Gordon Cope talks to Graham Sim, GE Power and Water, about current and future trends in coal miner water management.
Water management is an increasingly important part of mine management with mines falling into two general catagories: water scarce and water rich. “Water scarcity places constraints on production; water-rich mines have issues with rainfall, storage and environmental impact of discharge,” said Graham Sim, global mining industry director for GE Power and Water.
When too much water is a problem
Water-scarce mines are located all over the world, but the most prominent geographical regions include arid regions of Australia, the Middle East, North America and Latin America. Because these regions, by definition, have higher rates of evaporation than precipitation, being dry has its advantages when it comes to discharged water. In the western US, for instance, vast adjacent areas of non-arable land allow opencast mine operators to pump the mine water into lined holding ponds, where waste contents passively settle out and the water harmlessly evaporates. Because no water is discharged to lakes or rivers, very little active processing is required.
Water-rich mines are commonly found in southeast Asia, the US, Canada, Colombia, Brazil and eastern Australia. Typically, these mines must contend with discharges of 2000 – 10 000 gal./min. The majority of mines use various passive systems in which mine water is pumped into lined ponds where total dissolved solids (TDS), coal particles and metals settle out. If the water is acidic, it is treated with lime to neutralise pH. The water is then returned to rivers and lakes through managed discharge, in which it is diluted with surface water during the rainy (high flow) season.
Some mines, however, have uniquely large amounts of chlorides, metals or other contaminants that are not readily handled by passive systems. Operators must therefore install an active system. “In terms of active treatment suppliers, there are a few global players (Veolia, Siemens and GE), but most of the sector is fragmented into regional players and it is not dominated by one supplier,” said Sim.
Active water treatment
Because of the diversity of conditions, active water treatment suppliers must determine the content (and range of contents during the course of a year). Technicians use gas chromatography (GC) and liquid chromatography (LC) for organic content. For metal analysis, inductively coupled plasma (ICP) is used. All are commonly coupled with mass spectrometry (MS) devices. A treatment stream is then devised to handle the contents (and their seasonal variability), volume and byproducts.
Pre-aeration is often the initial treatment step. This drives off the CO2 and lowers acidity. The mine water then goes to an aeration tank to oxidise iron and manganese ion. The mine water is then sent by gravity to a cone-shaped clarifier tank. There, dense solid particles settle to the bottom to form a sludge.
The water coming off the top of the clarifier tank is sent to a polishing pond for further settling. The final polished water is pH neutral, has suspended solids typically less than 10 ppm, and the iron, aluminium and manganese are at acceptable levels. It can be legally discharged to a surface receiving stream or reused to clean coal.
The sludge, which is a mix of metals, clay and coal dust, often contains toxic elements. It is treated as hazardous waste and must be disposed of in special dumps. Disposal costs are based upon weight and the price for safely containing the sludge can amount to millions of dollars.
By far the largest single ingredient of sludge, however, is water (about 99%). Disposal costs can be greatly reduced by removing as much water as possible. The first step is to add a lime slurry mixture in an aeration tank. This causes the iron, manganese and sulfates to come out of solution, precipitating directly on the surface of the alkalised sludge particles. This high-density sludge is now around 30% solids and is an ideal candidate for further weight reduction through dehydration, to the point where it reaches close to 100% solids.
Several years ago, GE installed an active water treatment system in CONSOL Energy’s Buchanan coal mine in West Virginia, which produces approximately 3 million tpa. “The mine had discharge water with elevated levels of sodium chloride,” recalled Sim.
GE devised a 1600 gal./min system that used a combination of ultrafiltration, RO and evaporation to purify and recycle 99.9% of the water. The processed water coming out of the system can be used at the mine’s preparation plant facility, reducing the need to obtain water from other sources. “The concentrated salt is also beneficially reused,” said Sim.
Several trends are emerging that will guide the short-term future of mine water management. “Sulphates are now more regulated along the lines of World Health Organization drinking water standards, which call for levels under 250 ppm,” said Sim. “Treatment of water with elevated levels of calcium sulphate often results in scaling and plant performance challenges. We are seeing the increased use of electro dialysis reversal (EDR) for high scaling applications, which places an auto-reversible current across the membrane that keeps the negative and positive ions from depositing.”
RO systems, used to treat both brackish source water and also mine discharge water, are being upgraded. “Many water treatment plants to date have struggled due to inadequate pre-treatment. We are now seeing lime clarifiers being replaced with absolute barrier membrane technology, such as ultrafiltration,” said Sim. “We are currently installing an ultrafiltration system at a large mine in eastern Australia, treating 19 million litres/d.”
There is a trend toward remote monitoring and control of water treatment systems. “Many mines are in isolated areas where trained personnel are scarce,” explained Sim. “Mines are looking to supplier-operated remote systems to improve the accuracy and efficiency of advanced treatment processes.”
In terms of CAPEX, the cost of installing water treatment systems has come down exponentially over the last several decades, but the sector is now at the point where it is difficult to take significant further cost reductions. “The next big wave is OPEX; the energy cost of pumping, for instance, can be in extreme circumstance up to 80% of costs,” said Sim. “Improving pump design is a key focus.”
In the longer term, as companies seek out quality ore supplies, they are moving into increasingly remote locations. “Power availability is a big issue,” said Sim. “GE is working to lower power consumption and we see growth in linking water treatment plants with renewable energy sources, such as using wind and solar in their water treatment plants. There is not much happening now, but we expect to see an increase as operators struggle with power availability.”
In the coming decades, few doubt that mine water management is expected to become a greater headache for operators. However, it is also an opportunity to help establish meaningful benefits in ways that extend beyond mere compliance. Some operators, such as CONSOL, are using their processed water to aid in hydraulic fracturing, a stimulation technique that releases large volumes of natural gas trapped in the coal. Other companies are looking to find ways to convert the leftover salt brines into industrial chemicals.
Still others are realising that sound water management policy supports their social licence to operate. “Public pressure is a significant driver,” noted Sim. “People are concerned about how water is used and disposed of, so demand that companies manage and recycle water, as well as report their efforts.”
Operators who proactively expand mine water management will find not only economic and environmental payoffs, but social ones as well. In the current cultural zeitgeist where coal is seen as the least environmentally friendly of energy sources, every significant contribution helps.
The full version of this article appeared in the March 2014 issue of World Coal as: COPE, G., “The trouble with water”, World Coal (March 2014), pp. 53 – 56.
Written by Gordon Cope.
Edited by Jonathan Rowland
Read the article online at: https://www.worldcoal.com/mining/21042014/trends_in_coal_mine_water_management_coal/