 5
4000
< 1
< 5
< 5
1760
< 1
< 5
< 5
200
of Hg for
Even though the nature of the impurities and their levels change from mine to mine, the minerals containing these impurities are generally the same. This characteristic suggests that the effectiveness of the present treatment in removing impurities is, to a great extent, independent of the origin of the copper concentrate. Nevertheless, a detailed knowledge of the removal of impurities requires specific studies on particular concentrates. In general terms, it can be said that a treatment for one hour at 950°C with a vacuum of about 10-2 mm of Hg results in the complete removal of arsenic, antimony, and lead, and the removal of 95% of the zinc. Bismuth and selenium are also relevant impurities in copper pyrometallurgy, but the levels of these impurities in concentrate 1 are too low to study their removal. In contrast with concentrate 1, concentrate 2 is very dirty because it has 41,000 ppm of arsenic and 3,100 ppm of antimony. Despite its high content of copper and low content of iron, these high levels of arsenic and antimony do not permit this concentrate to be processed using standard pyrometallurgical procedures.
process for refining copper in solid state
409
Table 3 shows the results obtained when concentrate 2 is treated at 950°C for one hour in a vacuum of 10-2 m m of Hg. This concentrate presents no novelty with respect to concentrate 1 when it comes to removing arsenic, antimony, lead, and zinc. The residue of 230 ppm of arsenic is mainly due to the high level of this element contained in the original concentrate but, in any case, the percentage of removal is over the 99%. Although the residue is innocuous, it can be almost completely removed by increasing the time of treatment. A new result of great importance is the one associated with bismuth, since according to Table 3, it is completely removed from the concentrate. Unfortunately, most of the selenium remains in the concentrate. Nevertheless, the effectiveness of the treatment in removing selenium can definitely be improved by increasing the temperature and the time of treatment. TABLE 3
R e m o v a l o f impurities f r o m concentrate 2 with a v a c u u m of 10 -2 nun o f H g at a t e m p e r a t u r e o f 950°C and one h o u r of t r e a t m e n t
Element
II Cu I Fe
mmMl/Mmm memnnune m
nmnmnmmm
The fact that the final treated concentrate is free of impurities and contains a high level of copper and a low level of iron makes this treatment a very powerful one for this concentrate. These properties may lead to the obtainment of a semi-blister in a single step after smelting. Moreover, according to the data in Table 3 the treatment allows for the recovery of more than one third of the sulfur as elemental sulfur with a high degree of purity, since impurities such as zinc, antimony, and arsenic are deposited in places of higher temperatures.
ADVANTAGES OF T H E PROCESS The process for copper refining in solid state explained in section 2 presents advantages with respect to the current technology in the different steps considered in the pyrometallurgic procedure, namely, smelting, conversion, pyrorefining, electrorefining, acid plant, recovery of valuable elements and pollution control. To a great extent, the advantages derive from the simplifications due to the absence of impurities in the corresponding steps. Consequently, a reduction in copper production cost is obtained. This section deals with the advantages of the process by analyzing the different stages separately. Smelting
The smelting of the concentrate generates the following products: matte, dust, gases, and slag. The elimination of impurities prior to smelting leads to cost reductions in the treatment of each of them. Matte
The modem smelting process based on the use of oxygen is optimized by using the treatment described here, since the grade of the matte is no longer limited by the presence of impurities in the concentrate. Given that the grade of the matte depends basically on the quotient between the mass of oxygen and the mass of concentrate, the oxygen enrichment is another factor constraining the grade of the matte, since for normal concentrates an enrichment beyond 70% would produce too much heat [1] in the furnace. This constraint is overcome here, since the treatment removes nearly one third of the original content of sulfur, as shown in Table 3. In this case, the concentrate remains with a sulfur content of 24%, which allows a high enrichment of oxygen. Note that although the process suggested here removes an important part of the
41o
D. Villarroel
sulfur, this is partly counterbalanced by the fact that the concentrate would enter the flash furnace at temperatures of nearly 900°C.
Dust The most serious problem of the flash furnace is the generation of dust, which may represent nearly 10% of the original mass of the concentrate introduced into the furnace [6,7]. The dust must be recycled to avoid an unacceptable loss of copper. But given that the levels of impurities in the dust are much higher than in the original concentrate, the direct recycling of the dust is not possible for original concentrates with high levels of impurities [6]. Since under the treatment suggested here the impurities are removed prior to smelting of the concentrate, the dust can be directly recycled. The process also has an important effect on the combustion and the fusion of the particles in the concentrate, and may also affect the amount of dust. The kinetics of the reaction of the concentrate clearly differ from those of the currently used process, because the particles enter the flash furnace after losing the labile sulfur, presenting multiple cracks due to heating and desulfurization, and also because the concentrate enters at temperatures of nearly 900°C. The physical properties of the particles are very different in the currently used process of smelting, where the concentrate enters at a relatively low temperature and where the content of labile sulfur is still intact. The outburst of the particles in the concentrate at ignition [8,9] may constitute an important factor in the generation of dust. The currently used process generates multiple fragments that escape the high-temperature region of the furnace, thus preventing their complete fusion. Since the proposed process for removing impurities prior to fusion reduces the phenomenon of outburst and fragmentation of the particles, one may naturally expect a lower generation of dust.
