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ARSENIC IN THE ENVIRONMENT

Arsenic (As) is widely distributed in the environment via water, sediment, soil and, to a lesser extent, air and its release and deposition occurs from both natural and industrial processes. Arsenic’s mobilisation is dependent upon factors such as its occurrence and the type of the rock host, anthropogenic activities, weathering conditions and redox conditions of water and soil. Although living organisms and human beings are exposed to arsenic in daily life to some extent, intake, if any, of this toxic substance can pose a health risk. Increasing legislation has resulted in the need for industry to

ARSENIC - SOURCES, PATHWAYS AND TREATMENT OF MINING AND METALLURGICAL EFFLUENTS

Authors: Roger Bligh & Raul Mollehuara

major primary minerals, arsenic is found in realgar and orpiment in its reduced form, while it can be found in arsenolite oxidized 3. Other minerals bearing arsenic include loellingite, safforlite, niccolite, rammelsbergite, arsenopyrite, cobaltite, enargite, gerdsorfite, glaucodot, and elemental arsenic 4.

Coal minerals also can bear elevated concentrations of arsenic in the form of arsenite and arsenate. Weathering of rocks and sediments, hydrothermal ore deposits, volcanic eruptions, geothermal activities, forest fire, wind-blown dust and sea salt spray can be identified as natural processes distributing arsenic into the environment 4.

remove or reduce arsenic from its process solutions to acceptable levels before discharge to the environment. This article discusses the sources of arsenic, its treatment in mining and metallurgical effluents, and briefly outlines a case study from one particular site.

NATURAL SOURCES

Arsenic is a natural constituent of the continental crust with an average content of 2 – 3 g/t 1, 2. It occurs in more than 200 mineralogical species which approximates to 60% arsenates, 20% sulfides and sulfosalts with the remaining percentage as arsenides, arsenates, oxides, silicates, and arsenic in its native form. Among the

Fig.1. Sources and distribution pathways of Arsenic in the environment 1

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ANTHROPOGENIC SOURCES

Arsenic compounds are widely used as agricultural insecticides, larvicides, herbicides and wood preservatives. Almost 80% of the arsenic produced by humans is released to the environment in the form of impurities in pesticides. Coal combustion also volatizes and releases arsenic as gases and fine-grained aerosols to the atmosphere.

At present, few mining plants are set to remove arsenic contained in ore and typically large amounts of arsenic are released as wastes. Elevated concentrations of arsenic can be present in rocks, metal ores and concentrates, mine tailings, acid mine drainage, coal, peat and oil. The oxidation of arsenopyrite (FeAsS) in mine wastes is the common mechanism that distributes arsenic into the environment.

and metallurgical effluents can be broadly grouped into precipitation or non-precipitation methods.

Precipitation methodsPrecipitation methods are generally the most commonly used on mining and minerals processing sites on sites, depending on specific requirements. These methods are generally cheaper, simpler and less complex from an operational perspective:

Calcium arsenite/arsenate (Fig.2) � Lime is used to precipitate arsenic

present in both As(III) and As(V) states.

� Stable and constant conditions in the process are required 5.

Ferric arsenate (Fig.3) � Ferric iron is used to precipitate

arsenic as basic ferric arsenates or crystalline scorodite.

MINING AND METALLURGICAL EFFLUENT TREATMENTS

Generally, arsenic compounds have no significant market value, but they need to be removed from process streams because of environmental or process specific purposes.

Typically, arsenic from mining and metallurgical effluents is removed, with other metals, directly by lime precipitation with other metals as hydroxides. The selection of the process technology mainly depends upon the effluent quality and containing waste requirements (acceptable form, limits, long term stability), followed by other technical drivers such as the oxidation state of arsenic, input and output concentrations, oxidation requirements, etc.

The various technologies to remove arsenic from effluents in the mining

Figure 2. Typical flowsheet of a calcium arsenite precipitation process 7.

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Figure 3. Typical flowsheet for a ferric arsenate (scorodite) precipitation process 7.

� Produced solid residues more stable than calcium arsenite and arsenate precipitates 5.

Coagulation followed by precipitation

� Coagulant is added, usually salts of Al(III), Fe(III) or binary mixture.

� Arsenic is precipitated typically as arsenate.

� Coagulation may be followed by granular media or membrane filtration in some cases.

Biological precipitation � A combination of wetland and

humic materials trap arsenic. � Special attention must be given

to arsenic binding, as it can be released to the environment after organic matter decay or organism death 6.

