Turning Wastewater, Produced Water Into An Asset
Editor’s Note: This is a great story. Unfortunately, the company’s founder, Allan Provost, (my client) passed away in 2010.
Open pit copper mines, even in arid regions, can collect water in the mine pit from rain, snow, or groundwater intrusion. Pit water can accumulate in large volumes interfering with mining operations and flooding areas in and around the mine. Adding to the problem, if the mine uses a hydrometallurgical process such as in-situ leaching, leach solutions can accumulate excess water from rain or snow, diluting the copper and acid concentrations in the pregnant leach solutions (PLS) fed to the SX/EW plant. Concurrently, mines use large volumes of fresh, clean water in mining operations such as ore grinding and flotation. In fact, provision of fresh water represents a major operating cost to the mine.
Efforts by various mining operations to remove excess water in the leach solutions include rain water redirection, spray evaporation, and precipitation with lime. None of these methods recover the soluble copper in the leach solutions and only add to the cost of mining operations. Addition of lime generates large quantities of sludge that must be properly disposed of, adding to operating costs.
The challenges to this open pit copper mine in Mexico are to:
- Dewater the pit and reduce the leach solution volume
- Increase copper concentration in the leach solution for enhanced recovery
- Reduce acid costs by recovering acid solutions and avoid the cost of neutralization with lime
- Recover clean, fresh water for mining process use
As with any potential solution, there is the added challenge of minimizing capital and operating expenses.
The Pollution Solution
After considering various traditional water treatment technologies, the mine selected our client’s powerful treatment system. This process utilizes patented engineered membrane systems using thin film polymer filters to remove total dissolved solids from the water. High pressure pumps force PLS through these membranes elements where the water passes through the polymer film and the dissolved solids do not. This allows recoverable materials including acid and copper to be concentrated and collected while clean, low pH water is made available on the other side of the membrane for use as makeup water to the SX/EW plant or other uses.
Our client’s technology utilizes composite thin-film membranes with pore sizes different than the reverse osmosis membranes commonly used for sea water desalination. The pore size, membrane material, and operating conditions are all specifically engineered by our client to allow operation at lower pressures while producing high permeate flow rates and maximizing metal ion recovery. After passing the PLS through a proprietary treatment plant, the feed is divided into two streams: permeate and concentrate. The permeate contains low levels of dissolved solids and no suspended solids, offering a source of clean, fresh water for mining operations such as grinding and flotation. The concentrate contains most of the dissolved solids, including recovered copper and some suspended solids. The concentrate can then be fed to the SX/EW plant for recovery of copper otherwise lost.
As with most EMS® applications, a bench scale test was performed to prove the concept of utilizing membrane treatment on the pit water at the mine. The bench testing was then expanded to a pilot scale test on site to validate the process and collect plant design and economic data. With these results in hand, our client proposed the sale of a full-scale plant with the following objectives:
- Increase copper cathode production 14% by recovering copper otherwise lost in the mine wastewater.
- Realize a significant savings in process water costs by recovering clean permeate water.
- Recover the capital investment in less than 2 years of plant operation.
- Reduce the water level in the mine pit to assure continuous operation.
The Bottom Line
Upon commissioning of the system the mine began to realize the objectives they sought. More than 2.5 million gallons of clean water and 14,000 lb. of copper are recovered every day from wastewater streams that would otherwise cost millions to neutralize and treat for disposal. These include:
- An increase in copper concentration in the SX/EW feed water from 0.7 g/l of copper to 1.2 g/l for an increase of 71%. (Over 99% of the copper in the pit water was recovered.) This resulted in a >14% increase in copper cathode production.
- The water level in the pit began to drop at the rate of 3.5 meters per year or approximately or 3.6 million cubic meters per year, and this at the same time additional rain and groundwater infiltration was entering the pit. This water reduction in the pit provided an equivalent amount of clean, fresh water for mining operations such as ore grinding and flotation.
- Capital investment in the plant was recovered in 1.7 years.
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