Gases Usually, the sulfur dioxide generated during smelting and conversion is transformed into sulfuric acid, a process which requires a complex and elaborate stage of gas cleaning in order to eliminate impurities such as arsenic, antimony, and others [10]. The elimination of arsenic is particularly delicate, since this element not only affects the quality of the acid, but also give rise to problems of corrosion of some essential components of the acid plant. The removal of impurities before smelting simplifies the cleaning of the gases and reduces the loss by corrosion and wear in the acid plant; in addition, it generates high-quality sulfuric acid.
Slag The fact that the slag generated in the modern process of smelting contains a great amount of copper means that the slag must be treated if copper recovery is to be maximized. The present process for solid state copper refining will generate a slag which is free of impurities, thus simplifying its treatment. As a result of the treatment for the removal of impurities, part of the copper contained in the particles turn into whiskers, as can be easily shown with the help of an electronic microscope. This effect may contribute positively to diminishing the copper content in the slag.
Conversion The main purpose of conversion is to oxidize the iron and sulfur in the matte produced during the smelting. The sulfur leaves the converter as sulfur dioxide, while the iron is slagged with the help of fluxes; this yields blister copper with a purity of about 99%. The same products as in the smelting are generated in the conversion, but with the blister copper playing the role of the matte. Thus the process under consideration presents in the conversion stage similar advantages to those already mentioned in the case of the smelting stage.
Process for refiningcopper in solid state
411
In the currently used process, the impurities in the matte are partially volatilized and slagged during conversion. Given that in the process proposed here the matte is already free of impurities such as arsenic, antimony, bismuth, and others, this issue becomes irrelevant.
Fire refining The advantages of the process of removing impurities before smelting become particularly significant when the fire-refining stage is considered. The main objective of fire refining is to eliminate impurities such as arsenic, antimony, bismuth, and others, by means of oxidation and slagging. With the treatment described here, the blister is free of impurities, making this stage unnecessary, or at least reducing it to a process of eliminating the residual sulfur and iron present in the blister. The traditional fire-refining process involves several sequential operations involving long-lasting treatments. In particular, it needs special and costly fuels and fluxes [11,12]. The elimination of bismuth is practically impossible through fire refining, given that there is no appropriate flux for this impurity [12]. The bismuth must therefore be eliminated in the stage of electrorefining. Another problem of fire refining is that the copper is severely oxidized, which demands a later reduction step requiring special fuels. The process of oxidation and slagging also produces severe wear and corrosion in the refractory line in the furnace, as well as a loss of copper and valuable elements in the slag. All of these drawbacks mentioned above disappear in the solid state process for copper refining proposed here.
Electrorefining The pretreatment of the concentrate under discussion also simplifies the stage of electrorefining, thus reducing costs. Since in this case the anode is free of impurities such as arsenic, antimony, and bismuth, there is no possibility of formation of compounds due to the reaction of the arsenic with antimony and bismuth, which have been considered as a major source of cathode contamination [13]. Moreover, the lack of impurities in the electrolyte makes the complex process of electrolytic purification almost completely unnecessary during the electrorefining stage. In particular, the problems associated with the generation of the dangerous arsine disappear. When the impurities are removed prior to smelting, the amount of floating anode slimes decreases significantly, thus improving the physical quality of the cathodic deposit. In particular, the problem of the formation of dendrite associated with the presence of antimony disappears. [14] Furthermore, when the electrolyte is contaminated with floating anode slimes, the current density becomes a rather critical operational parameter, because a high current density leads to a dragging effect of impurities, with the consequential cathode contamination. This effect is very weak if the electrolyte has low levels of impurities. Thus, if the concentrate is subjected to the present treatment, it seems possible to increment the current density beyond the normal values currently in use.
Anode slimes In electrorefining, the anode slimes represent an important byproduct, because they contain valuable elements such as silver, gold, platinum, palladium, selenium, and tellurium. One important step for the recovery of these elements consists in the oxidation and slagging of impurities such as arsenic, antimony, bismuth, and lead. This procedure requires very special fuels and fluxes, and is such that part of the valuable elements are lost in the slag. These drawbacks disappear when the impurities are removed according to the present treatment. Also, this treatment simplifies the steps involved in the recovery of the different valuable elements. The removal of impurities prior to fusion also reduces in a significant way the amount of anode slimes, [15] a fact which also contributes to a simplification in the recovery of the valuable elements. In connection with this, it should be noted that according to some authors, [16] the amount of anode slimes is basically determined by the amount of lead in the anode. The pretreatment considered here completely removes the
412
D. Villarroel
lead from the concentrate, so that a significant reduction in the anode slime may be expected.
