The self-heating of sulfides: Galvanic effects

Under certain conditions of moisture and oxygen, sulfides can spontaneously heat or self-heat. In this paper, we test a hypothesis that galvanic interaction between some sulfides can promote self-heating. Using a self-heating test procedure, four cases were examined. Case 1 (specimen sulfides) and case 2 (concentrates) comprised four sulfides: pyrite, chalcopyrite, sphalerite and galena. They were tested individually and as 50:50 weight mixtures. The individual samples and mixtures of low rest-potential difference did not self-heat but mixtures of high rest-potential difference did. In case 3, pyrite was mixed 50:50 with two materials that self-heated alone, pyrrhotite and nickel concentrate, and self-heating further increased. For case 4, mixtures of pyrite–sphalerite were tested as a function of particle size. Self-heating increased as particle size decreased and it was the fineness of the pyrite (high rest-potential or cathodic mineral) that governed the response. The increase in self-heating with high rest-potential difference and increasing particle fineness of cathodic mineral supports that galvanic interaction can contribute to sulfide self-heating. A possible mechanism based on the hydrogen sulfide hypothesis is proposed where the reduction reaction on the cathodic mineral surface is ferric to ferrous. The understanding gained will be of interest to those involved in mining, storage, shipping and disposal of sulfide minerals.

Graphical abstractFigure shows that individual sulphides, pyrite (Py), chalcopyrite (Cp), galena (Gn) and sphalerite (Sp) did not self-heat (report to zone 1 on self-heating risk potential chart) while mixtures did when rest-potential difference (ΔE) exceeded ca. 0.2 V. Case 1 refers to specimen minerals, case 2 to concentrates. This provides evidence for galvanic interactions contributing to self-heating.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Mixtures of sulfide minerals can self-heat, attributed to galvanic effects. ► Rest-potential difference and fineness of the cathodic mineral are key factors. ► Pyrite is common cathodic mineral thus galvanic-driven self-heating may be common. ► Oxidation–reduction cycle promoting H2S generation offered as explanation.