Titan Resources, the Australian-based nickel producer, has recently confirmed the economics of its BioHeap process, following a pre-feasibihty cost analysis of the exploration of two small low-grade nickel sulphide deposits in northern Western Australia.
Who Developed the Process?
The process has been developed over the past four years by a 75%-owned subsidiary of Titan, Pacific Ore Technology Ltd, with capital to help the project from the Australian Government’s R & D Start Grant programme.
The BioHeap process is a form of bioleaching. Traditionally, this is carried out by naturally occurring micro organisms, usually thermophiles, which are micro organisms usually found in acidic environments produced by the oxidation of sulphur - for example in and around hot springs, volcanic regions and sulphide-rich areas. While most living organisms derive energy for growth and reproduction from organic carbon, thermophiles grow on inorganic matter and are harmless to living creatures. Their diet consists of pyrite, arsenopyrite and other metal suphides, such as chalcocite and chalcopyrite.
How Does the Bioleaching Work?
The exact mechanisms by which these micro-organisms oxidise sulphidised materials is not precisely known, although both chemical and biological forces work together to oxidise the metal sulphide to form acid-soluble sulphates. Precious metals, which are not soluble, remain with the residue. Iron, arsenic and base metals, such as copper, cobalt and zinc pass into solution, which can then be separated from the residue and be treated by conventional processing methods. The residue created by the process may contain precious metals, which can be recovered by cyanidation.
Types of Bioleaching
There are currently two methods of bioleaching - tank bioleaching and heap bioleaching. The latter involves crushing the ore, stacking it on plastic mats and spraying it with a dilute sulphuric acid solution containing bacteria and nutrients. The solution drains through the heap and is recovered and resprayed over the heap. When the solution is considered rich enough, it is drained off and the metals extracted by conventional processes.
Advantages of the BioHeap Process
Although both bacterial oxidation and non-bacterial heap leaching are established in the gold and copper industries, their combined use in nickel extraction is unique. The BioHeap process treats crushed ore, so avoiding the need for fine grinding and concentration steps usually needed for most bacterial oxidation techniques. This in turn leads to cost savings which allows the processing of lower grade nickel sulphide resources, which may currently be uneconomic to exploit.
How Does the BioHeap Process Perform?
Small samples of the disseminated nickel sulphide ore from the company’s Radio Hill mine were subjected to bacterial leach amenability treating during 1998. The results demonstrated that greater than 90% of the contained nickel, copper and cobalt could be extracted. The BioHeap process was then tested using disseminated ore - ore that had a content of 0.73% Ni and 0.87% Cu.
How Does the Bacteria Work?
The bacterial culture works in the temperature range 45-60°C, at pH levels of less than 2 and requires air, from which it fixes carbon from carbon dioxide and oxygen for oxidation reactions. This occurs both directly and indirectly - in the direct method the bacteria attaches to the sulphide mineral and oxidises Fe and S moieties in the mineral to release metal ions into solution. The indirect bacterial attack on the mineral involves the bacteria in solution oxidising ferrous ion to ferric ion. Ferric ion is a strong oxidising agent and this in turn attacks the sulphide mineral.
Evaluation of the BioHeap Process
The actual BioHeap test was constructed on the apron of the Radio Hill tailings dam using ore from the Mount Sholl deposit, and consists of three pads, sand filtration bunds, drainage gutters from each pad and a holding pond. The entire area is lined with HPDE.
Two heaps have initially been erected, one a 5,000 tonne ore heap and the second a 5,000 tonne waste heap. The former was inoculated with the bacterial cell culture, while the later was used in conditioning the pregnant liquor. Aeration pipes are located one metre from the base of the heaps and run the length of the heap (figure 1). These are connected to a manifold and low-pressure blowers used to aerate the heaps. Covering the heaps is a network of irrigation pipes and drippers used for acid irrigation, bacterial inoculation and nutrient irrigation of both heaps.
Figure 1. The BioHeap trial, showing aeration pipes.
Following bacterial inoculation, the heaps are continually irrigated with the pregnant liquor stream and with a nutrient solution. As the oxidation of the ore progresses, so the base metal tenure in the pregnant liquor stream increases. Once a desired level of metal in solution has been achieved, a bleed stream is removed from the system and the dissolved metals removed using conventional precipitation, electrowinning and solvent extraction methods. By 22 weeks of operation, nickel recovery was 74%, well above the company’s estimate of 70% after nine months.
Total capital costs for the venture were estimated at Aus$15 million, including preparation of the leach pads, bacterial breeding facilities and ion exchange and precipitation equipment. This capital cost equates to a unit comparison figure of less than US$2 per pound of nickel produced per annum over the two year life of the project.
While the actual production costs of nickel using BioHeap will vary for different ore deposits and for different scales of operation, the results of the study are consistent with Titan’s projections for BioHeap as a process with low capital and operating expenditures and inherent technical and environmental advantages over conventional flotation and smelting.