Recovery of other elements The present process allows the recovery of several other elements which may not be as valuable as those in the anode slimes, but nevertheless have a commercial value. Among these elements are: antimony, bismuth, lead, zinc, tin, cadmium, and mercury, all of which are currently discarded. The recovery of these elements may be of economic interest, especially in the case of concentrates with high contents of them. The process also removes significant proportions of selenium, tellurium, germanium, rhenium, and gallium, but the effectiveness of the recovery of these elements is still unknown. The recovery of elemental sulfur deserves special consideration. As is known, the sulfur dioxide generated in the smelting and conversion stages must be processed in order to avoid the environmental pollution that would result if it were freed directly to the atmosphere. In the smelter, this gas is transformed in sulfuric acid, which has a wide variety of applications into industrial processes. However, the production of sulfuric acid presents some problems when the smelter is far from the acid market, or when the potential for sulfuric acid production is far beyond the local needs for acid, as is the case in Chile. The problems are related to the high storage and transport costs associated with sulfuric acid, costs that sometimes make difficult, if not impossible, to reduce the environmental contamination by means of sulfuric acid generation. In Chile, the world's largest producer of copper, this problem has not yet become critical, because an important part of the concentrate is exported without further processing, and also because some of the sulfur dioxide generated is not transformed into sulfuric acid, but rather released into the atmosphere. Almost all the sulfuric acid that is produced is being used in hydrometallurgic processes for recovering copper from oxides minerals coming from mines that are not very far from the smelters. However, this picture will change dramatically in the future, since the oxides mines will be exhausted in the course of a few decades. Moreover, the environmental regulations will become stricter, and the release of sulfur dioxide into the atmosphere will no longer be possible. The process discussed here removes nearly one third of the sulfur content of the concentrate as elemental sulfur, which means that sulfuric acid production will also be reduced to one third. Elemental sulfur does not present the disadvantages of sulfuric acid, so, it would be ideal if most of the sulfur in the concentrate could be recovered as elemental sulfur. The present treatment paves the way for this possibility. In fact, part of the concentrate which is free of impurities can be treated with hydrogen at high temperatures to produce hydrogen sulfide, which can be combined with sulfur dioxide to obtain elemental sulfur according to a wellknown technology. It is known that when the concentrate is attacked with hydrogen, the iron in the residue can then be leached almost completely and selectively with respect to the copper by means of hydrochloric acid [17]. In order to avoid an excessive production of sulfuric acid in Chile in the future, it will likely be necessary to combine the current pyrometallurgic process with an hydrometallurgic process such as the one described here.
Pollution control The pyrometallurgy of copper leads to serious environmental problems due to the production of toxic substances such as arsenic, cadmium, mercury, and lead. The arsenic is removed as arsenic trioxide, which is poisonous and difficult to discard because of its high solvability in water. This problem becomes more complex if the concentrate has high levels of arsenic, as happens, for example, in Chuquicamata, where the discarding of arsenic residues of the acid plant is somewhat complicate. The arsenic contaminates the slag, the dust, and the residual gases as well. The process presented here completely removes the arsenic from the concentrate in a way which is innocuous to the environment, since it is removed mainly as elemental arsenic or as sulfides, which are practically insoluble in water.
Process for refiningcopperin solid state
413
As for cadmium, mercury, and lead, since they are completely removed prior to the fusion of the concentrate, the current problem of heavy metals pollution disappears. The discarding of the slag therefore causes no harm to the environment. The solid-state process for copper refining discussed herein can be implemented with the different smelting and converting processes currently in use. It is particularly appropriate, however, to the flash smelting followed by the Kennecott-Outokumpu Flash Converting Process, since it is then possible to take full advantage of a high enrichment of oxygen. Furthermore, this technology allows a rigorous control of environmental pollution, as can be seen in the modern Utah smelter where sulfur capture is as high as a 99.9%, making it the cleanest smelter in the world.
CONCLUSIONS Thermal decomposition of copper concentrates for one hour at a temperature of 950°C in a chamber with a vacuum of 10-2 mm of Hg allows the complete removal of harmful impurities such as arsenic, antimony, bismuth, lead, and zinc. This treatment may lead to the refining of copper in solid state prior to smelting at an industrial scale, a process which would introduce great simplifications in the different stages of copper pyrometallurgy. Besides, if the gasified substances are forced through a tunnel with a decreasing temperature profile, it will be possible to recover over one third of the sulfur content of the concentrate as elemental sulfur of high purity. Furthermore, the treatment allows the recovery of elements such as zinc, lead, antimony, cadmium, and mercury. The design of a special industrial furnace that allows the treatment of the concentrate according to this process will be presented in a forthcoming paper. This furnace processes the concentrate in a continuous way and the concentrate is heated with the help of the thermal energy contained in the fusion and conversion gasses.
ACKNOWLEDGMENTS The author wishes to thank the Comisi6n Nacional de Investigaci6n Cicntffica y Tecnol6gica de Chile (CONICYT) for the grant FONDECYT N ° 1960179.
