A geometallurgical study of the phosphates of the Dorowa carbonatite complex, Zimbabwe (Université de Liège)

Abstract

Phosphorus is an essential plant nutrient that cannot be substituted in food production or manufactured synthetically (Johnston, 2000). Recycling of phosphorus from human excreta and urban wastewater is also currently unfavorable due to low phosphorus yields, low phosphorus concentration in the feed sources, and high energy and reactant consumption during processing to produce sludge-based fertilizer (Pradel & Aissani, 2019). Therefore, phosphate rock remains the only economical source of the phosphorus required to manufacture phosphate-containing chemical fertilizers, which are needed to improve crop yields, thereby feeding the growing world population. In 2019, 240 million tonnes of phosphate rock were mined worldwide, and more than 90% of the mined phosphate rock was used to manufacture different types of fertilizer and animal feeds (US Geological Survey, 2019). To manufacture fertilizer, phosphate rock is first beneficiated to reduce the content of calcium, magnesium, iron, aluminum, silica, fluorine, chlorine, sodium, potassium, heavy metal impurities. The phosphate industry generally requires a phosphate rock concentrate containing: P2O5 > 30 wt.%, CaO/P2O5 ratio smaller than 1.6, and MgO < 1 wt.% (Sis & Chander, 2003). After beneficiation, the phosphate rock concentrate is reacted with sulfuric acid to produce single superphosphate fertilizer directly or wet-process phosphoric acid, which is a critical ingredient in the manufacture of different types of fertilizers and animal feed supplements. According to Kelahan et al. (2019), upon entering the phosphoric acid wet process, phosphate rock impurities dissolve releasing ions, which cause numerous process and final product-related problems. Therefore, the control of phosphate rock impurities going into the wet process of phosphoric acid production is critical. This study aimed to identify, sample, and subject the different ore types found in the south and north orebody of the Dorowa carbonatite complex in Zimbabwe to flotation test to determine which ore types could yield phosphate concentrates with Fe2O3 and CO2 contents greater than 2.14% and 2% respectively. Dorowa Minerals Limited currently exports phosphate rock concentrate to a phosphate chemistry leader called Prayon in Belgium. The phosphate concentrates exported to Prayon are required to contain; P2O5 > 30 wt.%, Fe2O3 < 2.14 wt.% and CO2 < 2 wt.%. Historical Dorowa phosphate concentrates exported to Prayon satisfy the Fe2O3 and CO2 restrictions imposed on the concentrate. However, variations in the ore types fed into the processing plant at Dorowa can result in the set restrictions not being met. Therefore, the different ore types found in the south and north orebody were subjected to batch flotation tests with the fatty acid and alkyl succinamate collectors currently used in the Dorowa Minerals Limited processing plant. A particle size by size analysis of key flotation streams was conducted to establish the optimum P2O5 grade liberation size range, the size range at which P2O5 is lost to the tailings, and the size ranges in which iron impurities contaminate the Aero 845, Betacol and final concentrate. A full chemistry determination of the process water used at Dorowa was conducted, and seasonal variations in water quality were also established.