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The improvement of the environmental and methods of arsenic control in mining industry.

Introduction

Arsenic is one of the most common potentially toxic trace metals (actually it is a metalloid) in the mining industry. Mine pits and underground working, waste rock piles, tailings and other ponds, and spent leach piles are of particular concern, because these are the areas in which toxic contaminants are most commonly found (Subramanian, 2002).

In gold mining industry the important effect of groundwater contamination is acid generation due to sulfide minerals. For iron recovered from sulfide-bearing ores, acid generation due to the oxidation of sulfides (e.g., pyrite and pyrrhotite) in the ore body, host rock and waste material may be of concern (Ferguson and Gavis, 1972). On the other hand, the regulations for the release of arsenic to the environment are becoming more stringent with increasing public awareness of the toxicity of arsenic and better understanding of its impact on the environment. The development of an integrated approach to effectively control the arsenic is the key to a profitable and sustainable operation, in full regulatory compliance.

Wherever possible, the separation of arsenic minerals in their original forms during mineral beneficiation presents an attractive solution to the arsenic control issue. There are two possible routes that could be followed:

* Separate the arsenic minerals from the rest of the ore (by any suitable methods such as flotation, gravity, etc.) and reject them to the tailing streams, if the arsenic minerals are barren. In that case, whenever possible, the arsenic minerals should be put back into the mine where they came from

* Separate the arsenic minerals from the rest of the ore and treat them using the process guaranteeing the best arsenic control, while the rest can be processed using other technologies.

Arsenic control

Temperature processing

During the roasting of arsenopyrite concentrates for their gold recovery, flue dust containing 60-70% arsenic trioxide is collected through the gas handling system. This flue dust is mostly consumed in the production of chromate copper arsenate (CCA) as wood preservative. The future market of CCA is uncertain in the long term, and therefore alternative methods of controlling the arsenic must be developed. A typical example is the work conducted by DIAND (Diand, 2001) to handle the ~230,000 tones of dust generated by the Giant Yellowknife mine in the NWT. The approaches which have been or are presently evaluated involve encapsulation of the dust into a stable matrix (bitumen, cement, vitrification) or ground freezing (Sollner and Ferron, 2003).

Hydrometallurgical processing

The chemical approach to control arsenic is mainly based on the formation of stable arsenic compounds or arsenical sludge through the hydrometallurgical processing of arsenic- containing materials. The commercially practiced processes for chemical fixation of arsenic includes the precipitation of calcium arsenite or arsenate, arsenic sulfide ([As.sub.2][S.sub.3]) arsenical ferrihydrite and crystalline scorodite. Among them, the scorodite process has been widely accepted as currently the most suitable method for stabilization of arsenic in terms of its high arsenic content, low TCLP arsenic solubility and environmental stability, if disposed appropriately.

The effect of bacteria on weathering of arsnides such as Fe-As has been investigated while none of the reactions have yet been investigated in natural water systems, conditions comparable to those occurring in order systems do occur in water as shown in fig 1 (Kalinowski etal,2000).

It is known that scorodite compounds are favorably formed in acidic solution of Fe (III) and As (V) at high temperature (eg. 160[degrees]C), i.e. under autoclave conditions. This allows it possible for the scorodite process to be conveniently integrated with sulfide oxidation and metal extraction processing in a single autoclave to treat arsenic- bearing ores. The most successful application of the scorodite process is the autoclave processing of arsenopyrite gold ore. Under autoclave conditions, the iron sulfide (FeS, Fe[S.sub.2]) and arsenopyrite (FeAsS) minerals are oxidized to liberate gold which can be efficiently recovered in the subsequent cyanidation stage. Simultaneously the Fe (III) and As (V) ions formed from the oxidative dissolution of sulfide minerals react to precipitate stable crystalline scorodite (FeAs[O.sub.4] x 2[H.sub.2]O) for safe disposal in the cyanidation tailing ponds. The same type of process has been briefly used commercially for cobalt arsenide minerals and is being considered for the treatment of arsenic-bearing copper ore (Subramanian, 2002).