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0892-6875/99/5 - - see front matter
0892-6875(99)00020-5
PROCESS
FOR
REFINING
COPPER
IN SOLID
STATE
D. VILLARROEL Departamento de Ffsica, Facultad de Ciencias Ffsicas y Matemfiticas, Universidad de Chile, Blanco 2008, Santiago, Chile
(Received 10 August 1998; accepted 28 December 1998)
ABSTRACT
A process for removing impurities from copper concentrate prior to smelting is presented here. The concentrate is subjected to thermal decomposition in a vacuum chamber at temperatures of about 950°C. This pretreatment allows the complete removal of arsenic, antimony, bismuth, lead, zinc, as well as of other impurities from the copper concentrate. © 1999 Published by Elsevier Science Ltd. All rights reserved
Keywords Industrial minerals, Sulphide ores, Pyrometallurgy, Mineral processing, Pollution
INTRODUCTION In copper metallurgy, the removal of impurities is critical to the production of high-quality copper. In the copper pyrometallurgy technique, which accounts for nearly 80% of the world's production of copper, the removal of impurities is attained through a tortuous process which is significant in the final cost of production. Given the constant increment in impurities in copper ore and the decrease in the content of copper as existing mines are exploited, it can be reasonably expected to have an even more critical situation in the future. Currently, a large number of mines around the world - - such as the Chuquicamata mine in Chile - - face this problem. The removal of impurities is carried out by volatilization and slagging during the smelting, conversion, and fire-refining stages, a process that ends with electrorefining. Despite the significant improvements introduced in the past decades to the smelting and conversion stages, especially due to the invention of the flash furnace, the methods of removing impurities have not changed, and have become a serious limitation to the modern processes of fusion and conversion. The main advantage of flash fusion lies in the generation of high-grade mattes, which can reach levels higher than 70%, but this requires the removal of impurities such as arsenic, antimony and bismuth, which is very difficult and costly. In particular, for concentrates with high levels of impurity it is more convenient to limit the grade of the matte [1,2,3]. The difficulty in removing impurities has thwarted the metallurgist's dream of obtaining blister copper in a single step. The difficulty in removing impurities from blister copper was an important factor working against the Noranda process of direct coppermaking [4], which was finally transformed into a rather traditional process in which the concentrate is smelted in one unit and then treated in the standard Peirce-Smith converter.
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D. Villarroel
A new process for the removal of impurities in the context of pyrometallurgy is presented in this paper. This process presents dramatic differences from the current process in that impurities such as arsenic, antimony, bismuth, lead, zinc, and others are removed prior to the smelting of the concentrate. The process consists of the gasifidation and removal of the impurities by subjecting the concentrate to thermal decomposition in a vacuum chamber at a temperature of about 950°C. [5] The pressure gradient induced by the vacuum pump forces the multicomponent gaseous mass through a tunnel with a decreasing temperature profile, thus allowing a stepwise condensation and recovery of the different substances. This paper deals mainly with the practical aspects of the process, especially those concerned with its effectiveness in removing impurities such as arsenic, antimony, bismuth, lead, selenium, and zinc. The advantages of the process over the current technology are also emphasized. The design of the furnace to carry out the process at an industrial scale is not discussed here. Also not treated in this paper are other fundamental aspects which will require a more detailed study.
THE REMOVAL
OF IMPURITIES
A simplified layout of the equipment used in the laboratory for the removal of impurities is shown in Figure 1. The electric furnace 1 is of the tubular type with three sections, each of which can be independently controlled by means of thermocouples and microprocessors. The concentrate in the form of dust is deposited in a quartz crucible located in the left-hand side of quartz tube 2, the zone with the highest temperature. A decreasing temperature profile is generated along the quartz tube with the help of thermal barriers. The samples can be extracted from the quartz tube by means of spherical joint 3, where the seal is obtained simply with silicone, given that the vacuum - measured with vacuum gauge 4 - is not very high, around lo-* millimeters of mercury. In order to extract the sample at the end of the process, the pressure inside of tube 2 is set at the atmospheric pressure by means of vacuum valve 5. The substances that remain in a gas state at room temperature are condensed in liquid nitrogen condenser 6. These gases are mainly SO,, H,S, and steam, all of which are harmful to mechanical vacuum pump 7. The toxic gases are freed to the atmosphere by means of vacuum valve 8 in condenser 6, which can be physically isolated from quartz tube 2 and vacuum pump 7 by means of valves 9 and 10, respectively.
,,‘_“_~_~_“~‘__‘~“.~ .._._ _._ .__.._ _____._ ..._._.._....__.-.-...,-_._._.__.. ___._.____.__._..___.---.-..--.-_-.--...---...-.--. -.
)
1
tD Fig.1 Experimental
layout for the thermal decomposition
I
in vacuum of the concentrates.
This paper deals with two types of concentrates coming from two different Chilean mines. These concentrates will be referred to as concentrates 1 and 2 respectively. Concentrate 1 is relatively clean, with concentration of arsenic of about 3,000 ppm, and will be discussed in first place.
