Mercury and Selenium recovery from Dust flues of Zinc Refineries

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By: Adolfo Rios Pita G.

 

Spanish-Espanol

Thornhill (USA 1910) applied a wet extraction method on cinnabar ores. This method consisted in leaching cinnabar concentrates with sodium sulphide and sodium hydroxide.

Zinc refineries roast spharelite ( Zinc sulphide ) to obtain a calcine of leachable zinc oxides. In this process, where fluid bed kilns are used for this calcination dusts are produced that contain variable amounts of mercury , selenium and other minor metals. These dusts are removed in cottrell dust collectors and a gas washing tower.

In 1987, when we were working at the Zinc Refinery of Cajamarquilla ( located near Lima City, capital of Peru, South America), these flues (actually wet muds) were stocked untreated.

At that time the owner of this plant was Minero Peru, the state owned mining enterprise, owner also of a gold mine where mercury was needed for gold amalgamation and we thought we could apply Thorhill's method to these muds.

We succeded in recovering almost all the mercury and selenium from these muds.

TREATMENT

 

We assumed that part of mercury metal was in these dust flues as HgSe and the rest of it as elemental mercury, so we applied the following reactions:  

 

Previous treatment (Temperature: Room,)

a)HgSe + Na2S = Na2Hg.Se.S

Mercury extraction

b) 3Na2Hg.Se.S + 8NaOH + 2Al + 4 H2O = 3Hg� + 2Na ( Al ( OH )4 + ( OH2 )2 ) + 3Na2S + 3Na2Se

Selenium extraction

c) Na2S + Na2Se + 2H2SO4 = 2Na2SO4 + H2S + H2 + Se�

Silver ( ~ 60 oz / mt ) and lead precipitate with mercury and they are separated when washing mercury with dilute nitric acid.

Procedure: In practice what is really done is leaching wet mud with 1:1 Na2S and 1:0.25 NaOH ( weighed on a dry basis ), about an hour at room temperature and with proper agitation; then discarded aluminum cathodes sheets are inmersed in this pulp. Reaction is exothermic and violent, but can be controlled through varying the depth of aluminum sheets inmersion. Reaction reaches a maximum temperature of 92 degrees centigrade. Al sheets are left overnight and the day after mercury is separated from the mother liquor.

Mother liquor is then treated following equation "c" until pH lowers to about 3.0. In this step H2S gas evolves and in the case that instead of using sodium sulphide we use only sodium hydroxide H2Se would be produced.