Frank J. Lincoln

Mechanochemical milling-induced reactions between gases and sulfide minerals. I. Reactions of SO2 with arsenopyrite, pyrrhotite and pyrite

Abstract The mechanochemical milling of arsenopyrite, pyrite and pyrrhotite in the presence of SO 2 has demonstrated the potential for inducing gas/solid reactions at low temperature. Both reactants and milled products were characterised by powder X-ray diffractometry and by scanning electron microscopy of polished sections. The gas compositions were determined by mass spectrometry. Milling in an atmosphere of SO 2 results in the conversion of arsenopyrite to pyrite via an intermediate pyrrhotite phase. Milling pyrite or pyrrhotite results in a continuous cyclic process where pyrite decomposes to pyrrhotite, which then undergoes resulfurisation back to pyrite.

The decomposition of monazite by mechanical milling with calcium oxide and calcium chloride

Abstract The decomposition of monazite mineral with calcium oxide and calcium chloride during mechanical milling has been investigated. Various milling times, gaseous atmospheres and charge ratios were tested to optimize the reaction conditions. Powder X-ray diffractometry, scanning electron microscopy and energy dispersive spectrometry were used to characterize the decomposition products. It has been found that the monazite was effectively decomposed in 12 h, under an inert argon atmosphere with a 15:1 charge ratio, producing rare earth element oxychloride and oxide, calcium chlorophosphate (chlorapatite) and thorium dioxide. About 95% of the monazite milled under these conditions may be extracted into 6 M nitric acid solution. Lowering the charge ratio or changing the milling atmosphere to air slows down the reaction rate and reduces the dissolution fraction. When sufficient air (oxygen) was present in the vial during milling, a cerium and thorium dioxide solid solution was formed, which is detrimental to acid leach.

Mechanical milling of auriferous arsenopyrite and pyrite

Abstract The behaviour of pyrite and arsenopyrite in the process of mechanical milling and its potential for exsolution of gold were examined. Both reactants and milled products were characterised by powder X-ray diffractometry and by scanning electron microscopy of polished sections. In the presence of pyrite, arsenic-rich arsenopyrite can readily be transformed to sulphur-rich arsenopyrite, a process which does not readily occur by heating methods. The transformation mechanism can be explained by relating it to similar structural relationships between pyrite, marcasite and arsenopyrite. Detailed structural refinement of powder XRD data by the Rietveld method confirmed changes in unit cell parameters of arsenopyrite as a result of changes in stoichiometry. While the milling process may possibly liberate gold from the sulphide structures, the fracturing and welding processes occurring during milling can also potentially lock the gold up in the large aggregate arsenopyrite.

The decomposition of NdPO4 during mechanical milling

Abstract The decomposition reactions of neodymium phosphate, NdPO 4 , during mechanical milling (MM) have been studied. It has been found that under optimum conditions, NaOH, Ca(OH) 2 , CaO with CaCl 2 , and CaO with Ca(OH) 2 decomposed NdPO 4 completely, producing respectively Nd(OH) 3 with Na 3 PO 4 , Nd(OH) 3 with Ca 5 (PO 4 ) 3 (OH), NdOCl with Ca 5 (PO 4 ) 3 Cl and Nd 2 O 3 with Ca 5 (PO 4 ) 3 (OH). CaO by itself caused partial decomposition of NdPO 4 to Nd 2 O 3 , with the formation of calcium neodymium phosphate oxide, Ca 8 Nd 2 (PO 4 ) 6 O 2 . No chemical reactions occurred between NdPO 4 and CaCl 2 , NaCl or Na 2 CO 3 during milling. Powder X-ray diffractometry and transmission electron microscopy have been used to characterize the decomposition products. The Gibbs free-energy changes of all the reactions under study are calculated. It appears that the free-energy changes at room temperature (25 °C) and ambient pressure (1 atm) are crucial in determining whether reactions can occur during MM.

Weathered Ilmenite: Diverse Mechanisms of Sintering and Association with Contaminants

We briefly review the nature and provenance of extremely weathered ilmenite, then investigate its fate when it is 10 % of a blended feed in the reduction kilns of the Becher process. There it is bound, unintentionally and intermittently, in sinter, which is discarded. It takes with it discrete aluminosilicate grains as well as contaminants, adsorbed and occluded within the ilmenite. Two distinct sintering mechanisms are identified. A layered wall accretion forms purely by solid state reaction, and is thickest early in the kiln. It occasionally detaches under its own weight or is eroded, and is tolerated between scheduled shutdowns for maintenance. However, if the weathered ilmenite and accompanying silica escape earlier wall accretion, a transitory aluminosilicate melt may later promote sintering of this material within the bed, adhering catastrophically to constrictions and obstructions on the kiln wall. The mechanisms and controlling factors are discussed.