APPLICATION OF ADVANCED WATER TREATMENT TECHNOLOGY
IN A PRECIOUS METAL REFINERY
Louis J. Kosarek, Director
Trace Metal Data Institute
El Paso, Texas 79912
Parallel with increased secondary recovery of silver values from by-products, discarded
metalware, and scrap, hydrometallurgical circuits utilizing aqueous chemistry are being
developed as based upon economics and pollution control regulations. Concurrent with the
utilization of these precious metal recovery circuits, specifically silver, the relatively new
technologies of ion exchange, reverse osmosis, and other membrane technologies are being
applied along with the previously used approaches such as: chemical precipitation and
cementing to ultimately minimize pollution discharge by optimizing water recycle. The
emphasis of this report is to incorporate technology and flow logistics in a manner which
improves production and metal recovery and minimizes the required capital output for
strictly pollution control methodology.
SILVER CHEMISTRY
Ionic Silver
Silver values contained within secondary sources can be dissolved by the use of nitric
acid. Nitric acid will dissolve silver as a monovalent cation (Ag ). Other acids such as sulfuric
or hydrochloric will not significantly dissolve silver because cool sulfuric acid dissolution
will result in the formation of a silver sulfate precipitate while hydrochloric acid will cause
the formation of silver chloride precipitate [ 1 ]. An aqueous solution which is to be used for
dissolution of silver by nitric acid must be devoid of certain halides because the halides will
cause the formation of the white, gelatenous, silver chloride; the creamy silver bromide; or
the yellow silver iodide [2]. The solubility of the most common silver halide. silver chloride,
is governed by a solubility product of 1.5 x 1CT10 which results in a soluble silver level of
1.4 mg/1 [3]. Because of the relative insolubility of silver chloride, the pH of a silver nitrate
solution can be adjusted using hydrous ammonia but cannot be adjusted using caustic because of residual chloride contamination [4]. Cationic silver in solution will precipitate as
the hydroxide species at a pH above 9.0. The level of silver within a nitric acid bath can be
determined volumetrically using a thiocynate titration [5] or using atomic absorption.
Complexed Silver
Silver can be dissolved into solution by the use of complexing agents [2). The primary
complexing agents used to dissolve silver are cyanide and thiosulfate but other agents such
as: ammonia, sulfite, and EDTA will also aid in the dissolution of silver [6]. Aqueous silver
cyanide (Ag(CN)2_) is a divalent anion which retains a linear symmetry [7]. Aqueous silver
thiosulfate (Ag(S203)2~3) is predominantly a trivalent anion, can be present as a higher valence anion, and retains a square planar configuration (6,71. Both silver cyanide and silver
thiosulfate are very soluble [3], are not precipitated in the presence of halides or thiocynate
[6], and can both be precipitated as the sulfide species [8,9). The stability of the cyanide
and thiosulfate complexes of silver are limited in pH wherein the cyanides will be converted
to toxic hydrogen cyanide gas at a pH below 10.5 [10] and the thiosulfates will decompose
to elemental sulfur and sulfur dioxide and subsequently sulfite plus sulfate at a pH below
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