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Title: The hydrothermal precipitation of arsenical solids in the Ca-Fe-AsO4-SO4 system at elevated temperatures
Author: Swash, Peter Michael
ISNI:       0000 0001 3493 6497
Awarding Body: University of London
Current Institution: Imperial College London
Date of Award: 1996
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Hydrothermal precipitation experiments were carried out in sealed test tubes to investigate the characteristics of solids precipitated from Ca-Fe-As04-S04 solutions at temperatures up to 225°C. The solids precipitated from solutions were examined by studying the individual Fe-AsC > 4, Ca-AsCTj and Fe-Ca- As04 systems at low (<1) and at elevated pHs (>3). Precipitation of solids in the Fe-AsC > 4-SC)4 system at pH1:1. The family of Type-2 compounds show close similarities and are usually produced from starting solutions with Fe:As >1:1 at temperatures >175°C. These compounds are light brown in colour and are composed of crystals up to 50(j.m, they easily accommodate sulphate into their lattice and have compositions which approximate to:- Fe3(As04)2(0H)x(S04)y (where x and y = 0 to 1). The formation of the Type-1 compound is common in the temperature range 150 - 225°C from solutions with ratios of Fe:As of <1:1. Type-1 compounds often have high solubilities and are found as white, fine grained solids (<2|im), usually having an Fe:As ratio of 0.7 to 0.9 with a stoichiometry approximating to:- Fe2(HAs04)3.zH20 (where z = 0 to 4). The overall results of the hydrothermal precipitation experiments, using simulated hydrometallurgical process solutions in test tube experiments and in a 4L autoclave, have shown that it is possible to precipitate >95% of the contained arsenic from solution. High temperatures are preferable (>175°C) as this promotes the growth of crystalline arsenical compounds. For optimum conditions the Fe:As ratio in the solution must be around 1:1 to satisfy the Fe:As requirements of the precipitated compound (scorodite or the Type-2 compound, 1:1 or ~1.5:1 respectively) and most of the arsenic is removed within 30 minutes. When higher Fe:As ratios are used, the rate at which the compounds are precipitated is reduced. In the Fe-AsC > 4 system at pH5 using an Fe:As ratio of ~1:1, a crystalline compound; designated Type-3 (approximating to Fe2(FIAs04)x(As04)y), is precipitated from arsenical-ferrihydrite sludges at temperatures above 125°C. Only at elevated pHs (>3) do calcium arsenate compounds begin to precipitate, and during neutralisation of iron-rich solutions arsenic preferentially combines with iron rather than calcium. In sulphate-rich solutions calcium will precipitate as gypsum rather than combine with arsenate to produce calcium arsenate compounds. From precipitation work carried out in the calcium-arsenate system, it was found that at temperatures below 100°C, the solids are partly hydrated and are usually composed of one of the following:- pharmacolite, haidingerite, or guerinite (CaHAsC > 4.2H20, CaHAs04.H20 and CasH2(As04)4.9H20, respectively). In the temperature range 100 - 200°C at pH's<8, the solids contain only constitutional water and are composed of a weilite-type compound (CaHAsC > 4). Solids precipitated at pH's above 8 and at temperatures above 100°C precipitated johnbaumite (Cas(As04)30H). Above 200°C, the predominant solid approaches a Ca3(AsC > 4)2 composition which contains only minor amounts of constitutional water. This work has examined a wide range of solution compositions and it has been found that all the hydrothermally prepared calcium arsenate-type compounds have very high apparent solubilities (>1000mg/L). Through experimental observation and consideration of some theoretical concepts a preliminary assessment of the long term behaviour of crystalline arsenic bearing metallurgical wastes in the environment has been made. Through empirical solubility testing and comparison with natural analogues it can be predicted that crystalline scorodite should have a low solubility for prolonged periods of time. Since scorodite is commonly found in many weathering zones and in most climatic regions of the world it is considered to be the most stable arsenate compound formed in nature and may be suitable for arsenic disposal purposes.
Supervisor: Monhemius, A. J. Sponsor: Mineral Industry Research Organisation (MIRO)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Heavy metal pollution; Metallurgical wastes