Introduction
The last few decades have been a serious environmental challenge for the metallurgical industry, especially for copper smelters, due to pressures from public opinion and the numerous environmental regulations imposed. Regulation of sulfur dioxide emissions has forced smelters to treat their gases directed to sulfuric acid plants to remove dust particles and any volatilized arsenic. During the smelting-converting process some arsenic also reports in the blister copper. It must be removed from the refined product otherwise it will deleteriously influence its properties. Because of its toxicity, arsenic has received great attention in the metallurgical industry. Environmental regulations limit the arsenic emitted into the atmosphere from each smelter. As a consequence there is a significant increase in the amount of arsenic residues, which will have to be managed in a safe way, whether for its recovery or disposal.
Arsenic Recovery
Arsenic is recovered, as a by-product of processing certain complex ores that are mined mainly for copper, cobalt, gold, and its supply is dependent on the demand for these metals. In the past years, the market for arsenic compounds such as arsenic trioxide and arsenic metal has not grown, thus creating a surplus of arsenic production. In 1998, U.S. Geological Survey estimated the arsenic trioxide production at about 41,000 metric tons (Reese, Jr., 1999). Another arsenic compound is commercial-grade (99% pure) arsenic metal, which is produced through the reduction of arsenic trioxide. It is believed that China accounted for nearly all the world's production of commercial-grade arsenic metal. High-purity arsenic, 99.9999% or greater, is also produced for use in the semiconductor industry (Reese, Jr., 1998). Table I summarizes the principal world refiners of arsenic by country (Valenzuela, 2000).
Table 1
The United States is probably the major consumer of arsenic (between 20,000 and 40,000 tons annually), which is dependent upon foreign suppliers. The end-use distribution of arsenic in recent years in the United States has been about 87% in wood preservatives, 5% in agricultural chemicals, 3% in glass manufacturing, 3% as metallic arsenic in nonferrous alloys, and 2% in other uses (Resse, Jr., 1998, 1999). Recently much attention is paid to the properties of gallium arsenide (GaAs), which is a semiconductor material similar to silicon with certain unique properties, such as high frequency operation for microwave circuits and optical properties for fiber optic applications (Kramer, 1998).
As it was shown in Table I, there are several countries recovering arsenic from copper and cobalt concentrates or arsenic-bearing copper smelter dusts. However, information about most of these operations is not available in the literature (e.g., China, Peru, Bolivia, Morocco, France, Georgia, etc.)
Roasting has been applied since 1981 at El Indio mine in Chile for As2O3 recovery and thus making its concentrate more acceptable as smelter feed (Figure 1). In Namibia, the copper concentrate produced at Tsumeb mine, having 6-7% As, is also treated prior to the smelter in an arsenic plant (2000 t/y of As2O3, 99%) consisting of four roasters with condensing chambers and a common baghouse. The Tsumeb Copper Company however, was liquidated in 1998 (Coakley, 1999). In Philippines, the copper concentrate produced at Lepanto plant is treated at the PASAR smelter by means of fluidized bed roasting, producing 2,000 t/y of white arsenic and a calcine having 1% As. However, the Lepanto Mining Company closed its plant in 1997 due to the exhaustion of its ore (Lyday, 1998). In the United States, most arsenic was recovered from copper concentrates for sale between 1910 and 1985, after which all domestic production of arsenic ceased when ASARCO, the sole remaining producer, closed its copper smelter and associated arsenic plant in Tacoma. In this plant, the copper concentrate, having 4% As, was roasted prior to smelting and arsenic was then captured using electrostatic precipitators and condensing chambers. The final product was a crude As2O3 of minimum 95% purity (Edelstein, 1994).
Figure 1
Japan's copper smelters have recovered arsenic from smelter dusts. In the 1980's, the
Furukawa Company produced 300 t/y of As2O3 at its Ashio copper smelter. However, Furukawa
permanently closed its smelter in 1988. The Sumitomo Metal Mining Company also produced
As2O3 at its Toyo copper smelter. The arsenic plant had the capacity to produce 720 t/year
of 99.9% pure As2O3 (Figure2).
Figure 2 y Figure 3
The production cost of Sumitomo's process is high because 2.6-3 tons of copper oxides are
required to produce one ton of arsenic trioxide. Arsenic recovery is very low, about 55%
because of recycling. Also, the operating conditions are complicated because of its high
temperature, high slurry and sulfuric acid concentrations, and their long reaction periods:
cementation time, 4 h; oxidation time, 10 h. To simplify the Sumitomo's process, Beijing
General Research Institute of Mining and Metallurgy has developed an aqueous oxidation
process (Figure 3) using oxygen at 150° C and 550 kPa to leach arsenous sulfide residues
from Guixi smelter. Experimental results show satisfactory arsenic and copper extraction,
97.7 and 97.4%, respectively (Kaixi, et al., 1998). The Sumitomo Metal Mining Company also
developed in 1984 a purification technique to produce 18 t/year of high purity arsenic
metal from As2O3 to fulfil the demand of the semiconductor field (Toyabe, et. al., 1988).
