Sodium ethyl xanthate

Production

Xanthates are prepared by the action of metal alkoxides on carbon disulfide:

CH₃CH₂ONa + CS₂ → CH₃CH₂OCS₂Na

Properties

Sodium ethyl xanthate is a pale yellow powder. It is relatively stable at high pH and rapidly hydrolyses at pH <9 at 25 °C. It is the conjugate base of the unknown strong acid with pK_a of 1.6 and pK_b estimated as 12.4 for the conjugate base. Sodium ethyl xanthate easily adsorbs on the surface of solid sulfides.

Decomposition in water

Sodium ethyl xanthate decomposes in water increases with decreasing pH and with increasing temperature. Three pathways are proposed:

A. Dissociation into xanthic acid and then to carbon disulfide and alcohol.

C₂H₅OCS₂Na + H₂O → C₂H₅OCS₂H + NaOH C₂H₅OCS₂H → CS₂ + C₂H₅OH

B. Oxidation to dixanthogen.

2 C₂H₅OCS₂⁻ + H₂O + 1/2O₂ → (C₂H₅OCS₂)₂ + 2 OH⁻

C. Hydrolytic decomposition

6 C₂H₅OCS₂⁻ + 3 H₂O → 6 C₂H₅OH + CO₃²⁻ + 3 CS₂ + 2 CS₃²⁻

Reactions A and B are minor and require acidic conditions. Reaction C proceeds in neutral or alkaline pH and is self-accelerating, as it is catalysed by the alcohol formed as a product. Its rate increases with concentration of the reagents and with temperature, from 1.1%/day at 20 °C to 4.6%/day at 40 °C for a 10% solution at pH=10. A decrease in pH from 10 to 6.5 increases the decomposition rate from 1.1%/day to 16%/day. Decomposition is also accelerated by the presence of metals, such as copper, iron, lead or zinc, which act as a catalyst.

Detection

Sodium ethyl xanthate can be identified through optical absorption peaks in the infrared (1179, 1160, 1115, 1085 cm⁻¹) and ultraviolet (300 nm) ranges. There are at least six chemical detection methods:

1. Iodometric method relies on oxidation to dixanthogen by iodine, with the product detected with a starch indicator. This method is however is not selective and suffers from interferences with other sulfur-containing chemicals.
2. Xanthate can be reacted with a copper sulfate or copper tartrate resulting in a copper xanthate residue which is detected with iodine. This method has an advantage of being is insensitive to sulfite, thiosulfate and carbonate impurities.
3. In the acid-base detection method, a dilute aqueous xanthate solution is reacted with a copious amount of 0.01 M hydrochloric acid yielding carbon disulfide and alcohol, which are evaluated. The excess acid and impurities are removed through filtering and titration.
4. In the argentometric method, sodium ethyl xanthate is reacted with silver nitrate in a dilute solution. The resulted silver xanthate is detected with 10% aqueous solution of iron nitrate. The drawbacks of this method are high cost of silver and blackening of silver xanthate by silver nitrate that reduces the detection accuracy.
5. In the mercurimetric method, xanthate is dissolved in 40% aqueous solution of dimethylamine, followed by heating and titration with O-hydroxymercuribenzoate. The product is detected with thiofluorescein.
6. Perchloric acid method involves dissolution of xanthate in water-free acetic acid. The product is titrated with perchloric acid and detected with crystal violet.

Sodium ethyl xanthate can also be quantified using gravimetry, by weighing the lead xanthate residue obtained after reacting SEX with 10% solution of lead nitrate. There are also several electrochemical detection methods, which can be combined with some of the above chemical techniques.

Applications

Sodium ethyl xanthate is predominantly used in the mining industry as flotation agent for recovery of metals, such as copper, nickel, silver or gold, as well as solid metal sulfides or oxides from ore slurries. This application was introduced by Cornelius H. Keller in 1925. Other applications include defoliant, herbicide and an additive to rubber to protect it against oxygen and ozone.

The mechanism of flotation enhancement is as follows. The polar part of xanthate molecule attaches to the ore particles with the non-polar hydrocarbon part sticking out and forming a hydrophobic layer. Then the particles are brought to the water surface by air bubbles. Only a small amount of about 300 g/tonne of ore is required for efficient separation. The efficiency of the hydrophobic action increases, but the selectivity to ore type decreases with increasing length of the hydrocarbon chain in xanthates. The chain is shortest in sodium ethyl xanthate that makes it highly selective to copper, nickel, lead, gold and zinc ores. Aqueous solutions (10%) with pH=7–11 are normally used in the process.

In 2000, Australia produced up to 10,000 tonnes of sodium ethyl xanthate and imported about 6,000 tonnes, mostly from China. The material produced in Australia is the so-called 'liquid sodium ethyl xanthate' that refers to a 40% aqueous solution of the solid. It is obtained by reacting carbon disulfide with sodium hydroxide and ethanol in a closed process. Its density is 1.2 g/cm³ and the freezing point is −6 °C.

Safety

Sodium ethyl xanthate has moderate oral and dermal toxicity in animals and is irritating to eyes and skin. It is especially toxic to aquatic life and therefore its disposal is strictly controlled. Median lethal dose for (male albino mice, oral, 10% solution at pH~11) is 730 mg/kg of body weight, with most deaths occurring in the first day. The most affected organs were the central nervous system, liver and spleen.

Since 1993, sodium ethyl xanthate is classified as a Priority Existing Chemical in Australia, meaning that its manufacture, handling, storage, use or disposal may result in adverse health or environment effects. This decision was justified by the widespread use of the chemical in industry and its decomposition to the toxic and flammable carbon disulfide gas. From two examples of sodium ethyl xanthate spillage in Australia, one resulted in evacuation of 100 people and hospitalization of 6 workers who were exposed to the fumes. In another accident, residents of the spillage area complained of headache, dizziness and nausea. Consequently, during high-risk sodium ethyl xanthate handling operations, workers are required by the Australian regulations to be equipped with protective clothing, anti-static gloves, boots and full-face respirators or self-contained breathing apparatus.