[FIGURE 1 OMITTED]

In actual autoclaving operation, the Fe(III)/As(V) ratio in solution depends not only on the total contents of Fe and As in the autoclave feed, but also on their oxidation and dissolution kinetics, which are in turn affected by their mineralogy and the autoclave conditions applied, such as oxygen pressure, temperature and acidity. Hence, in the preparation of the autoclave feed, both the iron to arsenic ratio and their mineral types should be considered. The iron to arsenic ratio in the autoclave feed and the operating conditions favorable for the production of stable arsenic- bearing solid phases may differ with ore types and need to be determined by experiment.

Arsenic removal and fixation

Low arsenic streams are generated from mining activities throughout the world. Those effluents widely differ in pH, arsenic concentration and speciation, the type and content level of other components, such as iron and other metals, and can be primarily classified into the following types, in terms of their sources.

* Underground and surface water from historical landfill, sediment or contaminated soil in mine sites closed or under current operation

* Liquor discharges from metallurgical processing of a variety of arsenic-containing ores

* Acid mine drainage from a range of mines

* Effluent from tailing ponds in closed or currently operated mines

Lime neutralization to a high pH (~12) was widely practiced for the treatment of effluent to remove arsenic, due to the convenience in its operation. But it is no longer considered acceptable in terms of the high As solubility and the environmental instability of the produced calcium arsenite or arsenate sludge.

The co-precipitation of As (V) with Fe (III) is considered an environmentally more acceptable method for the treatment of arsenic effluent. Arsenic (V) can be readily removed to low level (e.g. <0.1 mg/L) at Fe (III)/As (V) ratio of >3 and pH around 4, and the produced arsenical ferrihydrite sludge can be disposed in an environmentally safe way. Over the past decade, this method has gradually replaced the lime neutralization method and become the primary treatment method for low arsenic stream.

Recently, a new method called mineral-like precipitation has been investigated. This method is based on the formation of a solid solution compound ([Ca.sub.10] [([As.sub.x][p.sub.y][o.sub.4])sub.6] [(OH).sub.2]) at night pH of 12 with the addition to the effluent of phosphoric acid([H.sub.3][Po.sub4]) and lime (CaO); it can achieve low residual arsenic level (<50 ppb), and produce a stable sludge.

Adsorption is a widely recognized technology for the removal of arsenic from water, e.g., the naturally occurring low arsenic-containing underground and surface water. The adsorbents available include alumina, natural or artificial minerals, and ion exchange resin. This method needs adsorption or elution of arsenic to reactivate the adsorbent and the stabilization of desorbed or eluted arsenic in a environmentally stable form.

For the effluent from mine site and metallurgical processing, the adsorption technology is less likely to be an efficient and cost-effective solution due to the relatively high level of arsenic and other species which may compete with arsenic for absorption sites or contaminate the surface of the adsorbent particles.

Biological treatment of arsenic-containing effluents is based on the biological formation of arsenic sulfide, i.e. reduction of arsenic (V) to As (III) by bacteria and formation of insoluble arsenic sulfide complex under anaerobic conditions. It could be practiced in a bioreactor or anaerobic/wetland cell, and appears to have the capability to remove arsenic in the effluent to low levels that exceed or meet the site discharge criteria (Harris, 2000). The biological treatment appears to be an attractive approach for naturally occurring low arsenic-containing underground and surface water or as a polishing step in arsenic effluent treatment. As compared with arsenical ferrihydrite precipitation method, the biotechnogical method does not need oxidation of arsenic (III), but it has other disadvantages. Firstly, it is more sensitive to the chemical composition and temperature of the effluent to be treated and usually expensive biotreatabilty and optimization testing is needed for a given effluent to determine its suitability and efficiency. Secondly, the arsenic removal kinetics in the bioprocess is much slower than that in a chemical process. Thirdly, the environmental stability of the formed insoluble arsenic sulfide sludge and the possible effect of the bacteria in effluent discharged to the environment are still uncertain. These disadvantages might make the bioprocess less competitive for the treatment of large volume effluent from mining industry with relatively high levels of arsenic and other contaminant species.

