Abstract
The leaching process of cobalt using a wide range of experimental variables is described. The treated cobalt samples were from the Kalumbwe Mine in the south of the Democratic Republic of Congo. In this study, a predictive model of cobalt recovery using both the Taguchi statistical method and an artificial neural network (ANN) algorithm was proposed. The Taguchi method utilising a L25 (55) orthogonal array and an ANN multi-layer, feed-forward, back-propagation learning algorithm were adopted to optimise the process parameters (acid concentration, leaching time, temperature, percentage solid, and sodium metabisulfite concentration) responsible for the high recovery of cobalt by reducing sulfuric acid leaching. The ANN was built with a neuron in the output layer corresponding to the cobalt leaching recovery, 10 hidden layers, and 5 input variables. The validation of the ANN model was performed with the results of the Taguchi method. The optimised trained neural network depicts the testing data and validation data with R2 equal to 1 and 0.5676, respectively.Significance:
 
 We statistically investigated the main factors (acid concentration, leaching time, temperature, percentage solid, and sodium metabisulfite concentration) that affect the cobalt(III) leaching performance using both the Taguchi method and artificial neural network model. This allowed us to ascertain that it is indeed possible to leach cobalt(III) from oxide ores and to identify the optimum leaching conditions.
Highlights
Cobalt’s utility in green energy and modern industry makes it very important and the price of cobalt is growing rapidly due to its high demand
CColVl WCoi x where rCo is the cobalt recovery (%), CCol is the concentration of cobalt contained in the pregnant solution (g/L), Vl is the volume of the leaching solution and WCoi is the weight of cobalt in the material (g)
Using the Taguchi L25(55) orthogonal design of experiment and considering the acid concentration, leaching time, temperature, solid percentage, and sodium metabisulfite concentration as controllable parameters, the optimised conditions for the leaching of cobalt were calculated as 100 g/L for acid concentration, 60 min for leaching time, 65 °C for temperature, 15% for solid percentage, and 10 g for sodium metabisulfite concentration
Summary
Cobalt’s utility in green energy and modern industry makes it very important and the price of cobalt is growing rapidly due to its high demand. The cobalt is present in both bivalent and trivalent states. The dissolution of cobalt oxide ores may be accomplished in sulfuric acid media but cobalt in a trivalent state leaches only in the presence of reducing agents such as sulfur dioxide (SO2)[9], sodium metabisulfite (Na2S2O5) known as SMBS, metallic copper powder, and ferrous ions[2,8,10,11]. The dissolution rate of cobalt was reported to be faster than that under standard acid leaching.[12] In contrast, using SO2 could engender environmental issues due to gaseous emissions.[13] The use of SO2 derivatives such as sodium sulfite (Na2SO3) or sodium metabisulfite (Na2S2O5) reduces or eliminates the environmental risks.[13]
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