MATERIALS HANDLING
scale on membrane spacer before (a) and after cleaning (b) with Genmine C15 or C17 for 3 hours
Figure 1. Microscope pictures (x4 magnification) showing CaSO4
IN MINING T
Controlling CaSO4 scale in AMD Minewaters. By Gregory Gibson, Max Fazel and Stephen P. Chesters
he mining industry is the fourth largest consumer of water worldwide. Te extraction of precious metals such as gold
(33,000 litres/ounce), copper (18,000 litres/kg) and aluminium (8,000 litres/
kg) are some of the biggest contributors. [1]
Te location of these mines is also of
importance as many are located in water- scarce areas. As well as water shortage issues the environmental aspect of mining is one that is increasingly becoming more prominent, with recent mining disasters at Samarco and Brumadinho offering a stark reality of the consequences of poorly managed and maintained sites.[2,3]
During
the extraction procedure various chemical processes occur, with the formation of acid mine drainage (AMD) a particular problem. AMD forms in pyrite-rich mines beginning a chain of reactions that lead to high levels of iron and sulphate.[4]
If
left untreated, this process will continue leading to a reduction in pH and an increase in the total dissolved solids (TDS), with this waste usually stored in
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dams for future treatment. Tis in itself is an environmental problem as there have been instances in the past of uncontrolled spillages, or the collapse of dams such as seen with Samarco and Brumadinho that are catastrophic to the surrounding areas, often resulting in the pollution of nearby rivers and in these instances there have been multiple fatalities. By employing desalination techniques such as reverse osmosis (RO) it is possible to treat AMD waters for reuse onsite; in both a process capacity for further metal extraction or even domestically in drinking water for the workers.
Szyplinska stated that over 90% of mine water can be reused if treatments such as RO and micro-filtration are applied.[1]
Due to the high levels of
dissolved metals in solution coupled with the typical low pH values seen, most mine waters can be quite challenging in terms of their ability to scale and foul the membrane surface. In this short review the methods of controlling scaling will be discussed, particularly the formation
of CaSO4
(gypsum) scaling as well as
the ability to remove scale from the membrane surface by cleaning.
is the most difficult scale to deal with as it is pH independent and therefore a change in pH will have no effect on the ability to prevent scale precipitation on the membrane surface. Te simplest method of preventing scale formation is to know the water chemistry and what species are likely to exceed their saturation point; and then based on these findings an appropriate antiscalant can be used. An antiscalant works at the sub-stiochiometric level by one or more closely inter-related mechanisms of threshold inhibition, crystal distortion and dispersion. Treshold inhibiton prevents the precipitation of a salt once it has exceeded its solubility product (Ksp), crystal distortion affects the growth and ordering of a crystal causing an irregular shaping and weak structure and dispersion works by the inhibitor chemisorbing onto the crystal surface, giving additional charge, resulting in repulsion and subsequently dispersion.[5] Genesys International has formulated a number of different antiscalants
SCALING OF MEMBRANES AND METHODS OF CONTROL Te ability to control the build up of scale within an RO membrane plant is of vital importance for the successful and continuous operation of the plant. CaSO4
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