In copper smelting, it’s inevitable that the plant produces electrostatic precipitator dust (ESP), but what do you do with it?
Typically it is just stockpiled and left there, but this presents a serious environmental and health hazard.
It’s a liability.
But the ESP dust also contains a number of base metals including copper and gold, which aren’t being recovered and are simply lost.
At Xstrata Copper Mount Isa it developed a processing mechanism which took out two birds with one stone.
It created the ESP dust recovery program, which devised a method to safely and efficiently recover the value add elements while at the same time disposing of the non-valuable materials.
In creating the plan, it faced a number of problems.
During smelting, ESP dust is collected from the smelter gas stream in an electrostatic precipitator, where process water is added and the dust is either processed through the site’s ESP Acid Leach Plant or is simply diverted to the stockpile.
The dust itself is an "environmental hazard, presents a serious hygiene risk to people, creates a dusting problem, and takes up valuable real-estate on the mining lease when stockpiled," Xstrata explained.
On top of this, dust is continually generated during the smelting process and over 16 years of operations a rather large stockpile has grown.
There also safety issues when dealing with arsenic solution
While these are serious hurdles, the revenue stream from the recovered metals makes it a viable opportunity.
However the actual recovery process brought up a new set of issues – that there is only a very narrow window in pH available which would allow for the gold and copper to exist in a pregnant leach solution (PLS) while at the same time causing non-valuable solids to be separated out and disposed.
This narrow pH range is between 2.2 and 2.5 pH.
Xstrata Copper developed its plan to deal with two sources of the dust; from the live smelting operations and its historic stockpiles, which over 16 years had grown to around 130 000 tonnes.
Extensive research went into an acid leach process over several years, and for the process to be effective it retained an extremely tight control of pH to separate out the minor elements while also keeping the creation of arsine gas (a highly toxic, colourless and virtually odourless gas) to a minimum.
During the neutralisation stage, the increase of the pH to 2.4 removes nearly all arsenic from the solution, eliminating the potential for arsine gas.
The neutralisation system itself is BATCH filled by the acid leach system, keeping the tank level between 40 per cent and 50 per cent so that only minimum water is added.
During this batching cycle pH fluctuates, so a set points higher than 3 is needed to obtain an average pH of 3 in the thickener as lime addition will not keep up with the concentrated acid addition at the waste slurry tank.
At a pH of 2.4 precipitations begins, with a severe drop in the levels of arsenic and iron, with a small decline in copper.
Neutralisation is then performed by a Milk of Lime transfer system.
While the smelter had solved the ESP dust problem created during smelting, how would it deal with the large stockpile of dust left?
A reclaim and transportation method was also developed, as due to the composition of ESP dust traditional flotation circuits and direct smelting were not feasible.
So it created a purpose built 15m3, 316 stainless steel agitated mixing bowl mounted on a trailer (Agi-truck), where the dust is made into a slurry and then transported.
The dust reclaimed from the stockpile is transported and discharged into the leaching process in either the waste slurry tank along with the newly generated ESP dust from the smelter, is then neutralised and the copper and gold recovered; or put into the copper solution tank in the leaching plant after neutralisation and cementation, is thickened, dried and then blended in preparation for direct smelting again. Since commissioning the ESP leach and Agi-truck process, Xstrata Copper Mount Isa’s smelter has had a copper recovery rate of up to 98 per cent.
It forecast that by 2016 it will have consumed all arising and stockpiled ESP dust.
As a result of this, an additional 20 000 tonnes of copper and 18 000 ounces of gold will be recovered that would have otherwise been lost, Xstrata explained.