J.T. Woodcock

Perxanthates — A new factor in the theory and practice of flotation

Abstract A compound with a UV absorption maximum at 348 nm was observed in Mount Isa copper flotation plant solution. This spectrum was similar to that of the product of reaction of xanthate and peroxide in dilute, alkaline aqueous solution. The compound was termed perxanthate (more correctly “O”-alkyl dithiomonoperoxycarbonate). A new compound, ammonium sec-butyl perxanthate (C4H9 OCSSO·NH4), was prepared by reacting potassium sec-butyl xanthate and hydrogen peroxide in dilute alkaline solution, acidifying, extracting into iso-octane, and precipitating with anhydrous ammonia. Solutions of this compound were compared with solutions containing the Mount Isa compound. Each compound was found to have the same UV absorption spectrum in a given solvent (alkaline aqueous, acid aqueous, chloroform, iso-octane, iso-amyl alcohol, and n-butyl acetate), but the spectra were different in different solvents (especially in alkaline and acid aqueous solutions). Both compounds could be extracted from acid, but not alkaline, aqueous solutions by organic solvents, and both had similar IR and mass spectra. It was concluded that the perxanthate in plant solution resulted from reaction of xanthate with peroxide derived from reduction of oxygen during flotation. This lends credence to the electrochemical theory of flotation and has some important theoretical and practical implications.

Decomposition of alkyl dixanthogens in aqueous solutions

Abstract Alkyl dixanthogens, (ROCSS) 2 , decompose in aqueous solution in the presence of nucleophiles in many ways. It is proposed here that in alkaline solution the principal methods of decomposition of ethyl dixanthogen are by simultaneous attack of OH − ions on the sulphur-sulphur bond to give products which include xanthate ion (ROCSS − ) and peroxide (H 2 O 2 ) and on the carbon-sulphur bond to give products which include monothiocarbonate ion (ROSCO − ), sulphide ion (S 2− ), and sulphur (S 0 ). Above pH 12 reaction is complete in a few minutes, and more monothiocarbonate than xanthate is formed. At pH 9 the reaction takes over 20 h and more xanthate than monothiocarbonate is formed. The primary products react further to give various ions which depend in part on the pH of the system. In alkaline solution some of the xanthate and peroxide react to give perxanthate (ROCSSO − ). In acid solution both xanthate and monothiocarbonate decompose rapidly; CS 2 is formed from xanthate and OCS from monothiocarbonate. In the presence of other nucleophiles at pH 9.2, dissolved dixanthogen decomposes much more quickly than with OH − alone, and other reactions occur. With thiosulphate a higher proportion of xanthate is formed together with some xanthyl thiosulphate and monothiocarbonate but no perxanthate. With sulphite (in the absence of oxygen) or cyanide the products include xanthate and monothiocarbonate but no perxanthate. With sulphite in the presence of oxygen, perxanthate is also formed. Suspensions of dixanthogens react slowly but in a similar fashion to dissolved dixanthogens. Longer-chain dixanthogens are much less soluble than ethyl dixanthogen but, in general, react in a similar way. Higher temperatures increase the rate of decomposition by OH − . This work has various implications in operating plants.

Control of laboratory sulphidization with a sulphide ion-selective electrode before flotation of oxidized lead-zinc-silver dump material

Abstract Concentration of lead-zinc-silver dump material was accomplished by flotation of a “sulphide” concentrate (galena and marmatite) followed by sulphidization and flotation of an “oxide” concentrate (anglesite). A decant wash was sometimes used between sulphide flotation and sulphidization. In this work sulphidization and oxide lead recovery was studied. Sulphidization by slug additions of Na 2 S gave relatively uncontrolled sulphidizing conditions. Controlled-potential sulphidization, using a sulphide ion-selective electrode (ISE), resulted in good control and improved metallurgy. Optimum E S value (i.e. potential of ISE versus SCE) for three stage flotation was found to be −600 mV. Sulphidization time at −600 mV over the range 1–5 min had little effect on oxide lead recovery, but 3 min was the optimum. Flotation time was important and three 10 min stages (total 30 min) were needed for satisfactory lead recovery. Decant washing before sulphidization gave a further improvement in metal-lurgy and a decrease in Na 2 S consumption. Excess S 2− in solution displaced xanthate from mineral surfaces, and the amount displaced varied with E S . At E S values more negative than −600 mV, maximum displacement occurred. Xanthate residuals up to 50 mg/l were found in the third sulphidization/flotation stage. Further investigation of xanthate residuals is needed. It was concluded that a sulphide ISE would be invaluable in optimizing sulphidizing conditions, especially when flotation feed character was variable.

Comparison of methods of gold and silver extraction from Hellyer pyrite and lead-zinc flotation middlings

The Hellyer massive sulphide deposit in north-west Tasmania, Australia, which is a complex fine-grained Cu-Pb-Zn-Au-Ag-pyrite-arsenopyrite orebody, has recently been brought into production. Various base metal flotation concentrates are produced for sale. However, a substantial proportion of the gold and silver report in flotation middlings or tailings and is currently not recovered. Research into a number of methods for extraction of this gold and silver was conducted on various plant products, which had different mineralogy, and which assayed 1.5–3.3 g/t Au and 27–144 g/t Ag, as well as 0.7–8.0% Zn and 1.1–11.4% Pb. Product sizings were 90% minus 38 ¼.m.

Formation and recognition of alkyl xanthyl thiosulphates in sulphide ore flotation liquors

Abstract Alkyl xanthyl thiosulphates (R.OCSS.S2O3−) (RXT−) are formed in solution by mild oxidation (e.g. by I2) of solutions containing both xanthate and thiosulphate. They can also be formed by reaction of Cu2+ with xanthate and thiosulphate, reaction of dixanthogen with thiosulphate, and by reaction of xanthate with tetrathionate; these last three reactions can occur in flotation pulps in slightly acid or alkaline solutions (pH 5–10). Alkyl xanthyl thiosulphates are stable in acid and neutral solution; the solutions have a UV absorption maximum at 289 nm. In strongly alkaline solution (pH 12) RXT− decomposes within a few minutes to yield a xanthate (mostly) plus a little perxanthate. At pH 10 this decomposition to xanthate takes about 48 h. At pH 7–9 RXT− is relatively stable. RXT− is not extracted from aqueous solution with common solvents (chloroform, iso-octane, cyclohexane, or ether). It forms a water-insoluble adduct with cetyltrimethyl-ammonium bromide (CTAB); this adduct can be extracted into chloroform, and the extract has a UV absorption maximum at 296 nm. RXT− was found in solutions from the gangue-sulphide flotation section at Renison Ltd, the zinc flotation circuit and the copper flotation circuit at Mount Isa Mines Ltd, and the lead flotation section of The Zinc Corporation Ltd. The presence of RXT− in operating flotation plants has various practical and theoretical implications.

Investigation of silver extraction from a silver-sulphur flotation concentrate from the electrolytic zinc plant of Companhia Paraibuna de Metais, Juiz de Fora, Brazil

A silver-sulphur flotation concentrate produced from the hot acid leach residue in the electrolytic zinc plant of Companhia Paraibuna de Metais was investigated for hydro-metallurgical silver extraction. This concentrate assayed 8320 g/t Ag, 20.6% Pb, and 43.7% S. It consisted mainly of lead sulphate and elemental sulphur with numerous minor components. The form of the silver was not precisely established and several forms appeared to be present.