Three-Dimensional Predominance Volume Diagrams: The Ni-As-S-O System
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TION
THE pryometallurgical processing of sulfide minerals involves either the roasting to the sulfate or oxide for aqueous dissolution and recovery by electrowinning or oxidation to the oxide followed by carbothermic reduction. The roasting thermochemistry of individual sulfides is visualized with the use of two-dimensional predominance area diagrams (PADs) also known as Kellogg diagrams[1] and roaster diagrams.[2] The presence of arsenic in sulfide ores adds to the complexity of roasting since it forms compounds and volatile gases. The cost of processing arsenic-containing deposits may be offset by the presence of associated precious metals.[3–8] Examples of arsenic-containing minerals include cobaltite (CoAsS), nickelite (NiAs), gersdorffite
STANLEY M. HOWARD is with the Department of Materials and Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701. Contact e-mail: [email protected] Manuscript submitted October 20, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
(NiAsS), and enargite (Cu3AsS4), which has received much attention from Safarzadeh, Moats, and Miller.[9] Roaster diagrams and PADs, such as those by Safarzadeh and Howard,[2] cannot adequately describe roasting behavior in M-As-S-O systems, but three-dimensional predominance volume diagrams (PVDs) can do so by providing a means of visualizing phase equilibria encountered during the roasting of arsenic-bearing sulfides. Recently, Safarzadeh and Howard[10] and Safarzadeh, Miller and Howard[11] showed how PVDs are used to visualize roasting processes in the Cu-As-S-O system. The Predominance Volume Diagram Calculator software for generating the PVDs used in these papers has been under development for some time with the plan to make it available more widely to those having an interest in the subject. Compared with preceding papers this paper provides a more detailed description of the methodology of computing a PVD, of relationships PVDs share with ternary and quaternary phase diagrams, and of the reaction path suggested by the PVD and an associated isobaric surface.
II.
PVD GEOMETRICAL RELATIONSHIPS
The degrees of freedom in a chemical system may be determined by the Gibbs Phase Rule given as
F ¼ CP þ 2 where
C ¼ ðnumber of componentsÞ ðnumber of independent reactionsÞ ðnumber of independent restricting equationsÞ ½2
P ¼ ðnumber of phasesÞ
½3
In a four-component system such as (M, As, S, O) at fixed temperature C ¼ ðM; As; S; OÞ4 ðnoneÞ0 ðT ¼ fixedÞ1 þ B 1þB ¼ 3B ½4 where the subscript denotes the value of each quantity within the parentheses and B equals the number of diagram boundaries invoked (each one an independent restricting equation). Therefore, if a gas phase is always assumed present, as for predominance diagrams, F ¼ 4 ðPCond þ BÞ
COMPUTATION
Every PVD consists of an interconnected network of line segments connecting the invariant points. Each point (invariant) on the PVD, except for the corners, has four line segments radiating from it; the corners have three. Initially,
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