Process for refining copper in solid state
407
A typical treatment begins at room temperature with a vacuum between 10-2 and 10-3 mm of Hg. The temperature increases at a rate of 20°C per minute up to a final temperature of 850°C, while a simultaneous pressure increment is induced by volatilization. The pressure shows a temperature dependence with maximums at 350°C and 550°C, corresponding to maximum rates of gas generation. The main gasified substance in terms of mass is sulfur, most of which gasifies below 600°C. The pressure is generally below one millimeter of mercury during the heating period, and starts decreasing to 10-2 mm of Hg a few minutes after reaching the working temperature of 850°C. Due to the decreasing temperature profile, the gasified multicomponent gaseous mass begins to condense as it propagates along the quartz tube, thus forming several deposits. Given their different colors, each deposit is clearly distinguished from the rest. Although this paper does not deal with the physical and chemical compositions of the deposits, on general ground, it is easy to infer their main constituents. The deposits are all in solid state, and form a film that is uniformly distributed around the internal periphery of the quartz tube. Four deposits are clearly identified after the treatment when concentrate 1 is treated for one hour at 850°C in a vacuum of 10-2 m m of Hg. The sulfur condenses below 200°C, forming two deposits to the right end of the quartz tube. The first deposit from right to left forms at temperatures ranging between room temperature and about 100°C, presenting a milky-white color; it corresponds to amorphous sulfur. The second deposit forms at temperatures ranging between about 100°C and 200°C, presenting a bright yellow color. The arsenic deposit forms at temperatures ranging between a little over 200°C and 550°C. This makes it possible for the arsenic deposit and the sulfur deposits to occur practically without overlapping. Finally, there is a white deposit that forms at temperatures ranging between about 500°C and 800°C, consisting mainly of zinc and antimony. It should be pointed out that, strictly speaking, the ranges of temperatures quoted for the different deposits are not specific to the process, since they depend on factors such as the temperature of treatment, amount of concentrate, length of the quartz tube, temperature profile, capacity of the vacuum pump, etc. When concentrate 1 is treated for one hour at 850°C in a vacuum of 10 -2 m m of Hg, the level of arsenic goes from about 3000 ppm to less than 1 ppm; this makes this treatment very effective in removing the arsenic. This result is noticeable because, in general, the arsenic in the copper concentrate is present mainly as enargite and arsenopyrite, as happens with concentrate 1. The effectiveness of the treatment will therefore be high, regardless of the mine from which the concentrate is taken. Regarding the effectiveness of the removal of impurities, it is interesting to study the influence of the three fundamental variables: temperature, time of treatment, and degree of vacuum. Table 1 shows (in ppm) the effectiveness of the treatment in removing arsenic, antimony, lead, and zinc, when concentrate 1 is treated at 900°C for one hour with a vacuum of about one millimeter of mercury. This treatment proves to be quite effective for the removal of arsenic, but is of practically no effect in the removal of antimony, lead, and zinc. The levels of lead and zinc in sample 2 are higher than in the original sample due to the fact that the concentrate has lost nearly 12% of its mass. It has already been noted that when concentrate 1 is treated at 850°C during one hour in a vacuum of about 10-2 mm of Hg, it ends up completely free of arsenic. Thus, according to Table 1, the pressure plays a significant role in the effectiveness of the process. Consequently, a vacuum of nearly 10-2 m m of Hg will always be used in further tests, but with different temperatures and times of treatment.
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D. Villarroel
TABLE 1
Removal of impurities from concentrate 1 with a vacuum of a few millimeters of Hg for one hour at 900°C
Element
Arsenic
Antimony
Lead
Zinc
Original sample
2700
260
860
5600
Sample 2 (Treated)
370
220
880
6100
Table 2 shows values for concentrate 1, where the data for the original sample has been repeated in order to facilitate comparisons. Sample 2 corresponds to a treatment at 900°C for one hour, so the temperature and the time of treatment are the same as in sample 2 in Table 1, but as just noted the vacuum here is now nearly 10-2 mm of Hg. The effectiveness of the treatment increases strongly with respect to sample 2 in Table 1, given that the arsenic, the antimony, and the lead are now almost completely removed. However, the effectiveness in removing the zinc is rather poor. For this reason, the time of treatment is extended to two hours in sample 3, with the temperature and the degree of vacuum remaining the same as in sample 2. In this case, the removal of zinc reaches nearly 70%. To better understand the removal of zinc, the temperature in sample 4 was raised to 950°C, with the same time of treatment and vacuum degree as in sample 2 in Table 2. Under these new conditions, it is possible to remove more than 95% of the original content of zinc from the concentrate. TABLE 2
Removal of impurities from concentrate 1 with a vacuum of different temperatures and times of treatment
Element Original sample Sample 2 (T = 900°C t = 1 hr) Sample 3 (T = 900°C t = 2 hr) Sample 4 (T = 950°C t = 1 hr)
II
U Arsenic
1 0 -2 m m
Antimony
Lead
Zinc
2700
260
860
5600
< 1
< 5
< 5
4000
< 1
< 5
< 5
1760
< 1
< 5
< 5
200
of Hg for
Even though the nature of the impurities and their levels change from mine to mine, the minerals containing these impurities are generally the same. This characteristic suggests that the effectiveness of the present treatment in removing impurities is, to a great extent, independent of the origin of the copper concentrate. Nevertheless, a detailed knowledge of the removal of impurities requires specific studies on particular concentrates. In general terms, it can be said that a treatment for one hour at 950°C with a vacuum of about 10-2 mm of Hg results in the complete removal of arsenic, antimony, and lead, and the removal of 95% of the zinc. Bismuth and selenium are also relevant impurities in copper pyrometallurgy, but the levels of these impurities in concentrate 1 are too low to study their removal. In contrast with concentrate 1, concentrate 2 is very dirty because it has 41,000 ppm of arsenic and 3,100 ppm of antimony. Despite its high content of copper and low content of iron, these high levels of arsenic and antimony do not permit this concentrate to be processed using standard pyrometallurgical procedures.