Non-precipitation methodsThese methods of arsenic removal are presently uncommon on mine sites due to high expense and complexity:

� Adsorption – arsenic is adsorbed onto materials such as phyllosilicates, silica, hydrous

The roaster complex, built by Outotec 7, has an integrated effluent plant which removes the arsenic content from the process water. Copper concentrate containing arsenic is processed in the roaster to produce a calcine with a low arsenic content and high copper concentration.

This is processed to obtain a high-purity cathode in the smelter and refinery.

The effluent is treated using calcium arsenite precipitation (Fig 2) and includes Outotec’s process concept and proprietary design for High Density Sludge (HDS) neutralisation in which sludge is recycled to improve lime consumption and treatment efficiency. The aim is to recover the maximum water possible for reuse in the concentrator plant.

Features of the effluent plant include; tailored OKTOP® reactors for precipitation, Outotec’s high rate thickener, efficient process control with analyser, sludge recycling in the process, optimised flocculant

oxides aluminium and iron. � Ion exchange – As(V) removed,

effluent must be of low sulphate (<50 mg/L) and low nitrate (<5mg/L).

� Membrane separation – reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF).

The two flowsheets (Fig 2 & 3), outline common precipitation methods in more detail. Outotec’s technology and equipment can be configured in multiple arrangements to meet specific process requirements.

MINISTRO HALES, CODELCO - NEAR CALAMA, CHILE

Codelco’s Ministro Hales (MH), originally known as Mansa Mina, has estimated total resources of 1.3 billion tonnes (at an average grade of 0.96 copper); 282 million tonnes of these ore reserves will be mined using open pit methods; by the end of 2013, the operation will have an output equivalent to an average 170,000 metric tonnes per annum of fine copper and 300 tonnes of silver.

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preparation and dosing system. Additionally, the moisture in the residue can be lowered further by using an Outotec Larox automatic pressure filter (PF). In this way, by enhancing the solid liquid separation process more water is recycled back to the concentrator.

CONCLUSION

In summary, arsenic occurs in various forms both as the result of natural and anthropogenic activity. Arsenic is being introduced to metallurgical processes at higher ratios as low grade and complex ores are being extracted and

Figure 4. Ministro Hales, Codelco - Effluent treatment plant

About the authors...Roger Bligh joined Outotec in 1989 and is currently the Head of Energy, Light Metals and Environmental Solutions in the South East Asia Pacific region. Roger has 30 years experience with process plant design, project management, commissioning and operation in alumina refineries, alumina calciners, in wet metallurgical processes including industrial waste water and in other thermal treatment processes. He holds a B E (Chem) from the University of Queensland, Process Metallurgy.

Raul Mollehuara is a consultant Senior Process Engineer for Outotec. He has 18 years experience in mineral and water process related roles within the mining industry, specialising in mine water projects.

once introduced in the process interferes with metal extraction and deteriorates the product quality. Therefore, most of the arsenic requires disposal and has become more of concern for sites due to increased environmental risk, disposal problems and the introduction of stringent environmental regulations.

OKTOP® Reactor

The treatment of site effluent is one area where there are many options for arsenic removal, so it is important to carefully review all of these options. Such a review will result not only in the most efficient and secure method of arsenic removal for sites but can also deliver the most cost effective solution.

References

1. Tanaka, T., Distribution of arsenic in the natural environment with an emphasis on rocks and soils. Appl. Organomet. Chem. 1988, 2 (283-295).

2. Cullen, W.; Reimer, K., Arsenic speciation in the environment. Chem. Rev. 1989, 89 (713-764).

3. Nriagu, J., A global assessment of natural sources of atmospheric trace metals. Nature 1989, 338 (47-49).

4. Bhattacharya, P.; Mukherjee, A.; Bundschuh, J.; Zevenhoven, R.; Loeppert, R., Arsenic in Soil and Groundwater Environment. Trace Metals and other Contaminants in the Environment 2007, 9 (127-156).

5. Van_der_Meer, T., Arsenic removal from effluents. Outotec reports 2011.

If you would like more information, click here to contactroger.bligh@outotec.com

6. Rubidge, G., Evaluation and optimisation of selected methods of arsenic removal from industrial effluent. Thesis faculty of Applied Science - Port Elizabeth Technikon 2004.

7. Outotec, Outotec references for precipitation process. Technical presentation - Industrial Water Treatment group. 2011.