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Carbon disulfide, xanthates and metham sodium


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Carbon disulfide

Carbon disulfide (CS2) is also known as carbon bisulphide and sometimes dithiocarbonic anhydride. It is a faintly yellow highly flammable liquid with a strong disagreeable odour.

Carbon disulfide is manufactured from hydrocarbons and sulfur and is a very flammable liquid which is therefore extremely hazardous to manufacture and transport.

Worldwide carbon disulfide is used to manufacture regenerated cellulose (viscose rayon and cellophane), carbon tetrachloride and organic sulfur compounds including xanthates used as flotation agents in mineral processing and Metham sodium soil fumigant.

Carbon disulfide is no longer manufactured in Australia . Worldwide carbon disulfide is produced in electric arc furnaces and more recently by reacting hydrocarbons and sulfur. World production is declining from 1m tpa along with its principal uses in the production of carbon tetrachloride and xanthates.

Carbon disulfide manufacturing plants at Botany NSW (ICI) and Rhodes NSW (Union Carbide) have ceased operation by the late 1980s. The carbon disulfide was used to produce carbon tetrachloride and xanthates.

In Australia carbon disulfide is used by Nufarm at Kwinana WA to produce metham sodium, a dithiocarbamate soil fumigant pesticide and since 1998, by Coogee Chemicals WA to produce xanthates.

Xanthates

Xanthates are salts of esters of carbonodithioic acids produced from carbon disulfide reacted with sodium or potassium hydroxide and an alcohol, most commonly ethanol, isobutyl alcohol or amyl alcohol. The most common alcohols used are ethanol and amyl alcohol with total Australian demand for both forms estimated at 15 000 tpa. (Valued at some A$25m p.a.)

Xanthates are commonly used to manufacture cellulose film (cellophane) and rayon, but in Australia they are exclusively used as a collector for the extraction of sulfide minerals.

Union Carbide once operated a small plant at Rhodes, New South Wales that closed in 1987. 

It is useful to note that xanthates are imported in a solid form to reduce transport costs. The cost of drying can be avoided by producing xanthates in a liquid form if close to the market providing a substantial advantage to local manufacturers. 

In 1998, Coogee Chemicals began the manufacture of ethyl- and plans for iso-butyl xanthates in a 5 000 tpa xanthates plant (close to Nufarm) where carbon disulfide is also used to produce metham sodium. Coogee Chemicals produces it in a liquid form of about 30 per cent solids. 

Another plant is due for commissioning in May 2001 by Coogee Chemicals at Mount Isa with handling facilities at Townsville. It will produce around 4000 tonnes per year of isobutyl xanthate with production in 25 tonne batches.

Metham sodium

Until 1999, Nufarm manufactured metham sodium from imported carbon disulfide and methylamine, and caustic soda from their nearby chloralkali plant.About 59 tonnes of carbon disulfide is required for each 100 tonnes of metham sodium. Since 2000, Coogee Chemicals manufactures it recognising the practicality of it already producing xanthates also made from carbon disulfide.

It is a highly reactive substance (the basis for its function as a fumigant) and must be transported as a liquid solution (about 50 per cent by weight). The bulk of the solvent provides a substantial measure of assistance enabling local manufacture in a small market with little other advantage.

Metham sodium is a dithiocarbamate soil fumigant pesticide that has experienced rapid growth with the phasing out of methyl bromide soil fumigant. It is made simply by reacting carbon disulfide with methylamine and caustic soda. About 59 tonnes of carbon disulfide is required for each 100 tonnes of metham sodium.

The Australian market in 1995 is estimated at 1 500 tonnes (dry weight basis) that requires around 900 tonnes of carbon disulfide. The market is believed to be growing rapidly and in 1995 was valued at around $3m per year.

Worldwide, metham sodium is normally a sideline product of xanthate manufacturers.

Metham sodium competes with methyl bromide as a soil fumigant though this is being phased out under international agreement as part of the Montreal Convention (for harming the ozone layer). Accordingly demand for metham sodium is expected to increase. Its growth, combined with investment in xanthate manufacturing in Australia, may create a market in Australia to support a carbon disulfide manufacturing plant.

Again like xanthates, metham sodium is produced as a 50 per cent solution which provides a substantial freight saving advantage for local manufacturers close to the market.

The major user is the Western Mining Corporation using about 600 tonnes of ethyl xanthate and 400 tonnes of potassium amyl xanthate. Their Mt Keith project (opening 1995) will require between 1 000 to 4 000 tonnes ethyl xanthate (currently estimated at 1 400 tonnes). Western Mining has indicated that up to 14 000 tonnes per year of xanthates could be required by year 2000 subject to the resolution of production economics.


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