In this edition of Australasian Mining Review we look at a range of ways mine sites around the world are making the most out of what is increasingly becoming a precious commodity in its own right – water.
Fresh water usage at Xstrata Copper’s Mount Isa Mines copper concentrator was reduced by 40% in 2012 through a series of initiatives, implying a saving of about 800 million litres, in an area where water is a scarce resource. Richard Harvey, Mount Isa Mines Copper Concentrator Manager notes a number of initiatives implemented in the concentrator to reduce water usage. “These measures included automated daily reports to track water use in all plant areas during the previous 24 hours and setting targets for the flow rate of water used in the operation of our large slurry pumps.”
Further significant reductions were achieved by recycling the water used to cool the ball mill as water for the pumps and installing flow control valves at the wet fill plant. As well as reducing the impact of the plant on the surrounding environment, the measures have also accrued annual savings of around A$1.1 million.
Water and chemical mass balance
MWH’s Zygi Zurakowski (Geotechnical Engineer) and Resa Furey (Market Analyst) contend “there is no simple recipe for managing water at a mine, integrating the skill sets of engineering and science disciplines is an important ingredient. Site-wide mine water balance models are a standard, and valuable, approach to managing and accounting for volumes of water, in, out and throughout a mine.
“Including aspects such as water quality data to the water balance framework creates a powerful management and strategic planning tool that can be used to evaluate the impact of environmental and operational changes as well as uncertainties at a mine site. Combining the expertise of the various engineering disciplines including the hydrological, geotechnical, geochemical, environmental and water treatment engineers into a single water management tool will contribute to a mine’s success and regulatory compliance.
“The need to couple chemical mass data with water balances is driven most often by two factors: first, impacted mine water discharged to the environment must meet regulatory water quality standards. Second, recycling water within the mine may impact (positively or negatively) mineral recovery, so tracking and forecasting of the water chemistry is crucial.
“Effectively-developed Water Balance (WB)/ Chemical Mass Balance (CMB) models track chemical concentrations and monitor water flows throughout the mine; they can evaluate whether treatment is necessary prior to discharge to the receiving environment, and if so, what the treatment system influent concentrations are which will subsequently effect treatment system design criteria. In addition, the combined models help operators understand the effects of recirculating chemical loads: on long-term treatment requirements, and process water quality. Importantly, these models can also be used to demonstrate if treatment systems can reliably meet discharge requirements. As regulations continue to target mining effluent, and requirements become stricter, savvy tools help operators and managers make improved decisions.
“CMBs are most often completed during design and evaluation of individual operational components such as waste storage facilities, heap leach pads, etc., and are often not considered in context of how the flows and chemical loads will impact processes throughout the mine. To combat this, the chemical mass data from the different mine components should be coupled with a site-wide water balance model early in the mine planning, and as the mine plan is updated, so too should the combined model.”
At a Latin American mine with challenging water management conditions, MWH engineers led a team to complete a combined WB/CMB tool. This was used to provide data for sustaining capital investments, water treatment process decisions and environmental compliance that would result from the addition of a leaching circuit to a mine with an existing grinding circuit and tailing storage facility.
The model helped to evaluate the following scenarios:
- how operational changes would affect the chemical make-up of the water reporting to a proposed water treatment plant
- the efficacy of the treatment plant
- the composition of the treated effluent
- how water recycling would affect mineral recovery.
The operators wanted to discharge a waste stream from one area of the site into the tailing storage facility, however the combined model showed that chemical concentrations in the tailing reclaim water would be extremely high and have a very negative impact on mineral recovery. Through the use of the combined WB/ CMB model, it was possible to discern acceptable chemical concentrations for both discharge to the environment and recirculation within the mine. Based on the collaboration between the various engineering disciplines, the team could provide water treatment and management recommendations which optimised both cost and mineral recovery.
In Australia, a mine was looking to dispose of water treatment plant sludge on-site. By using a WB/CMB tool, engineers were able to determine whether or not the sludge would remain stable and potentially leach if disposed at the bottom of the pit. The combined model provided data to help determine whether this disposal method was feasible and whether or not it would result in significant cost savings. This determination would not have been possible without the combination of geochemical and water balance modelling. The coupled model provided an understanding of the best method to dispose of the sludge and whether or not the metals would re-dissolve and require re-routing to the water treatment plant.