Arsenic Disposal
The disposal of arsenic has been accomplished in practice by the formation of metal arsenates and metal arsenites, e.g., of Ca2+, Cu2+ and Fe2+ because of their low solubility. Arsenic has been precipitated by adding lime to the solution, obtaining a calcium arsenate compound. However, the stability of this compound has been questioned because under the influence of atmospheric CO2, calcium arsenate decomposes to calcium carbonate and liberates arsenic oxide in the solution (Nishimura, et al., 1988). For a long-term stability, the formation of more stable forms has been studied (i.e., Cu, Zn, Co, Ba, Hg, etc) but at present the industry has adopted the ferric arsenate method known in the nature as scorodite. Table II summarizes the inorganic speciation of arsenic according Eh/pH representation (Sracek, 1998).
Table 2
The work by Krause and Ettel (1987) and Robins (1988) has shown that amorphous ferric arsenates with Fe:As ratio > 4 are very insoluble and stable on a long-term basis and that the presence of CO2 does not increase the solubility of arsenic. Also, Krause and Ettel (1989) have also established that crystalline ferric arsenate is approximately 100 times less soluble than amorphous ferric arsenate reported in the literature. The formation of crystalline ferric arsenate can be realize under two conditions:
- Autoclave conditions at 150°C: Scorodite has been synthesized during jarosite precipitation at 150°C from sulfate solutions by Dutrizac, et al. (1987) obtaining complete arsenic precipitation for solutions containing 5 g/L As(V). Precipitation of scorodite in the Fe-AsO4-SO4 system at pH<1 was carried out in the temperature range 150-225°c ranging fe/as molar ratio between 1:1 to 9:1 and precipitating 90% of arsenic (swash and monhemius, 1998). the formation conditions in all these cases relate to the use of autoclaves, which are considered to being a capital-intensive technology.
- Ambient pressure at 95°C: Immobilization of arsenic in As(III)-rich chloride and
sulfate solution and flue dust as scorodite by ambient aqueous oxidation (95°C) using H2O2
has been tested at McGill University by the Hydrometallurgy Group (Demopoulos et al., 1994,
Droppert, 1996). The approach involves supersaturation control by a neutralization
technique to avoid precipitation of any amorphous arsenic compounds. Precipitation of
crystalline scorodite (Figure 4) is induced by the addition of scorodite seed material into the arsenic-rich solution, slow rate of addition of Fe3+, where it never must be in excess. Thus, » 90% of arsenic precipitates for solutions containing between 5 to 10 g/L As(V).
Figure 4
On the other hand, hydrometallurgical processes have been implemented at the Saganoseki and
Kosaka smelter in Japan to treat the dust generated in their copper smelting operations.
The Saganoseki plant was constructed in 1982 and has been operating satisfactorily to
remove minor elements from copper smelting (Figure 5). 500 t/month of converter dusts can be treated in this dust treatment plant where arsenic is fixed as arsenic sulfide, then polymerized and stored in the smelter (Hino, et al., 1995).
Figure 5 y Figure 6
The hydrometallurgical plant at Kosaka smelter started its operation in 1975 (Mohri and
Yamada, 1976). The basic steps in the process (Figure 6) are leaching of the dust to recover lead sulfate in the residue, copper recovery from the solution, ferric arsenate precipitation as the most stable form for arsenic disposal, cadmium recovery as sponge cadmium and finally zinc recovery as zinc hydroxide from the solution.
Another process is also described by Godbehere, et al. (1995) to treat an arsenic-bearing
weak acid effluent and precipitator dust from the Horne Copper Smelter using iron and zinc
derived from acid mine drainage and precipitate arsenic as iron and zinc arsenite-arsenates.
Reference should be made to the process at Kennecott smelter for the treatment of smelter
dusts (Figure 7) where arsenic is precipitated as As2S3 and FeAsO4 (Gabb, et al., 1995).
Figure 7
In Chile, El Teniente Division developed a process to treat the dusts collected by the
electrostatic precipitators of the gas handling system at the Caletones smelter (Figure 8).
Chuquicamata Division also treats smelter dusts in a hydrometallurgical plant (Figure 9) recovering copper and disposing of the arsenic in the residues of the leaching operations where it will precipitate in situ as ferric arsenate (Farias, et al., 1996).
Figure 8 y Figure 9
Conclusions
This paper has provided a brief overview of different processes for recovery or disposal of arsenic from copper concentrates prior to smelting or from arsenic-containing copper smelter dusts.
Arsenic trioxide is the main commercial arsenic compound recovered, which is principally used for the production of wood preservatives. In the past years, the market for arsenical compounds has not grown due to the restrictions of the utilization of arsenic bearing products by environmental regulation. However, the growing use of cellular telephone technology has resulted to be a boom for gallium arsenide (GaAs) demand, mainly in the United States where in 1997, 22.4 tons of gallium in the form of GaAs were consumed.
On the other hand, when arsenic is not recovered it is removed from arsenic-bearing residues and stabilized in the form of a solid compound prior to disposal. The forms commonly used are calcium arsenate, arsenic sulfide or ferric arsenate. However, It is interesting to note that there have not been significant and innovative improvements in the methods for removing arsenic from process and effluents solutions in the last years. However, it will be interesting to test on industrial effluents the scorodite precipitation technique developed by the Hydrometallurgy Group of McGill University, possibly on a continuous pilot plant scale. This method does not require an autoclave; therefore, the capital investment can be appreciably reduced.
Acknowledgements
The author would like to acknowledge the helpfull comments and corrections made by Dr. Fathi Habashi at the Dept. of Mining and Metallurgy of Laval University.
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