Silica scale mitigation for high recovery reverse osmosis of groundwater for a mining process

The feasibility of silica removal in RO treatment of groundwater from a Western Australian mining and processing operation to prevent scaling and enhance water recovery was investigated. This study has shown that it is possible to decrease the silica concentration in RO concentrate to levels that would allow an overall water recovery of 90% to 95% using 10 g/L of regenerable activated alumina adsorbent. Regeneration of the adsorbent using 2% NaOH was found to be effective for at least three regeneration cycles. A preliminary costing of the high water recovery RO process using silica removal by adsorption indicated product water (permeate) costs of $5.6/kL and savings due to a reduction in brine volume from the current 40% of feed volume to 5–10% of feed volume. It also allows better utilisation of a scarce groundwater resource, allowing the production of up to 1.6 times more low salt water from a given volume of groundwater. These results warrant larger scale investigation of silica removal and adsorbent regeneration for high recovery RO processing for mining operations, and application of silica removal to RO treatment of other silica laden waters such as coal seam gas produced water. © 2014 Elsevier B.V. All rights reserved.

It was shown that it is possible to decrease the silica concentration in RO concentrate to levels that would allow an overall water recovery of 90% or above using high concentrations (10 g/L) of activated alumina adsorbent. Regeneration of the adsorbent using 2% NaOH as regenerant is effective for at least three regeneration cycles. Analysis of the 2% NaOH regenerant solution after use gave evidence that suggests that the alumina surface chemistry is modified by the regeneration process. The regenerated alumina was found to be less soluble and has a higher adsorption capacity than the un-regenerated alumina. The liberation of adsorbed silica from the regenerated alumina was also lower than for the un-regenerated alumina. These changes in surface chemistry were tentatively attributed to the formation of aluminium silicate on the alumina surface during regeneration. Regeneration was also found to release considerable quantities of calcium, suggesting the possibility of the formation of calcium silicate and/or calcium aluminium silicate on the adsorbent surface. These findings highlight the complexity of the chemical interactions involved in silica removal by adsorption, the feedwater specificity of the outcome of adsorption processes, and the need for a better understanding of chemistry involved in order to gain better control over the process. Preliminary costing of the high recovery RO process with an added silica removal stage indicates that this added stage leads to high product water costs ($5.6/kL), and is likely to significantly decreases the brine management costs due to the reduction of the brine volume from the current 40% to 5% of the feed volume. It also allows greater mineral production in situations where groundwater availability is a limiting factor. These results warrant larger scale investigation of silica removal using adsorption column, and adsorbent regeneration.