process for refining copper in solid state
409
Table 3 shows the results obtained when concentrate 2 is treated at 950°C for one hour in a vacuum of 10-2 m m of Hg. This concentrate presents no novelty with respect to concentrate 1 when it comes to removing arsenic, antimony, lead, and zinc. The residue of 230 ppm of arsenic is mainly due to the high level of this element contained in the original concentrate but, in any case, the percentage of removal is over the 99%. Although the residue is innocuous, it can be almost completely removed by increasing the time of treatment. A new result of great importance is the one associated with bismuth, since according to Table 3, it is completely removed from the concentrate. Unfortunately, most of the selenium remains in the concentrate. Nevertheless, the effectiveness of the treatment in removing selenium can definitely be improved by increasing the temperature and the time of treatment. TABLE 3
R e m o v a l o f impurities f r o m concentrate 2 with a v a c u u m of 10 -2 nun o f H g at a t e m p e r a t u r e o f 950°C and one h o u r of t r e a t m e n t
Element
II Cu I Fe
mmMl/Mmm memnnune m
nmnmnmmm
The fact that the final treated concentrate is free of impurities and contains a high level of copper and a low level of iron makes this treatment a very powerful one for this concentrate. These properties may lead to the obtainment of a semi-blister in a single step after smelting. Moreover, according to the data in Table 3 the treatment allows for the recovery of more than one third of the sulfur as elemental sulfur with a high degree of purity, since impurities such as zinc, antimony, and arsenic are deposited in places of higher temperatures.
ADVANTAGES OF T H E PROCESS The process for copper refining in solid state explained in section 2 presents advantages with respect to the current technology in the different steps considered in the pyrometallurgic procedure, namely, smelting, conversion, pyrorefining, electrorefining, acid plant, recovery of valuable elements and pollution control. To a great extent, the advantages derive from the simplifications due to the absence of impurities in the corresponding steps. Consequently, a reduction in copper production cost is obtained. This section deals with the advantages of the process by analyzing the different stages separately. Smelting
The smelting of the concentrate generates the following products: matte, dust, gases, and slag. The elimination of impurities prior to smelting leads to cost reductions in the treatment of each of them. Matte
The modem smelting process based on the use of oxygen is optimized by using the treatment described here, since the grade of the matte is no longer limited by the presence of impurities in the concentrate. Given that the grade of the matte depends basically on the quotient between the mass of oxygen and the mass of concentrate, the oxygen enrichment is another factor constraining the grade of the matte, since for normal concentrates an enrichment beyond 70% would produce too much heat [1] in the furnace. This constraint is overcome here, since the treatment removes nearly one third of the original content of sulfur, as shown in Table 3. In this case, the concentrate remains with a sulfur content of 24%, which allows a high enrichment of oxygen. Note that although the process suggested here removes an important part of the
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sulfur, this is partly counterbalanced by the fact that the concentrate would enter the flash furnace at temperatures of nearly 900°C.
Dust The most serious problem of the flash furnace is the generation of dust, which may represent nearly 10% of the original mass of the concentrate introduced into the furnace [6,7]. The dust must be recycled to avoid an unacceptable loss of copper. But given that the levels of impurities in the dust are much higher than in the original concentrate, the direct recycling of the dust is not possible for original concentrates with high levels of impurities [6]. Since under the treatment suggested here the impurities are removed prior to smelting of the concentrate, the dust can be directly recycled. The process also has an important effect on the combustion and the fusion of the particles in the concentrate, and may also affect the amount of dust. The kinetics of the reaction of the concentrate clearly differ from those of the currently used process, because the particles enter the flash furnace after losing the labile sulfur, presenting multiple cracks due to heating and desulfurization, and also because the concentrate enters at temperatures of nearly 900°C. The physical properties of the particles are very different in the currently used process of smelting, where the concentrate enters at a relatively low temperature and where the content of labile sulfur is still intact. The outburst of the particles in the concentrate at ignition [8,9] may constitute an important factor in the generation of dust. The currently used process generates multiple fragments that escape the high-temperature region of the furnace, thus preventing their complete fusion. Since the proposed process for removing impurities prior to fusion reduces the phenomenon of outburst and fragmentation of the particles, one may naturally expect a lower generation of dust.
