The formation of high density sludge from minewater with low iron concentrations

摘要

The high-density sludge (HDS) process is recognised as the preferred method of treating mine water containing high metal concentrations. The method is based on the addition of hydroxides to raise the pH and precipitate the metals as a sludge that settles to a much higher density than achieved by conventional alkali neutralisation processes. This is achieved in a two stage neutralisation process using recirculated sludge to act as a seed for further metals precipitation. HDS is formed by mixing either: · the alkali and recirculated sludge prior to adding the mine water, or; · the recirculated sludge and the mine water prior to adding the alkali. The choice between the two methods is project specific and can only be determined by pilot trials. A typical HDS plant will produce a sludge that settles to between 35 and 50 percent solids and can be mechanically dewatered to between 50 and 70 percent solids. In comparison the sludge produced by a conventional plant will settle to between 2 and 5 percent and can be mechanically dewatered to between 25 and 40 percent solids. The HDS process is therefore particularly advantageous where large amounts of sludge are generated or where sludge disposal costs are significant. Most HDS plants are treating waters dominated by iron chemistry and use slaked lime as a reagent. Current wisdom is that high density sludge cannot be formed from mine water dominated by non iron species nor using other alkali reagents.