Uranium reagent waste tanks.Think of a small pool of water filled with piranha. Now imagine tossing a side of beef into that pool. You'd be left only with bones. If bones are what you want, then this might be an acceptable process. Today's mining operators are looking for the bones. They use techniques that are similar to this analogy in form. Just replace the water and piranha with acid, and the beef with ore, and you have a rough description of today's extraction process.
Ore is obtained by either tunnelling underground or by striping off layer upon layer of the earth's soil and rock through open-pit mining techniques. Both methods bring ore to the surface so whatever precious metal or mineral is being sought can be extracted. The extraction methods employ powerful chemicals that strip away unwanted compounds, or ‘tailings.’ In the past, chemicals used in this process had the tendency to eat away the containers used to extract the minerals. Now, thanks to advanced technologies in reinforced plastic materials for these containers, minerals can be extracted using a number of chemistries and temperatures without the concerns connected with past methods.
The processes used today to extract precious metals from ore create a harsh environment that places extreme corrosive pressure on the containers used for this purpose. In the past, most processes used other metals, such as lead, to line the containers. But this method inherently led to problems with welds and seams giving way from the constant gnawing of acidic compounds.
Early breakthroughs in science, realised nearly 50 years ago, delivered a new family of resins that could coat leaching tanks and electrolytic cells, and successfully replace lead and other materials while providing a level of reliability never before seen in the mining industry.
Composite materials entered the mining industry, and although many mine operators were sceptical at first, today's corrosion-resistant composite materials are an integral part of mining. Composite, composite lining and polymer concrete, are important components in processes used to mine nickel, copper, zinc, manganese, cobalt and other metals.
A made-to-order match
An example of how mining techniques have improved from the introduction of composite material is the development of polymer concrete. This material is formed by mixing select aggregates and polymer resins. When combined, following a formula that mixes approximately 10% resin and 90% sand and additives, this bond provides an extremely powerful material capable of performing under extreme conditions.
The polymer concrete is often used to construct a cell or monolithic moulded unit that, at least for copper refining, is then filled with a sulphuric acid solution. The solution is subjected to electrolysis and copper is electrodeposited on a cathode made of copper. This all takes place at 65°C.
For a copper refinery in Tocopilla, Chile, Ashland's DERAKANE® 411-45 epoxy vinyl ester resin was combined with concrete to fabricate electrolytic cells. The solution these cells contain consists of a 14% concentration of sulphuric acid and a 4% concentration of copper sulphate at 149°F (65°C). These cells have been operating for nearly 20 years without failure.
Beyond polymer concrete
While the concrete mix provides one level of protection for mining operations, other processes, calling for higher corrosion resistance, require linings with special epoxy vinyl ester resins to complete the job. In processing zinc, for example, CEZinc of Valleyfield, Quebec, installed a new system to capture sulphuric acid during its electro-winning process.
The zinc production process generates a fine mist of corrosive sulphuric acid that must be removed from the work area and recovered. The system they installed gathers the acid into seven cooling towers where a demister located at the top of each tower removes the acid and returns it to the processing cells through a closed-loop system. The cooling towers were fabricated using Derakane 510C-350 resin coupled with a 3% antimony trioxide compound to provide additional ignition-inhibiting properties.
“Building the structure of acid-resistant material eliminates many maintenance concerns,” says Mike Agnew, CEZinc's director of operations. “If made of concrete or steel, there would have been a need for continual inspection and repair.”
Most metal mining operations consist of three major operational steps:
- extraction;
- benefication; and
- processing.
Extraction, as mentioned earlier, is removing ore material from a deposit and encompasses all activities prior to benefication. Benefication of ores includes crushing, grinding, washing, filtration, sorting, sizing, gravity concentration and flotation concentration. Processing activities follow benefication and include smelting and refining of concentrates to prepare a marketable product.
It is the refining process where composite materials have enabled mining operators to find efficiencies. As higher input costs continue to put pressure on margins, it is essential that mining operators continually look for economical ways to increase production.
According to an October 2005 report from the International Copper Study Group, the outlook for copper is steady. Production, led by Chile, which supplies 35% of the entire world's copper, is keeping up with current demand. Year-end carryover stock-piles are dwindling according to the study which reported that only 49 000 tons remained in inventory at the end of 2004 versus the 442 000 tons on hand when the year began.
In addition to epoxy vinyl ester resins being used to refine ore, these tough workhorse resins are also used in pollution scrubbers at the mining plants. At the tin smelting plant of Capper Pass Ltd in the UK, Derakane resin was used to line the inside wall of a scrubber. Two fires were documented inside the scrubber, but no damage to the Derakane lining was noted. The compound laminating the internal structure was examined after almost nine years of service with no deterioration.
Proven
“Derakane resin and HETRON® epoxy vinyl ester resins have really proven their worth in the performance driven corrosion resistant materials market,” says Thom Johnson, North American industry manager for Ashland Specialty Chemical's corrosion and fire resistant composite resin business. “This is most evident in mining operations where the chemical processes used are extremely corrosive to vessels and piping. Nothing holds up as well as Hetron resin and Derakane resins in these extremely aggressive environments.”
The epoxy vinyl ester family of resins provides broad corrosion resistance properties. They are seen in the industry as fabrication and installation friendly and they exhibit good mechanical properties. Many case studies have been written that support their use and longevity. Whether mining for minerals, metals, or gems, composite materials help mining operators find their bones of value. ♦
This article was published in the December 2005 issue of Reinforced Plastics magazine.