Effluent Treatment - Arsenic
June 26, 2003

Introduction Precipitation as Ferric Arsenate Precipitation Using Calcium Precipitation as a Sulphide References

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Introduction

Arsenic is present in metallurgical off-gases as As₂O₃ if the conditions are oxidizing which is generally the case.  As the gas passes through the scrubbing sections of the gas cleaning system the arsenic is removed from the gas and enters weak acid circulating system.  As₂O₃ dissolves slowly in weak acid to give HAsO₂ (arsenious acid).  HAsO4 (arsenic acid) is not present since it is oxidized to HAsO₂ by the ever present SO₂ which is dissolved in the weak acid stream.

Arsenic may also be present as As₄, As₂S₃ and As₂O₃ (undissolved).  All these forms of arsenic can be easily filtered and removed from the effluent stream.

The chart on the right shows the solubility of arsenic in sulphuric acid solutions as a function of temperature.   Arsenic is very soluble at low acid concentrations and high temperatures.  In order to remove arsenic from the effluent the form the arsenic is in must be changed.   Arsenic can be removed from weak acid effluent solutions in many forms.  Some forms are very insoluble and have excellent long-term environmental stability while other forms can be easily re-dissolved.

Arsenic can occur in two different oxidation states, as As³⁺ and As⁵⁺.  In order to ensure the maximum long-term stability of the precipitated arsenic compounds, the arsenic must be in the As⁵⁺ state.  A strong oxidant such as hydrogen peroxide can easily oxidize any As³⁺ to As⁵⁺.

Precipitation as Ferric Arsenate

Ferric (Fe(III)) arsenate (FeAsO₄·2H₂O) is a very stable arsenic compound.  It is soluble, but the solubility decreases rapidly as the ratio of iron to arsenic increases.  The solubility of ferric arsenate can be reduce several orders of magnitude if four to five times the stoichiometric amount of iron is present in a pH range of 3.0 to 7.0.  The chart on the right shows that at pH 4, the arsenic remaining in solution is about 8 mg/L at a Fe:As molar ratio of 1.5.   Increasing the Fe:As ratio to 2 results in a significant reduction of arsenic in solution to about 0.15 mg/L.  Further reduction of arsenic to about 0.02 mg/L can be obtained with a Fe:As ratio of 5.0.  The data represented in the chart is experimental and represents near equilibrium conditions.  Actual arsenic levels in operating systems will not achieve the same level of arsenic removal.  We can see that at all Fe:As levels the minimum arsenic level in solution is obtained at about pH 4.

The reaction to convert soluble arsenic acid to ferric arsenate occurs according to the following reaction:

Fe₂(SO₄)₃ + 2H₃AsO₄ => 2FeAsO₄ + 3H₂SO₄

Laboratory precipitation test at a Fe:As ratio of 4, at 33°C and a retention time of 2 hours resulted in arsenic concentrations of 0.2 mg/L at pH 3.5 to 5.0.   Higher precipitation temperatures has a negative effect on the solubility of arsenic.  Therefore, it is desirable to precipitate arsenic at lower process temperatures.  A chart showing results from this laboratory experiment are shown below.

In most cases the effluent stream to be treated will contain other base metals such as Cu, Cd, Ni, Co and Zn.  Experimental results indicate that the presence of excess ferric iron at the time of precipitation lead to increased arsenate stability.  Excess ferric iron also assisted in maintaining the stability of metal arsenates when carbon dioxide (CO₂) is present as is the case when effluent is stored in a pond or tailings area.

A typical block flow diagram of a arsenic precipitation process using ferric iron is illustrated below.

Block-diagram for the Fixation of As2O3 as Ferric Arsenate (FeAsO4)

Precipitation using Calcium

Precipitation of arsenic by calcium proceeds according to the following reactions:

2Ca²⁺ + 2AsO₂^- + 2OH^- = Ca(AsO₂)₂ Ca(OH)₂

4Ca²⁺ + 2AsO₄^3- + 2OH^- = Ca₃(AsO₄)₂ Ca(OH)₂

It has been documented in the literature that both calcium arsenite (Ca(AsO₂)₂) and calcium arsenate (Ca₃(AsO₄)₂) are not appropriate forms due to their lack of stability.  It was found that atmospheric carbon dioxide causes calcium arsenite and arsenate to decompose to calcium carbonate in alkaline solutions.  Barium arsenate, magnesium arsenate and strontium arsenate are affected by carbon dioxide in a similar manner.

As well, calcium arsenate is relatively stable at pH’s of 8 to 11 but readily decomposes at lower pH values.  If calcium arsenate is stored in a landfill slightly acidic rainfall or groundwater will result in calcium arsenate dissolving into solution.

Precipitation as a Sulphide

Precipitation of arsenic as arsenous trisulphide (As₂S₃) in an acidic solution can be done by the addition of a sulphide such as Na₂S, NaHS, or H₂S.  The resulting compound is stable and relatively insoluble which is contrary to most other metal sulphides which decompose to metal salts and H₂S in an acidic solution.  However, in alkaline solutions, arsenous trisulphide is fairly soluble.  Therefore, removal of the precipitated arsenous trisulphide by filtration must be done while the solution is still acidic.  Once the arsenic is removed, the solution can be neutralized for precipitation and removal of other impurities.  Disposal of arsenous trisulphide by landfilling or tailings area is a problem since there is the potential that it will go back into solution.

Arsenic may also precipitate as FeAsS which appears to have low solubilities over a broad range pH range.

References

Stefanakis, M. and Kontopoulos, A., Production of Environmentally Acceptable Arsenites-Arsenates from Solid Arsenic Trioxide, Arsenic Metallurgy Fundamentals and Applications, 1988 TMS Annual Meeting and Exhibition, Pheonix, Arizona, January 25-28, 1988.

Robins, R.G., Huang, J.C.Y., Nishimura, T. and Khoe, G.H., The Adsorption of Arsenate Ion By Ferric Hydroxide, Arsenic Metallurgy Fundamentals and Applications, 1988 TMS Annual Meeting and Exhibition, Pheonix, Arizona, January 25-28, 1988.

Papassiopi, N., Stefanakis, M., and Kontopoulos, A., Removal of Asenic from Solution by Precipitation as Ferric Arsenate, Arsenic Metallurgy Fundamentals and Applications, 1988 TMS Annual Meeting and Exhibition, Pheonix, Arizona, January 25-28, 1988.