Research On Extracting Nickel And Cobalt From Laterite Using Organic Acid

With the increasing demand of nickel and decreasing reserves of nickel sulpHides in the world, the research needed on the utilization of laterite is becoming more and more urgent. However, domestic research in this field is still at the starting stage. In this thesis, a new method using organic acid on leaching laterite that has a high content of iron and the leaching mechanism were studied. The laterite mineralogy, leaching reaction rationale and thermodynamics, optimization of leaching system and various technological parameters during the leaching process, dynamics of nickel and cobalt leaching, as well as the organic acid regenerative cycle were also analyzed and expatiated.Mineralogy analyses indicated that the main minerals in laterite are goethite and gibbsite. 69.92% nickel scatters into the goethite in isomorpHism and 13.53% nickel disperses among the manganese minerals. Nickel, iron and aluminum interweave together in the laterite. 43.08% cobalt exists in oxides, while 29.23% cobalt distributes in the high valent state manganese minerals.The organic acid leaching fundamental research results showed that the leaching effect of citric acid-ammonium bifluoride leaching system based on their coordinating leaching effectiveness. The acidic ammonium bifluoride solution can destroy the layered silicate skeleton of clay minerals. The citric acid has strong acidity and reducibility. It can dissolve the partial goethite and the carbonate, and reduce the high valent state manganese minerals, promoting the fettered nickel and cobalt to come out. In addition, the citric acid has the strong chelate ability. The acid radical will react rapidly with Ni²⁺ and Co²⁺, which were dissociated from the mineral crystal lattice, forming the stable nickel, cobalt citric acid complex compound. The Eh-pH chart of Metal-H₂O system indicated that FeOOH under the alkalinity condition is quite stable. When pH≤6.51, Eh=-0.4402～0.7708V, it starts to dissolve into Fe²⁺; When pH≤6.233, NiO transforms to Ni²⁺ in water; when pH=7.51, CoO starts to transform to Co²⁺. Under the acidic condition, nickel and cobalt can exist stably in two valent stage metallic ion shapes, causing Ni²⁺ and Co²⁺ to enter into solution.It was indicated that the tranditional acid leaching by using inorganic acids, including hydrochloric acid and sulfuric acid, is unable to get satisfactory performance in nickel and cobalt leaching without a large amount of iron ions produced during leaching. The four organic acid systems being studied in this thesis exhibit higher nickel and cobalt leaching with less iron ions produced than the inorganic acid leaching system. The Citric acid-ammonium bifluoride leaching system is the most effective one in extracting nickel and cobalt from the laterite with little iron being leached.Through the optimization of leaching process's various technological parameters, optimized conditions of the leaching process were obtained. For the citric acid-ammonium bifluoride leaching system, the nickel and cobalt leaching may reach 49.56% and 89.91% when the appropriate citric acid concentration is 30g/L, the ammonium hydrogen fluoride concentration is 10g/L, the extraction time is 2 hours, the leaching temperatures is 30℃, the rotational speed is 23 rpm and the liquid to solid ratio is 1/10.According to the research on the leaching dynamics, nickel leaching belongs to the internal diffusion reaction, while cobalt leaching belongs to the combined chemical diffusion and internal diffusion reaction.The analysis of metallic ion content in a leaching solution indicated that the enrichment ratios of nickel and cobalt in solution surpass the iron obviously and manifest good metallic ion-leaching selectivity of the organic acid, reducing the difficulty in the following purification of solution. Through research on the stability of complex compounds and experimental tests, the citric acid complex compound will have hydrolysis gradually along with the time extension. Then the citric acid obtains the regeneration while nickel and cobalt form precipitation in the form of Ni(OH)₂ and Co(OH)₂.