The Hydrothermal Precipitation of Arsenical Solids in the Ca-Fe-As04-S04 System at Elevated Temperatures by Peter Michael Swash B.Sc (Hons), M.Sc A thesis submitted for the degree of Doctor of Philosophy of the University of London and for the Diploma of Imperial College of Science and Technology and Medicine Department of Earth Resources Engineering, Royal School of Mines, Imperial College of Science, Technology and Medicine, University of London. M September 1996 Abstract 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- A s0 4 systems at low (<1) and at elevated pHs (>3). Precipitation of solids in the Fe-AsC>4-SC)4 system at pH<l was carried out in the temperature range 150 - 225°C using solutions of 0.5M total ionic concentration. The phases which were identified were hematite, scorodite, arsenical ferrihydrite, and two unknown compounds which have been designated the Type-1 and Type-2 compounds. The scorodite and Type-2 compounds were found to have very low solubilities (<5mg As/L by the US EPA criterion) and such materials may be considered suitable for disposal as wastes. Scorodite (FeAs04.2H20) is usually light to apple green in colour and is found as crystals up to 20pm. It is formed at temperature above 150°C, from solutions with Fe:As ratios of >1:1. 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 C. 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. ' ACKNOWLEDGEMENTS I would like to thank Dr A.J. Monhemius for initiating the arsenic research programme and for his help and supervision during this work. My international assortment of friends in the Hydrometallurgy Group are also thanked: Lisa and John B., Mike, Vic, Steve (N. Ireland), Miguel Diaz (Dominican Republic), Anwar (Pakistan), Nick (Holland), Nina (Norway), Heidi (Norway), Amanda, Luis (Brazil), Themis (Brazil), Ayhan (Turkey), Juliano (Brazil), Prof. R. Burkin, Marcella (Czech Republic), Hero (Cyprus), Concha (Spain), Juan (Spain), Julian (France), Ridge (Zimbabwe), Jan (Belgium), Ike (Nigeria), Gustavo (Colombia). Thanks also go to Carl, Guo and Yelin Tao (China), Irina (Russia), Majid (Iran), M. Ramsay, M. Gill, R. Sweeney, Ying Hi Lee (China) and Lay Poh Tan (Malaysia) for their input and technical contribution. Paulo, and Roberto are thanked for the cups of coffee they made me over the years! Special thanks also go to Bill Hopkin formerly of RTZ, Dr G. Ugarte and Diane Pinkstone who really got the RC59 Arsenic Stability Project going and who carried out a lot of the pioneering characterisation and precipitation procedures. Dr R. Bowell formerly of the British Natural History Museum is also thanked for his enthusiasm and contributions to this study. The author would like to thank the sponsors of the Mineral Industry Research Organisation (MIRO) RC59 Arsenate Stability Programme who not only provided generous financial support to the study but also gave practical direction to the work. MIRO members whose financial assistance is gratefully acknowledged include Barrick Gold Corporation, Billiton, Boliden, Borax, CODELCO, GENMIN, Lurgi, MINORCO, MINTEK, Noranda, Outokumpu, Placer Dome, RTZ and Union Miniere. Thanks also go to my former bosses at ZCCM - Nicola Kostic and Mike Pearl, MINTEK - "Doc" Hiemstra and Johan de Villiers, and at Anglo American Research Labs - Derek Robinson and Werner Glatthaar who inspired me and taught me the art of Applied Mineralogy. And finally to my wife and daughter; Chizuko and Kumiko, and of course to my mum and dad. "Youth is wasted on youth" TABLE OF CONTENTS ABSTRACT ACKNOWLEDGEMENTS TABLE OF CONTENTS.............................................................................................5 LIST OF FIGURES............. ........................................................................................ 8 LIST OF TABLES..............................................................................................................11 Chapter 1 Introduction 1.1 Background...... .................................................... 15 1.2 Objectives of the research....................................................................... ....16 1.3 Arrangement of the research.....................................................................16 1.4 Arrangement of the thesis..................... .................................................... 17 Chapter 2 Literature survey 2.1 Environmental and health aspects.......................................................... 20 2.2 Geochemistry of arsenic.............................................................................22 2.3 Arsenic in metallurgical processing......................................................... 26 2.3.1 Arsenical dusts......................................................... 26 2.3.2 Arsenical slags................................................ 26 2.3.3 Arsenical cements..........................................................................27 2.3.4 Miscellaneous.......................................... 27 2.3.5 Oxidation of arsenic trioxide............... 28 2.4 Low temperature neutralisation......... ..................................................... 28 ( 2.4.1 Lime neutralisation........................................................................28 2.4.2 Stability of the residues............. .................................................. 31 2.5 Hydrothermal precipitation...................................................................... 35 2.5.1 Theory...............................................................................................35 2.5.2 Nucleation and growth........................................................ 36 2.5.3 Hematite...........................................................................................38 2.5.4 Jarosite.............................................................................................. 38 2.5.5 Basic iron sulphate................... 39 2.5.6 Apatite minerals.....................................