Leaching and recovery of Mo, Ni and Co from metals recycling plants mineral sludges
In view of the on-going depletion of the natural resources taking place worldwide, the high price, high demand and future shortage of the primary mineral resources for Mo, Ni and Co it is extremely important to implement metals recycling/recovery/reuse from semi-finished products, by-products, secondary materials and wastes, including hazardous waste (i.e. spent catalysts, mineral sludges). Furthermore, there is a need to utilize more efficient technologies to recover metals from wastes/secondary resources in order to minimize capital outlay, environmental impact and to respond to the metal increased demand.
Among the different secondary resources, spent catalysts and mineral sludges generated at the spent catalysts recycling plants could be a very good secondary resource, as they contain high concentrations of different metals (especially Mo, Ni and Co). Therefore, they should be viewed as a resource, not as a waste.
In our study we are dealing with the catalyst, metallic oxide and battery recycling plant mineral sludge. This type of material contains high concentrations of different metals. However, to the best of our knowledge, speciation, leaching and recovery of Mo, Ni and Co from this type of material were not investigated before.
Mineral sludge was minutely characterized where pH, Loss On Ignition (LOI), Toxicity Characteristic Leaching Procedure (TCLP), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDS), Total Metal Content (TMC) and Sequential Extraction (SE) were performed.
Based on the mineral sludge characterization results the leaching rate and yields of Mo, Ni and Co from mineral sludge sample were quantified. Different leaching reagents (stand-alone acids (nitric, sulfuric and hydrochloric) and acid mixtures (aqua regia (nitric + hydrochloric (1:3)), nitric + sulfuric (1:1) and nitric + sulfuric + hydrochloric (2:1:1)) were investigated at changing operational parameters (solid to liquid ratio, leaching time and temperature), in order to understand the leaching features and select the suitable leaching reagent which achieves the highest metal leaching yields. Sulfuric acid (H2SO4) was found to be the leachant with the highest metal leaching potential. The optimal leaching conditions were a three stage successive leaching, temperature 80°C, leaching time 2 h and S/L ratio 0.25 g L-1. Under these conditions, the leaching yields from our mineral sludge sample reached 85.5, 40.5 and 93.8% for Mo, Ni and Co, respectively.
Target metals recovery from synthetic and real acidic leachate of a mineral sludge from a metal recycling was investigated with Na2S solution. At first, the operational parameters (metal sulfide ratio (M/S) 0.1 – 1, agitation speed 0 – 100 rpm, contact time 15 – 120 min, and pH 1 – 5) were optimized in batch conditions on synthetic metal leachate (0.5 M HNO3, Mo = 101.6 mg L-1, Ni = 70.8 mg L-1, Co = 27.1 mg L-1) with a 0.1 M Na2S solution. Additionally, recovery of the target metals was theoretically simulated with a chemical equilibrium model (Visual MINTEQ 3.0). The optimized Na2S precipitation of metals from the synthetic metal leachate resulted in the potential selective recovery of Mo as oxide at pH 1 (98% by modelling, 95.1% experimental), after simultaneous precipitation of Ni and Co as sulfide at pH 4 (100% by modelling, 98% experimental). Metal precipitation from the real acidic leachate (18 M H2SO4, Mo = 10,160 mg L-1, Ni = 7,082 mg L-1, Co = 2,711 mg L-1) of mineral sludge was performed with 1 M Na2S, and resulted in a maximal Mo recovery at pH 2 (50.3%), while maximal recoveries of Ni and Co were at pH 4 (56.0 and 59.7%, respectively). Real acidic leachate gave a lower metals recovery efficiency, which can be attributed to various factors such as changes in the pH, nature of leachant, co-precipitation of Zn and competition for S2- ions.