Gases Usually, the sulfur dioxide generated during smelting and conversion is transformed into sulfuric acid, a process which requires a complex and elaborate stage of gas cleaning in order to eliminate impurities such as arsenic, antimony, and others [10]. The elimination of arsenic is particularly delicate, since this element not only affects the quality of the acid, but also give rise to problems of corrosion of some essential components of the acid plant. The removal of impurities before smelting simplifies the cleaning of the gases and reduces the loss by corrosion and wear in the acid plant; in addition, it generates high-quality sulfuric acid.
Slag The fact that the slag generated in the modern process of smelting contains a great amount of copper means that the slag must be treated if copper recovery is to be maximized. The present process for solid state copper refining will generate a slag which is free of impurities, thus simplifying its treatment. As a result of the treatment for the removal of impurities, part of the copper contained in the particles turn into whiskers, as can be easily shown with the help of an electronic microscope. This effect may contribute positively to diminishing the copper content in the slag.
Conversion The main purpose of conversion is to oxidize the iron and sulfur in the matte produced during the smelting. The sulfur leaves the converter as sulfur dioxide, while the iron is slagged with the help of fluxes; this yields blister copper with a purity of about 99%. The same products as in the smelting are generated in the conversion, but with the blister copper playing the role of the matte. Thus the process under consideration presents in the conversion stage similar advantages to those already mentioned in the case of the smelting stage.
Process for refiningcopper in solid state
411
In the currently used process, the impurities in the matte are partially volatilized and slagged during conversion. Given that in the process proposed here the matte is already free of impurities such as arsenic, antimony, bismuth, and others, this issue becomes irrelevant.
Fire refining The advantages of the process of removing impurities before smelting become particularly significant when the fire-refining stage is considered. The main objective of fire refining is to eliminate impurities such as arsenic, antimony, bismuth, and others, by means of oxidation and slagging. With the treatment described here, the blister is free of impurities, making this stage unnecessary, or at least reducing it to a process of eliminating the residual sulfur and iron present in the blister. The traditional fire-refining process involves several sequential operations involving long-lasting treatments. In particular, it needs special and costly fuels and fluxes [11,12]. The elimination of bismuth is practically impossible through fire refining, given that there is no appropriate flux for this impurity [12]. The bismuth must therefore be eliminated in the stage of electrorefining. Another problem of fire refining is that the copper is severely oxidized, which demands a later reduction step requiring special fuels. The process of oxidation and slagging also produces severe wear and corrosion in the refractory line in the furnace, as well as a loss of copper and valuable elements in the slag. All of these drawbacks mentioned above disappear in the solid state process for copper refining proposed here.
Electrorefining The pretreatment of the concentrate under discussion also simplifies the stage of electrorefining, thus reducing costs. Since in this case the anode is free of impurities such as arsenic, antimony, and bismuth, there is no possibility of formation of compounds due to the reaction of the arsenic with antimony and bismuth, which have been considered as a major source of cathode contamination [13]. Moreover, the lack of impurities in the electrolyte makes the complex process of electrolytic purification almost completely unnecessary during the electrorefining stage. In particular, the problems associated with the generation of the dangerous arsine disappear. When the impurities are removed prior to smelting, the amount of floating anode slimes decreases significantly, thus improving the physical quality of the cathodic deposit. In particular, the problem of the formation of dendrite associated with the presence of antimony disappears. [14] Furthermore, when the electrolyte is contaminated with floating anode slimes, the current density becomes a rather critical operational parameter, because a high current density leads to a dragging effect of impurities, with the consequential cathode contamination. This effect is very weak if the electrolyte has low levels of impurities. Thus, if the concentrate is subjected to the present treatment, it seems possible to increment the current density beyond the normal values currently in use.
Anode slimes In electrorefining, the anode slimes represent an important byproduct, because they contain valuable elements such as silver, gold, platinum, palladium, selenium, and tellurium. One important step for the recovery of these elements consists in the oxidation and slagging of impurities such as arsenic, antimony, bismuth, and lead. This procedure requires very special fuels and fluxes, and is such that part of the valuable elements are lost in the slag. These drawbacks disappear when the impurities are removed according to the present treatment. Also, this treatment simplifies the steps involved in the recovery of the different valuable elements. The removal of impurities prior to fusion also reduces in a significant way the amount of anode slimes, [15] a fact which also contributes to a simplification in the recovery of the valuable elements. In connection with this, it should be noted that according to some authors, [16] the amount of anode slimes is basically determined by the amount of lead in the anode. The pretreatment considered here completely removes the
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D. Villarroel
lead from the concentrate, so that a significant reduction in the anode slime may be expected.
