New hydrometallurgical process for obtaining mercury from cinnabar ore
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I.
INTRODUCTION
THEkinetic study of cinnabar leaching in aqueous hydrochloric acid-potassium iodide solutions and the basic reactions in a new hydrometallurgical process for recovering mercury and obtaining sulfur in the elemental state in a closed circuit solution were considered in a previous paper, l The three reactions on which the process is based are: In the leaching: HgS + 4 I- + 2 H + Hgh= + H2S
[1]
The H2S generated may be removed by an air flow to minimize its concentration in the leaching solution; thus the equilibrium is shifted to the right. The HgI4- solution is treated electrolytically to obtain metallic mercury on the cathode and iodine on the anode: HgL= ~ H~ + I3 + I-
[2]
A small concentration of free iodide ion must be present to prevent HgI2 formation on the anode and solubilize the iodine. HgI2 + 2 I- ~ HgI}
[3]
12 + I- ~ I 3
[4]
The iodine solution reacts with the hydrogen sulfide resulting from the leaching reaction: I3 + H 2 S ~ S ~ + 2 H + + 3 I -
[5]
Through this reaction, the leaching solution returns to the same concentrations as at the beginning and may be recycled. Reactions [1] and [5] may take place at the same time during leaching. Thus the overall reaction is: HgS + I3 + I- Hcl) S ~ + HgI4
[6]
This makes regeneration of the leaching solution unnecessary.
C. NI]IqEZ, Head of Department of Metallurgy, and F. ESPIELL, Assistant Professor, Department of Metallurgy, are with the Facultad de Ciencias Quimicas, Universidad de Barcelona, Av. Diagonal 645, Barcelona-28, Spain. Manuscript submitted July 18, 1983. METALLURGICALTRANSACTIONS B
Reactions [1] and [6] are very fast at room temperature at concentrations of 4 to 5M for HC1 and 0.5 to 0.8M for total iodine in solution. HgI4 has a much larger formation constant than any other iodo or chloro complex of mercuricion. Equations [1] to [6] have therefore been written in terms of this species. No formation of elemental sulfur or iodine was observed on leaching cinnabar concentrates with reaction [ 1] and with high air flows bubbled into the liquid. The possibility of sulfide or iodide ion oxidation: 4 H + + 2 S- +
0 2 -"')
2 H 2 0 + 2 S~
1
2 H + + 2 I- + -~- O2---> I2 + H20
[7] [8]
is practically nonexistent because at high hydrochloric acid activities, like the ones used here, the concentration of sulfide ion in the liquid is greatly reduced. At the same time, a fast air flow removes the sulfhydric acid from the liquid quickly, and its concentration becomes practically nil. If some iodide ions are oxidized to iodine by the air flow, the latter will disappear according to the reaction [6] which may take place at the same time as reaction [1]. No significant differences have been observed between reacted fraction against time curves obtained in the same experimental conditions by using an air flow in the first case and a nitrogen flow in the second one. In industrial practice, the gas flow may be eliminated by working in the leaching stage with reaction [6]. In this paper a laboratory-scale study was made for reactions
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