Precipitate Flotation of
Complexed Cyanide
ROBERT B. GRIEVES, Professor and Chairman
DIBAKAR BHATTACHARYYA, Research Associate
Chemical Engineering Department
University of Kentucky
Lexington, Kentucky
INTRODUCTION
Inorganic precipitates can be floated to the surface of a suspension and physically removed in a foam rising above the suspension. A surface-active agent of
opposite charge to the precipitate is adsorbed on the surfaces making the particulates
hydrophobic and susceptible to gas bubble attachment. The initial charge of the
precipitate (and thus the type of surfactant to be used) is determined by exchange of
ions from solution with ions of the outermost layers of the crystal and/or with ions of
the interior, especially interstitial ions and "vacancies" (1). The ion preferentially
exchanged is generally the constituent ion of the precipitate present in excess in the
solution phase. Coordination between ionic species present in the precipitate and the
surfactant may be significant and can explain the flotation of a precipitate by an
ionic surfactant of the same charge (2). A single surfactant can be utilized as both a
collector and a frother.
Research on precipitate flotation has been limited to the past decade, beginning
with that of Skrylev and Mokrushin (3) and that of Baarson and Ray (4). A number
of studies have been carried out (5,6,7,8), including an excellent group of papers by
Pinfold and Mahne (9,10,11,12,13,14) and another by Rubin, et al. (2,15,16).
A series of investigations has been conducted of the foam separation of cyanide
complexed by ferrous iron. With a molar iron to cyanide ratio of 0.206 mole Fe/mole
CN, the soluble ferrocyanide ion reacts with a cationic surfactant to form a surface-
active, particulate complex which is readily floatable at pH 7. Upon increasing the
iron to cyanide ratio to 0.351, colloidal particulates are immediately formed
(Prussian blue), probably consisting of polynucleated [FeFe(CN)6]" and [FeFe
(CN)6]2-, which can be floated more efficiently than [Fe(CN)g]4-, considering the
quantities of ferrous iron and of surfactant that are required. With both Fe/CN ratios,
the effects have been investigated of pH, of initial cyanide concentration, and of
initial surfactant concentration (17,18). Complexed cyanide has been included in a
discussion of the use of foam separation as a technique for qualitative analysis (19).
A series of rate studies has been made, indicating that the rate of removal of
[FeFe(CN)g]2- can be fit by an approximately first order, reversible rate expression
(17).
Upon increasing the Fe/CN ratio to 0.50, particulates become visible to the
naked eye. With a constant initial surfactant concentration of 0.132 mM (1.32 x
10_4 molar), 98 per cent flotation of complexed cyanide can be achieved from an
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