Part VIII – August 1969 – Papers - The Hydrogen Reduction of Copper, Nickel, Cobalt, and Iron Sulfides and the Formation of Filamentary Metal

It has been shown that hydrogen may be made to serve as a rapid and eflicient reducing agent for Cu, Ni, Co, and Fe sulfides if a scavenging agent for hydrogen sulfide is intimately mixed with the sulfide particles being reduced. Accelerated reduction kinetics are demonstrated for nickel sulfide. Copper, nickel, and cobalt sulfides, when treated at certain temperatures in a combined reducing agent-scavenging agent system, are converted to voluminous masses of fibrous metal product. Studies have been carried out to determine the conditions which lead, on the one hand, to irregular poly crystalline fibers and, on the other, to long single crystal filaments a few microns in diameter. A mechanism is proposed to account for the formation of single crystal filuments. The sulfide minerals of Cu, Ni, Co, and Fe are an important source of these metals yet there has been comparatively little scientific effort devoted towards understanding reduction mechanisms of these minerals. This may be, in part, due to the fact that the most convenient reducing agents for carrying out such studies, viz., hydrogen and carbon, do not react appreciably with sulfides. We have found that the reaction of hydrogen with metal sulfides can be markedly accelerated by placing a scavenging agent for hydrogen sulfide in close proximity to the metal sulfide. A brief series of experiments demonstrating relative reduction rates is reported in this paper to illustrate the effect. With the reduction process thus accelerated we have observed an unusual type of reduction behavior on some of the sulfides investigated. Under certain conditions the metallic product of the reduction reaction takes the form of filaments growing outward from the sulfide particles. The present paper deals largely with efforts to classify the various types of growth forms observed. This study has shown that filamentary growths from sulfides take a much greater variety of forms than has heretofore been reported by Ercker,1 Hardy,2 and Nabarro and Jackson3 in their reviews of metallic growths from copper and silver sulfides. THERMODYNAMIC CONSIDERATIONS The thermodynamics for hydrogen reduction of metal sulfides is quite unfavorable. For the sulfides considered here equilibrium constants typically range from 10-3 to 10-5. These low equilibrium constants impose severe kinetic limitations on reduction since hydrogen sulfide must be transported out of the system at concentrations of only a few hundred ppm. Unless extremely high gas flow rates are employed the atmosphere surrounding any sulfide particle will always be essentially in equilibrium with the sulfide. If, however, one places an efficient scavenging agent for hydrogen sulfide in close proximity to the metal sulfide particles the concentration of H2S near the metal sulfide will be held to a very low value. This would permit the reduction reaction to proceed with little or no inhibition from a buildup of reaction product gas. It is well known that calcium oxide is capable of removing hydrogen sulfide from a hydrogen gas stream of low dew point.4 If a sufficient quantity of calcium oxide is mixed with the metal sulfide particles the reaction: CaO+H2S=CaS+ H2O [l] will substitute moisture in place of hydrogen sulfide in the gas stream and this will not affect, in a direct manner, the reaction: MeS +H2=Me + H2S [2] A convenient method of considering the thermodynamics of the combined reducing agent-scavenging agent system is to consider the atmosphere when the partial pressure of hydrogen sulfide is the same over both the metal sulfide and the scavenging agent, i.e., pH2S (1) =pH2S (2). As a consequence: pH2O (1) pH2(2) =K1K2 The chemical driving force for reduction will depend inversely upon the moisture content of the gas and will be 0 when, in the system, pH2O = pH2.K1K2. Table I lists values of the equilibrium constants for reduction and H2S scavenging reactions for a number of sulfides at several temperatures. Data are taken from Rosenqvist4,5 and Kelly.6 The equilibrium constant products calculated from this data show that the limiting level of gaseous reaction product has been increased by a factor of 10' to l04 as a result of substituting a reducing agent-scavenging agent system for a simple reducing agent system. One possible side effect which must be considered is the possibility that the moisture evolved in the scavenging reaction might cause the atmosphere in the system to be sufficiently oxidizing to favor the formation of oxide rather than metal. This possibility was examined by comparing the equilibrium constant products listed in Table I with equilibrium constants for hydrogen reduction of the respective metal oxides. It was found that for copper, nickel, and cobalt the combined reduction-scavenging reactions could not develop a sufficiently high oxidizing potential in the