“The recovery of by-products from mineral waste water is a promising approach from a technical, economic and environmental point of view.”
At a precious metal mine in Eastern Europe a cyanide mass balance was completed in conjunction with the water balance to fulfill regulatory requirements for permitting. One use of this model was to track the movement of cyanide throughout the operation. Going beyond the routine permitting requirement and the typical level of detail required, the combined WB/ CMB model traced the cyanide from cradle-to-grave in a predictive manner that accounted for consumptive use and chemical transformations. For example, the model addressed the fate of cyanide in the tailing impoundment including HCN volatilisation from the reclaim pond and attenuation of cyanide during seepage through the embankment foundation. Cyanide concentrations during both operations and closure were forecast. The WB/CMB also helped to improve the accuracy in air quality modelling by defining the source terms. Ultimately, the WB/ CMB helped reduce reviewer concerns regarding the use of cyanide at the mine.
Finally, at a closed US-based copper mine, a WB/ CMB and groundwater model was completed to evaluate the potential range of volume and quality of seepage from waste piles through a proposed seepage barrier wall. The combined tool was used to evaluate mixing different water flows at the site to get a more favourable influent composition for treatment. Based on results from the model, a pilot test program was developed. This process allowed MWH engineers to identify that one single treatment plant would achieve better quality effluent than the two treatment plants that were originally planned. Combining the impacted streams identified optimised process conditions; this simultaneously enhances the ability of the treatment plant to meet stringent discharge criteria and reduces operating requirements, both translating to cost savings.
Treating tailings
Blue Gold has been designing and building a solution to recover platinum, gold, silver, uranium, and other precious metals with LAREMUTEC (Laser Aided Methodology with Ultrasonic and Thermo-Electric Conductivity) process, while simultaneously using this technology to rid the water used in the mining process of any contaminants including toxins such as cyanide.
The process starts from the pond mine tailings or directly from the tailings tank, where a slurry pump transports the liquid into a hydrocyclone and it passes through electronic precipitators where heavier and lighter solids and water are separated and suspended solids and metals are removed.
Contaminated water then passes through the Ultrasonic Diffused/Dissolved Air Flotation (UDAF) process where micro bubbles produced by sonic waves breakdown the solution thus releasing free water molecules. Heavy sludge that settles in the bottom of the tank and lighter sludge that floats to the top are both removed. Collected water from the sludge concentrator flows to a smaller electrolysis unit to capture the remaining dissolved metals. The water discharged from the UDAF is then put through a centrifugal separator where remaining particles are separated and then polishing starts at the media filters. Final filtration occurs at the nanofilter where sub-micron particles and other volatile organic compounds that were not removed previously are collected. Polished water discharged from the nanofilter is then sent to the Ozonator for oxidising and disinfecting remaining pollutants, and once water is discharged and regional standards are met, water is stored at the final clarifier tank for distribution. If standards are not met at this tank, a diverter valve redirects the water back to the system for reprocessing.
The system is not only focused on wastewater treatment, but also on recovering dissolved precious metals. This is achieved by augmenting the use of other Blue Gold technologies that are applied in the ion exchange using nano resin. This particular nano resin, will absorb the dissolved precious metals. The resulting material can be sent to refiners for the extraction of the adsorbed metals. Laboratory tests have shown the efficiency of this technology is around 70%, however fine tuning in the field is required to confirm this efficiency. Due to the complex nature of the environmental factors of a specific project, the cost per cubic metre of the tailings needs to be evaluated on a case by case basis.
Stricter discharge limits
Tom Sandy, Michael Blois, and Marek Mierzejewski of CH2M Hill note “the management and treatment of waters for target pollutant and/or water quality parameters (e.g., heavy metals, acid mine drainage, scaling elements) has been a long-standing challenge in the mining sector, whether from active or closed mines. Relatively straightforward treatment through a variety of active and passive treatment processes has been successfully used over the years for simple pH neutralisation and heavy metals precipitation utilising a variety of alkaline agents. However, discharge requirements for both heavy metals and other water quality parameters have become more stringent and comprehensive. This includes heavy metals being regulated to nearly the method detection limit as well as the addition of other water quality constituents of interest (e.g., selenium, sulphates, chlorides, TDS, osmotic pressure (OP), conductivity, etc.).”
