Traditional Pyrometallurgical Treatment of Concentrate

Copper Applications in Mining & Extraction

By Tony Eltringham

The use of fire to extract metals - known as pyrometallurgy – pre-dates recorded history. Currently, just under 90 percent of the cathode copper in the world is produced by pyrometallurgy. There are multiple reasons: reactions are faster at higher temperatures; molten metals are easy to handle and alloy; reagents are cheap (wood, natural gas, flux); the metal and impurities separate easily; the precious metals stay with the primary metal; and the concentration of copper in a smelter is 20 times that in a hydrometallurgical process.

However, the ease of the process can be overcome by the cost of producing it. An increased need to lessen environmental impacts has raised the capital cost of the equipment and the operating costs, given the need to capture more than 99 percent of the sulphur dioxide generated.

BHP Technology Programs

Producing lower-cost copper is achieved by reducing operating and capital costs significantly. One way is not moving material with little or no value, such as waste rock. In order to produce copper at the lowest overall cost, BHP is investigating three branches of technology:

1. In Situ Leaching
2. Hydrometallurgy
3. Mini-Smelters

In Situ Leaching

"In situ" literally means "in place." With in situ mining, a diluted sulfuric acid and ferric sulfate solution is injected down holes drilled into the ore body. The solution flows through cracks in the rock under pressure, leaching the copper from the rock into the solution. The solution is then pumped to the surface to recover the copper, using solvent extraction techniques.

Tests show that recovery rates normally achieved with heap and dump leaching could also be approached with in situ mining. There are significant cost advantages of this operation which include: the surface need not be disturbed with anything other than pump and piping installations, no waste piles are created, start-up is relatively fast, equipment needs are reduced significantly, fluid control is more easily automated than solid batch processes, and we can mine deep, relatively low grade and complex ore bodies.

Hydrometallurgy

Hydrometallurgy is focusing on the two routes to remove copper from the ground:

The leaching of the "oxide" materials is relatively easy and relies on dump, vat and agitation leaching using dilute sulfuric acid solutions.

Leaching sulphide materials, whether in ore or concentrate, requires a chemical oxidizing agent – ferric ions (Fe3+). These special ferric ions can be generated by reactions with air. The oxidation can be assisted by either pressure (as in an autoclave) or more commonly with bacteria. The importance of the bacteria and our understanding of their potential only introduces an extra variable into the process. But different copper minerals require different conditions to extract the copper.

Leaching of ore is more temperature restrictive. With leaching concentrate in a sulfuric acid medium, temperature becomes a variable so combinations of temperature and bacteria or temperature and pressure are alternatives available to us.

However, sulfuric acid is not the only reagent that can dissolve copper from concentrate. For example, BHP has already patented a process using ammonia to dissolve part of the copper concentrate. (This is the basis of the Coloso cathode production plant in Chile.)

However, there are a number of challenges for these processes: the iron must be fixed in a stable environmentally-friendly form, an acceptable copper shape must be produced, and an economical way to capture the precious metals must be found.

Hydrometallurgical routes have become more popular to research because: energy costs are lower; more pollution-conscious communities require "zero discharge" type conditions; whether for air or water, as smelting processes are made "leak proof" the impurities become a problem by concentrating in the copper; hydrometallurgical processes can separate impurities better; the operating temperatures are lower making pilot plants easier to operate; sulphur dioxide is a gas that has to be made into acid. (Elemental sulfur is formed in hydrometallurgical processes.)

Mini-Smelters

The capital costs of smelters and refineries are very high compared to operating costs. One approach has been to operate these plants with a large 300,000-350,000 tonnes copper capacity.

However, the larger the smelter, the more likely it is that concentrate transportation will become an additional cost factor. Also, many mines need acid for leaching the oxide mineralization in the vicinity.

A breakthrough occurred in the smelting process when the industry as a whole recognized the value of "decoupling" the matte-making part of the smelter from the matte treatment part. This could allow matte to be shipped (at half the overall cost of concentrate) to either a pyrometallurgy converting stage or a hydrometallurgy stage (as in the nickel industry).

The advantage of this break in the flow is that a small, cheaper matte-making furnace could be placed at the mine site where acid could be generated and used.

The advantage of decoupling at the beginning of the treatment route is that the furnace can operate restricted only by the quality of the output (rather than quantity, which happens in a directly coupled process). This simplifies and reduces the cost of ancillary equipment. This also leads to the development of the concept of two or three mini-matte makers feeding one matte treatment facility which can be more capital effective.

Summary

The future success of copper mining lies in the three alternative methods of copper extraction: in situ, new hydrometallurgical routes and mini- smelters. Economics will continue to be the driving force that challenges us companies to change technology in order to stay profitable.

—Tony Eltringham, BHP Copper, Vice President Special Projects, Growth and Technology, San Francisco, California, USA.

This article is reprinted with permission from the publication,
On CU, July – September, 1997, Vol. 1, No. 4.

On CU is the quarterly publication of BHP Copper, a business group of The Broken Hill Proprietary Corp., Ltd. © 1997 BHP Copper

BHP Copper 550 California Street San Francisco, California 94104

Also in this Issue:

• How Do They Do That? How Copper is Made
• Traditional Pyrometallurgical Treatment of Concentrate
• How Do They Do That? In Situ Mining
• Millions of Dollars Saved With Non-Traditional Shutdown Procedures
• How Do They Do That? Bringing Copper to Market
• Big Blue Goes Copper
• How Do They Do That? Wringing Sulfuric Acid out of Sulfur Dioxide Emissions

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