Leaching of cinnabar with HCl-thiourea solutions as the basis of a process for mercury obtention
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NH +
f I C--S--S--C tl tl
NH
HgC12 " 2CS(NH2)2 + H2S
) 2CS(NH2)2 + S~ + 2H +
NH
No change was found in leaching power after four complete cycles in laboratory-scale experiments. The single byproduct is elemental sulfur.
I.
INTRODUCTION
IN recent papers L2 the kinetics of cinnabar leaching in acid complex-forming reagents like HC1 and KI, HC1 and KBr and HBr aqueous solutions have been discussed. Since the pK values of HgI4 and HgBr4 are, respectively, 30 and 22, it may be assumed that, if a ligand which forms a complex with a mercury ion with a pK higher than 22 is present in strongly acid media, dissolution of cinnabar may take place. Of the different organic ligands which form complexes with a high pK value, thiourea appears to be one of the most interesting. It forms HgC12 9 2SC(NH2)2 with a pK equal to 24. 3 Metal-thiourea complexes are usually prepared by the Kurnakov method. 4 Complex formation during leaching of the precious metals with thiourea solutions is described by many authors. 5'6'7 In the case of mercury, given the high pK value of HgC12 9 2SC(NH2)2, complex formation should be possible. Further studies on the possibility of an extraction method for mercury from cinnabar ore by leaching with strongly acid thiourea solutions are useful only where there is the possibility of recycling thiourea, such as is the case when using hydrochloric acid-iodide. 8'9 The reaction between cinnabar and thiourea in hydrochloric acid solutions is: HgS + 2HC1 + 2SC(NH2)2 --~ HgCI2 9 2SC(NH2)2 + H2S [1] / -
C. NUNEZ, Professor and Director, F. ESPIELL, Professor, and M. CRUELLS, Assistant Professor, are with the Departmentof Metallurgy, University of Barcelona, 08028 Barcelona, Spain. Manuscript submittedJanuary4, 1985.
METALLURGICALTRANSACTIONSB
The HzS generated prevents chemical degradation of the reagent and passivation of cinnabar. The solubility of HgC12 9 2SC(NH2)2 depends on the temperature and the HC1 and thiourea concentrations. It may be precipitated from leaching solutions by cooling or addition of HC1 gas if necessary. The purpose of this work is to study the kinetics of reaction [1] and to investigate the possibility of a hydrometallurgical process for obtaining mercury based on the leaching reaction mentioned above.
II.
MATERIALS AND PROCEDURE
In the leaching kinetics study, spheres of various sizes carved from pure cinnabar ore from Almad6n (Spain) were used. The chemical analysis of the ore sample is shown in Table I. Inclusions of small amounts of sili'ca crystals ( 2C--SH
+ S ~ + 2H ยง
II
NH
10 g/I
'U
NH2
+ H2S
9
U
NH
[6] In order to propose a process for obtaining mercury by electrolysis from ore leaching solutions with HCl-thiourea, we have studied the determination of certain basic parameters, such as the potential to apply and the current efficiency. Experiments were performed in a 250 cm 3 magnetically stirred open electrolytic cell. The temperature of the electrolyte was kept at 25 ~ The cathode was made of copper with mercury surface amalgamation; the anode was graphite
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