Maintaining product stability after it has been disposed

Effective management of the disposal of arsenic- bearing materials requires both a full Characterization of the arsenic-bearing materials to dispose of and the understanding of the environment for their disposal. Currently, there are different testing methods to evaluate the stability of arsenic-bearing materials, including the EPA Toxicity Characteristics Leaching Procedure (TCLP), but those testing methods do not give sufficient consideration to the interaction between the arsenic material and the environment in which it is disposed of. Therefore, they are usually helpful in making a preliminary judgment on whether an arsenic-bearing material can be considered for disposal in a given environment, but do not adequately assess its long-term stability. For those materials which can pass the preliminary stability testing, such as arsenic-bearing minerals, crystalline scorodite and arsenical ferrihydrite (Fe(III)/As(V)>4) formed from metallurgical processing, improved methods must be developed to asses the long-term stability. The methods should involve complete characterization of the materials (including chemical, mineralogical analysis and physical properties) and full consideration of the interaction between the arsenic materials and the environment so that favorable disposal conditions can be identified and predictions of behavior can be made.

Given the range of arsenic-bearing materials and the diversity of the potential disposal environments at the mine and mill sites, the long- term stability of the arsenic-bearing materials for disposal and the environmental risks posed by the disposed materials are still likely to be evaluated on the case-by-case basis.

Conclusions

To improve the environmental performance and the sustainability of the mining industry, an integrated approach is necessary to insure the arsenic control at each phase of the mine life, in a proper and responsible way. Maximize arsenic rejection in its original minerals at the mine and mill, through careful evaluation of the arsenic minerals and optimization of mineral processing, is an attractive option whenever possible. The environmental and economic impact of this option is often not fully appreciated and needs more attention. Currently, the primary approach for arsenic control is to stabilize arsenic as a stable product through metallurgical processing. The most environmentally acceptable methods includes crystalline scorodite formation for the treatment of arsenic-rich materials and arsenical ferrihydrite precipitation with Fe (III)/As (V)>3 for the treatment of low-arsenic stream. However, the long-term stability of the both materials in a given environment depends on the method and conditions applied for the disposal. A comprehensive protocol has to be designed for the full understanding of the interaction between the materials and environmental surroundings so that an effective and durable management of arsenic disposal can be achieved.

References

[1] Subramanian, V, Environmental Hazards in South Asia, Capital publishing company, Kolkata, New Delhi, 259-263 (2002).

[2] Fergusen, J.F and Gavis, J, A Review of the Arsenic Cycle in natural waters. Wat.Res, 6, 1259-1274, (1972).

[3] Diand, C, Management of Arsenic trioxide bearing dust at Giant Mine, Yellownife, Northwest Territories, (2001).

[4] Sollner, C. J. Ferron, R. Massimi, Soil stabilization of soluble mine waste, to be presented at the 6th International Conference 2003 Acid Rock Drainage, Australia, (2003).

[5] Kalinowski, B. E , Routh, J, Bhattacharya, P, Jacks, G, Mazumdar, L, Ahmed, K.M, Biochemical aspects of arsenic in Tala region, Satkhira district southwest Bangladesh, 31st International Geological Congress, Rio de Janeiro, Brazil, (2000).

[6] Harris, G.B, The removal and stabilization of arsenic from aqueous process solutions: Past, Present and future, Edited by C. Young, SME, Littleton, 3-20 (2000).

Seyed Morteza Moosavi Rad (1) and Zahra Izadkhah (2)

(1) Department of Geology, University of Mysore, (India) Email: moosavi200@yahoo.com

(2) Department of Botany, University of Mysore, Manasagangotri, (India)
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Author:Rad, Seyed Morteza Moosavi; Izadkhah, Zahra
Publication:International Journal of Applied Engineering Research
Article Type:Report
Geographic Code:1USA
Date:May 1, 2009
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