Dealing with a lack of water for process plants

Snowden has recently completed a feasibility study for a gold ore processing project. The project is located in a hot desert climate without any good and acceptable, available water source. The site has extreme heat during the day (45^oC^), an abrupt drop in temperature at night (10oC), and slight, erratic rainfall. The entire year’s rainfall may consist of one or two torrential rain events that rapidly disappear into the soil to be trapped above the layers of impervious bed rock. Although the average rainfall is probably less than 40 mm per year, the region may not experience rainfall for several years.

The simplest options to provide water to this process plant site were the use of underground water, the transport of fresh water from nearest source, the transport of saline or sea water from the nearest source or to use treated sewage water from the nearest source.

The underground water source was far too small in volume and too saline to be considered. Transport of fresh water to the site was too expensive and also too scarce. Sea water was identified as being located too far away to be economically transported or pumped to site. Thus, the use of treated sewage water as raw water for the plant was the only real option to be considered. Snowden was advised that treated sewage water could not be used as potable water due to cultural reasons. Thus, it was decided to provide potable water to site from the nearest town by means of tanker transport, however this potable water was also saline. Carbon filtration and reverse osmosis treatment was therefore required as purification steps to provide the site with potable water and supply quality water to the plant for eye wash stations and safety showers.

Following this, a detailed assessment on the use of treated sewage water as raw water to the process plant was conducted. The treated wastewater considered was available from three stages of the sewage treatment plant of the nearest city, all differing in quality and total contaminants. It was found that the sewage treatment stage producing the highest quality of water contained residual chlorine which needed to be reduced below acceptable levels so that it did not degrade equipment or consume an unreasonable amount of free cyanide. At this level it would also ensure that there was no negative influence on the  CIP circuit, thus blinding the active sites of the activated carbon. Test work conducted showed that the leach kinetics and gold recoveries proved insensitive to the type/quality of effluent water used in the process. However, it had an influence on the oxygen uptake and on the carbon activity in the CIP plant.

Installation of an on-site carbon filter to remove chlorine from the sewage water was recommended. This would reduce the risk of lower gold recoveries and prevent corrosion of plant equipment and also remove any organic contaminants in water that could cause a negative effect on carbon adsorption circuits of gold plants. Thus, with a simple cleaning and polishing step, treated or partially treated effluent water could be used as raw or potable water for process plants (see schematic flow diagrams below). This water was relatively inexpensive and readily available from regional centres. While the initial installation cost of transferring this water to a site by pumps and pipeline was expensive and would have added to the initial capital cost and eventual annual operating cost, the results will be beneficial in the long run.