Recovery of other elements The present process allows the recovery of several other elements which may not be as valuable as those in the anode slimes, but nevertheless have a commercial value. Among these elements are: antimony, bismuth, lead, zinc, tin, cadmium, and mercury, all of which are currently discarded. The recovery of these elements may be of economic interest, especially in the case of concentrates with high contents of them. The process also removes significant proportions of selenium, tellurium, germanium, rhenium, and gallium, but the effectiveness of the recovery of these elements is still unknown. The recovery of elemental sulfur deserves special consideration. As is known, the sulfur dioxide generated in the smelting and conversion stages must be processed in order to avoid the environmental pollution that would result if it were freed directly to the atmosphere. In the smelter, this gas is transformed in sulfuric acid, which has a wide variety of applications into industrial processes. However, the production of sulfuric acid presents some problems when the smelter is far from the acid market, or when the potential for sulfuric acid production is far beyond the local needs for acid, as is the case in Chile. The problems are related to the high storage and transport costs associated with sulfuric acid, costs that sometimes make difficult, if not impossible, to reduce the environmental contamination by means of sulfuric acid generation. In Chile, the world's largest producer of copper, this problem has not yet become critical, because an important part of the concentrate is exported without further processing, and also because some of the sulfur dioxide generated is not transformed into sulfuric acid, but rather released into the atmosphere. Almost all the sulfuric acid that is produced is being used in hydrometallurgic processes for recovering copper from oxides minerals coming from mines that are not very far from the smelters. However, this picture will change dramatically in the future, since the oxides mines will be exhausted in the course of a few decades. Moreover, the environmental regulations will become stricter, and the release of sulfur dioxide into the atmosphere will no longer be possible. The process discussed here removes nearly one third of the sulfur content of the concentrate as elemental sulfur, which means that sulfuric acid production will also be reduced to one third. Elemental sulfur does not present the disadvantages of sulfuric acid, so, it would be ideal if most of the sulfur in the concentrate could be recovered as elemental sulfur. The present treatment paves the way for this possibility. In fact, part of the concentrate which is free of impurities can be treated with hydrogen at high temperatures to produce hydrogen sulfide, which can be combined with sulfur dioxide to obtain elemental sulfur according to a wellknown technology. It is known that when the concentrate is attacked with hydrogen, the iron in the residue can then be leached almost completely and selectively with respect to the copper by means of hydrochloric acid [17]. In order to avoid an excessive production of sulfuric acid in Chile in the future, it will likely be necessary to combine the current pyrometallurgic process with an hydrometallurgic process such as the one described here.
Pollution control The pyrometallurgy of copper leads to serious environmental problems due to the production of toxic substances such as arsenic, cadmium, mercury, and lead. The arsenic is removed as arsenic trioxide, which is poisonous and difficult to discard because of its high solvability in water. This problem becomes more complex if the concentrate has high levels of arsenic, as happens, for example, in Chuquicamata, where the discarding of arsenic residues of the acid plant is somewhat complicate. The arsenic contaminates the slag, the dust, and the residual gases as well. The process presented here completely removes the arsenic from the concentrate in a way which is innocuous to the environment, since it is removed mainly as elemental arsenic or as sulfides, which are practically insoluble in water.
Process for refiningcopperin solid state
413
As for cadmium, mercury, and lead, since they are completely removed prior to the fusion of the concentrate, the current problem of heavy metals pollution disappears. The discarding of the slag therefore causes no harm to the environment. The solid-state process for copper refining discussed herein can be implemented with the different smelting and converting processes currently in use. It is particularly appropriate, however, to the flash smelting followed by the Kennecott-Outokumpu Flash Converting Process, since it is then possible to take full advantage of a high enrichment of oxygen. Furthermore, this technology allows a rigorous control of environmental pollution, as can be seen in the modern Utah smelter where sulfur capture is as high as a 99.9%, making it the cleanest smelter in the world.
CONCLUSIONS Thermal decomposition of copper concentrates for one hour at a temperature of 950°C in a chamber with a vacuum of 10-2 mm of Hg allows the complete removal of harmful impurities such as arsenic, antimony, bismuth, lead, and zinc. This treatment may lead to the refining of copper in solid state prior to smelting at an industrial scale, a process which would introduce great simplifications in the different stages of copper pyrometallurgy. Besides, if the gasified substances are forced through a tunnel with a decreasing temperature profile, it will be possible to recover over one third of the sulfur content of the concentrate as elemental sulfur of high purity. Furthermore, the treatment allows the recovery of elements such as zinc, lead, antimony, cadmium, and mercury. The design of a special industrial furnace that allows the treatment of the concentrate according to this process will be presented in a forthcoming paper. This furnace processes the concentrate in a continuous way and the concentrate is heated with the help of the thermal energy contained in the fusion and conversion gasses.
ACKNOWLEDGMENTS The author wishes to thank the Comisi6n Nacional de Investigaci6n Cicntffica y Tecnol6gica de Chile (CONICYT) for the grant FONDECYT N ° 1960179.
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