Abstract: Membrane replacement can be a major operating cost of a membrane plant. During the development of a novel desalination technique (the SPARRO process) for treating calcium sulphate scaling mine waters the expected life of tubular cellulose acetate membranes operating in the seeded-slurry mode was investigated.During four operating phases of the plant over a five-year period more than 9 000 h of operating data were obtained. Performance data showed that each operating phase was dominated by either membrane fouling or membrane hydrolysis. Membrane fouling was observed to begin near the front-end of the membrane stack and proceed towards the back. Hydrolysis, on the other hand, occurred first in the tail end of the stack and moved backwards towards the Front end modules. Although two detailed membrane autopsies were carried out no definitive statement can be made in respect of the causes of either membrane hydrolysis or membrane fouling. However, suggestions are presented to explain the observed fouling phenomenon in relation to the turbidity of the pretreated feed water and the presence of chlorine. It is proposed that the presence of radioactive isotopes in the mine water which become concentrated in the process contributes to the observed membrane hydrolysis. A membrane life of up to two years is projected for an improved pretreatment arrangement.

Abstract: Where mining proceeds below the water-table-as it has extensively in Britain and elsewhere-water ingress is not only a hindrance during mineral extraction but also a potential liability after abandonment. This is because the cessation of dewatering that commonly follows mine closure leads to a rise in the water-table and associated, often rapid, changes in the chemical regime of the subsurface. Studies over the past two decades have provided insights into the nature and time-scales of these changes and provide a basis for rational planning of mine-water management during and after mine abandonment. The same insights into mine-water chemistry provide hints for the efficient remediation of pollution (typically due to Fe, Mn and Al and, in some cases, Zn, Cd, Pb and other metals). Intensive treatment (by chemical dosing with enhanced sedimentation or alternative processes, such as sulphidization or reverse osmosis) is often necessary only during the first few years following complete flooding of mine voids. Passive treatment (by the use of gravity-flow geochemical reactors and wetlands) may be both more cost-effective and ecologically more responsible in the long term. By the end of 1999 a total of 28 passive systems had been installed at United Kingdom mine sites, including examples of system types currently unique to the United Kingdom. Early performance data for all the systems are summarized and shown to demonstrate the efficacy of passive treatment when appropriately applied.

Abstract: For the past 40 years various approaches have been developed to treat acid waters coming from abandoned as well as operating mining operations. System designs have evolved to meet increasingly stringent discharge permit limits for treated water, as well as to provide solid disposal within economic constraints. A treatment system for remediation of acid mine drainage (AMD) or acid groundwater (AG) requires two main steps: 1. The addition of chemicals to precipitate dissolved metals contained in the waters, and if necessary, to coagulate the precipitated solids ahead of physical separation. 2. Physical separation of the precipitated solids from the water so the water can be lawfully discharged from the site. Choosing the appropriate technology and equipment results in the most efficient plant design, the lowest capital outlay, and minimum operating cost. The goal of these plants is to discharge liquids and solids able to meet standards. The separation of solids from liquids can be achieved through various means, including gravity settling, flotation, mechanical dewatering, filtration and evaporation. As important as the liquid solids separation unit operations are, they are driven by the chemistry of the water to be treated. The content of the dissolved solids will influence the quality and quantity of the solids produced during precipitation. Thus the two aspects must be integrated, with chemistry first, then mechanical engineering. This presentation will provide an overview of a number of liquid solids separation tools currently being used to treat AMD-AG at several sites in the USA. It will also discuss how their operations are impacted by the chemistry of their particular acid water feeds. The tools used include clarifier-thickeners, solids contact clarifiers, dissolved air flotation, polishing filters, membrane filters, and mechanical dewatering devices (belt and filter presses, vacuum filters, and driers).