Treatment of both heavy metals to lower levels and other water quality constituents of interest require different and more sophisticated water management and treatment approaches to maximise benefits and minimise costs. There are a variety of water treatment and brine management technologies in various states of readiness to meet these requirements, with some more or less selective for constituents of concern. These CH2M Hill practitioners list:
- Membrane base (e.g., Reverse Osmosis (RO), Nanofiltration (NF), Ultrafiltration (UF), Electrodialysis Reversal (EDR), Electrodialysis (ED) et al.)
- Ion exchange (IX) (e.g., weak and strong base/acid polymeric resins, liquid
- Chemical treatment (e.g., zero valent iron precipitation methods for sulphate and TDS)
- Active biological treatment (e.g., anoxic anaerobic attached and suspended growth biochemical reduction processes
- Passive biological treatment (e.g., vertical flow biochemical reactors and horizontal flow wetlands).
“In cases where specific water quality parameters (e.g., selenium, cadmium, sulphate) are only targeted versus a broader range of water quality parameters (e.g., osmotic pressure, conductivity, osmotic pressure), the challenge is finding technologies that are specific to the constituent of interest. CH2M Hill has found that it is more cost effective to find treatment technologies (e.g., IX, chemical, biological) that are water quality parameter specific versus technologies (e.g., RO, NF, EDR) that treat a broader range of parameters, given brine and residuals management issues. However, when this is not possible given the water quality criteria for factors like TDS, conductivity and OP, or there is a high potential for future more comprehensive water quality criteria for these same parameters, then more non-selective treatment technologies will inevitably need to be considered. Each of these water treatment technologies will require brine and/or residuals management and disposal technologies (e.g., thermal evaporation/crystallisation, thickening, and dewatering). Depending upon the selectivity, brine management and reconstitution may be potentially bigger issues than the water treatment itself.
“Careful consideration of these treatment and residuals management aspects must go into determining a water management strategy, and methods for equalisation and diversion of surface and subsurface water discharges are key to minimising treatment requirements and costs. In particular, management of wet weather flows (e.g., storm water) requires special consideration.
They conclude that “overall, the industry needs to be aware that the trend of being required to meet more stringent water quality criteria requires a higher level of water treatment and focus on the brine management reconstitute issue, demanding more sophisticated water management strategies to minimise the cost of that treatment.”
Economic by-product recovery
At the SME Annual Meeting in Denver, K. Tabra and O. Gaete of Arcadis presented ‘Ways to deal with mine/plant effluent residues: a roadmap process’. They concluded that “chemical precipitation is a well-accepted technology which offers a solution for metal removal requirements and achieves stringent discharge limits that are protective of public health and the environment. It is a flexible technology that can address metal contamination in waste water at mine sites. This technology can be used in conjunction with other treatments or by itself, depending on effluent characteristics and treatment targets. Its immediate results, efficiency, easy implementation and monitoring are the main advantages. However, the active nature of the process (use of chemical reagents), energy inputs, operation and maintenance can lead to relatively high treatment cost.
“The proposed roadmap is an effective tool to address the challenges of mine waste water active treatment and to evaluate the possible recovery of valuable elements. It consists of a first stage of diagnosis and optimisation of the system in order to reduce water make up and improve effluent quality. A second stage of characterisation is needed to define treatment targets based on local standards and historical information. The proposed treatment is defined by laboratory tests on waste water samples. These tests determine reagent consumptions, design parameters and a first approach to the costs required for treatment alternatives and possible recovery of valuable elements. Economic and efficiency analysis of different alternatives dictate the chosen treatment.
“In general terms, lime technology has lower treatment costs at the expense of voluminous contaminated sludge generation. Precipitation with sulphides or co-precipitation with ferric chloride allows a more effective and controlled removal of contaminants and their possible recovery for commercial purposes. Also, by these means the volume of sludge generated is considerably smaller.
“Fresh water usage at Xstrata Copper’s Mount Isa Mines… was reduced by 40% in 2012 through a series of initiatives, implying a saving of about 800 million litres, in an area where water is a scarce resource.”
“The recovery of by-products from mineral waste water is a promising approach from a technical, economic and environmental point of view. This recovery can account for a portion of the treatment cost and in some cases generate profits. However, detailed laboratory investigations should be carried out to correctly evaluate the economic and environmental feasibility and benefits.”
This article first appeared in International Mining (www.im-mining.com) and has been re-published with kind permission.
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