Adsorption of guar gum and CMC on pyrite

Abstract

The depression of pyrite in sulfide flotation by two types of polysaccharides, CMC and guar, was investigated using micro-flotation tests, zeta potential measurements and adsorption tests. Environmentally and from an occupational safety standpoint – this would be a better processing option than cyanide. The range of variables tested included molecular weight of guar, degree of substitution of CMC, pH and the concentration of calcium ions in the flotation process water. The guar gums depressed pyrite considerably even at low dosages. An optimum dosage of 10 ppm was recommended. The CMC’s were not as effective as the guar gums, and required much higher dosages to achieve pyrite depression. Within the range of CMC’s tested, lower degree of substitution was more effective than high degree of substitution. This was presumably due to lower electrostatic repulsion between the negatively charged CMC and pyrite. When calcium ions were introduced with CMC’s, the adsorption of the CMC was considerably enhanced. No such effect was observed for guar gums. Both CMC and guar gums demonstrated Langmuirian behavior in their adsorption isotherms. Two different adsorption mechanisms were displayed. The guar gum adsorption was attributed to hydrogen bonding and Bronsted acid–base interaction. The CMC was considered to occur through electrostatic interaction in the presence of calcium ions, and Bronsted acid–base interaction depending on pH.

Introduction

Pyrite is the most widespread and abundant of naturally occurring metal sulfides. It is commonly present in base metal sulfide bearing ores and frequently appears in coal as a major source of sulfur in coal. Pyrite lowers the quality of base metal concentrates and increases the amount of sulfur compounds produced in the base metal extraction processes. Therefore, depression of pyrite is essential in the concentration of base metal sulfides by flotation. This can be achieved by floating in alkaline solutions using highly selective inorganic modifiers such as cyanides, sulfites, ferro-cyanides, etc. However, the use of inorganic depressants have raised concerns on environmental grounds. Hence, natural, bio-degradable, non-toxic agents such as polysaccharides are worth investigating as alternative selective depressants. Kydros et al. (1994) have reported separation of pyrite/sphalerite mixtures with copper sulphate, ethyl xanthate and white dextrin. Valdivieso et al. (2004) showed that dextrin can be as an effective depressant of pyrite as cyanide.

Polysaccharides are a promising class of strong depressants that could replace the highly toxic inorganic modifiers in the differential flotation of sulfide minerals. However, although they have been reported as selective depressants in the differential flotation of sulfide minerals (Laskowski et al., 2000) a general lack of understanding of the interaction mechanism between the polysaccharides and sulfide mineral surfaces hinders the development in this area. It has been reported that these polymers adsorb through interactions with metal-hydroxy species on the mineral surfaces (Laskowski and Liu, 1999a, Laskowski and Liu, 1999b) but other mechanisms such as hydrogen bonding, hydrophobic bonding, electrostatic interaction have also been proposed (Steenberg and Harris, 1984, Miller et al., 1983, Rath et al., 1997). Therefore, the differences in the pH ranges for metal hydroxylation, and differences in the affinities of polysaccharides towards hydroxyl species of different metals, may result in selective adsorption and subsequent flotation.

In this study, adsorption behavior of two types of polysaccharides, guar gum and carboxymethyl cellulose (CMC) having different chemical and structural properties are investigated using micro-flotation tests, zeta potential measurements and adsorption tests. In these experiments the effects of depressant type and dosage, pH, oxidation and the presence of Ca²⁺ ions on flotation behavior of pyrite were investigated.