Registration Dossier

Environmental fate & pathways

Additional information on environmental fate and behaviour

Administrative data

Endpoint:
additional information on environmental fate and behaviour
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)

Data source

Reference
Reference Type:
study report
Title:
Unnamed
Year:
1995

Materials and methods

Test guideline
Qualifier:
no guideline followed
GLP compliance:
not specified
Type of study / information:
Environmental fate and Environmental exposure of sodium ethyl xanthate

Test material

Reference
Name:
Unnamed
Type:
Constituent
Type:
Constituent

Results and discussion

Any other information on results incl. tables

Environmental exposure

Sodium ethyl xanthate is used as a collector during the processing of sulphide oresby flotation, a process that involves addition of the reagent to aqueous slurries ofcrushed and finely ground ore contained in flotation tanks. Air is blown through theslurry. In general, a series of such tanks is used. During the use of sodium ethylxanthate in the flotation process the mineral particles become separated as a frothfrom the tailings, which settle at the bottom of the flotation tank.

The froth (float) is collected and dried, either in air under ambient conditions or atelevatedtemperatures in an oven,* while the tailings are conveyed as a slurry to atailings dam where they settle, dry and consolidate. Spills and washings wouldalso be directed to tailings dams. Tailings typically have a solids content of about30%. In some operations, tailings may be intercepted in settling tanks so thatwastewater containing low concentrations of xanthates can be recovered for reuse inflotation. Xanthates in the tailings are not monitored, but most would be expected to be

retained in the froth. Xanthate residues in the ore concentrate are expected to decomposeduring drying or smelting.

 

Environmental fate

Hydrolysis will be a significant factor in determining the environmental fate ofsodium ethyl xanthate. In neutral or mildly alkaline solutions, sodium ethylxanthate decomposes to the alcohol, carbon disulphide, sodium carbonate andsodium trithiocarbonate, the two salts arising from neutralisation of carbondisulphide with the sodium hydroxide liberated. In more strongly alkaline media,hydrogen sulphide is liberated. However, strongly alkaline conditions are unlikelyto be encountered under the conditions of use in the mining industry. The half-life atpH 7 at 25°C is reportedly about 260 hours, increasing to over 500 hours in the pHrange 8 to 11.

 

Sodium ethyl xanthate is hydrolytically unstable when exposed to acidicconditions, reverting rapidly to ethanol, carbon disulphide and caustic soda, andtherefore will not persist in the acidic environment of tailings dams. If dischargedto waterways, the chemical would be likely to persist for at least some days,hydrolysing only slowly in this more neutral environment. However, it is notexpected to bioaccumulate in view of its ionic character.Sodium ethyl xanthate is not expected to contaminate the environment where oretailings are confined to well constructed tailings dams. Most will be retained onsulphide minerals and destroyed when they are dried after flotation. Minorresidues that remain associated with tailings will be destroyed by hydrolysis intailings dams.

Sodium ethyl xanthate residues in tailings have been discharged directly to waterwaysin exceptional circumstances. Until recently (1994), this practice was allowed to occur atthe Mt Lyell copper mine in Queenstown, Tasmania, which operated under a ministerialexemption from State discharge requirements. Although currently closed, indications arethat the Mt Lyell copper mine may be reopened by December 1995. Any resumptions inoperations will require the construction of a tailings dam as the Tasmanian Department ofLand and Environment has indicated that exemptions allowing discharge to rivers willnot be issued. The Queen River into which discharges occur, and the King River betweenMacquarie Harbour and its junction with the Queen, no longer support aquatic life. TheAustralian Mining Industry Council advised the Environment Protection Agency in January1995 that all Australian mines currently operating concentrators have tailings dams.

Applicant's summary and conclusion

Conclusions:
Hydrolysis will be a significant factor in determining the environmental fate of sodium ethyl xanthate. In neutral or mildly alkaline solutions, sodium ethyl xanthate decomposes to the alcohol, carbon disulphide, sodium carbonate and sodium trithiocarbonate, the two salts arising from neutralisation of carbon disulphide with the sodium hydroxide liberated. In more strongly alkaline media, hydrogen sulphide is liberated. However, strongly alkaline conditions are unlikely to be encountered under the conditions of use in the mining industry. The half-life at pH 7 at 25°C is reportedly about 260 hours, increasing to over 500 hours in the pHrange 8 to 11.
Executive summary:

Sodium ethyl xanthate is hydrolytically unstable when exposed to acidic conditions, reverting rapidly to ethanol, carbon disulphide and caustic soda, andtherefore will not persist in the acidic environment of tailings dams. If dischargedto waterways, the chemical would be likely to persist for at least some days,hydrolysing only slowly in this more neutral environment. However, it is notexpected to bioaccumulate in view of its ionic character.Sodium ethyl xanthate is not expected to contaminate the environment where oretailings are confined to well constructed tailings dams. Most will be retained onsulphide minerals and destroyed when they are dried after flotation. Minorresidues that remain associated with tailings will be destroyed by hydrolysis intailings dams.