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San Luis Project: process optimization

Paper from the Proceedings of the Second International Conference on Mine Water Solutions in Extreme Environments held in Vancouver, Canada, April 12-15, 2015. (downloadable PDF)

Authors: Stephen E. Rogers and Jason Kerstiens, ARCADIS, USA; William Lyle, Newmont Mining Corporation, USA

Published by InfoMine Inc. 2015.
ISBN: 978-0-9917905-7-9

Abstract

The San Luis Project in southern Colorado addresses the reclamation of a gold mine that closed in the 1990s. As groundwater filled the backfilled excavations, seeps into the Rito Seco were observed. These were deemed to be unpermitted discharges to surface water by the Colorado Department of Public Health and Environment (CDPHE).

A series of interceptor wells was installed to halt the movement of the presumed groundwater plume, and several extraction wells were installed in the backfilled material. Both the interceptor wells and the backfill wells feed into an equalization pond for the Rito Seco Water Treatment Plant (WTP). The original Rito Seco WTP included chemical precipitation and reverse osmosis (RO) to remove dissolved constituents from the groundwater, most notably manganese (Mn+2), sulfate (SO4-2), and fluoride (F-). The facility was conceived as a zero discharge facility, with chemical precipitation of the RO concentrate before recycling it back through the treatment process.

The original Rito Seco WTP suffered from numerous problems. Within the first six months of operation, the original RO membranes fouled with gypsum (calcium sulfate). The facility never achieved its design requirement of 22 liters per second (L/s). Initial operations were plagued by changing water quality as a consequence of the concentrate recirculation, excessively high chloride concentrations that attacked the RO pressure vessels, and declining recovery as recirculating salts accumulated in the system.

The treatment process at Rito Seco was reconfigured to a single pass lime/soda ash chemical precipitation process. RO system recovery was increased from 65% to 90%. The mine implemented a concentrate disposal process to evapotranspire the concentrate along with tailings underflow. The RO influent was of such high quality that a significant sidestream could bypass the RO to be recombined prior to discharge, thus increasing WTP throughput to approximately 38 L/s and overall efficiency to 94%. This allowed the WTP to operate intermittently, reducing operating personnel and power requirements and improving efficiency in chemical usage.

This paper describes the difficulties in operating the facility as designed, the process changes implemented to improve plant performance, and the successful WTP operations since the process optimization improvements were implemented. An indication of the success of the operations is the service life of the RO membranes, now fifteen years in service.


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