The cementation of nickel onto iron at elevated temperatures

Much research has been undertaken to find economic ways of recovering nickel from limonitic and silicate deposits. In one process, nickel ions are precipitated from aqueous solution at elevated temperature using metallic iron. A fundamental investigation into the reaction mechanism and kinetics has been undertaken using a rotating disc and a particulate geometry. Using a rotating disc geometry, the Influence of initial nickel ion concentration, reaction temperature, degree of stirring, and oxygen potential were studied. At low oxygen potentials, the rate of nickel precipitation is con¬trolled mainly by a surface chemical step. At temperatures greater than 100° C, the rate at which mass Is transferred to the reaction sites becomes important also. In the pH range 4.5 to 5.5, and at low oxygen poten¬tials, nickel precipitation onto metallic iron is hindered by an adsorbed layer of hydrogen atoms blocking much of the iron surface. As the oxygen potential is raised, oxygen acts as a depolarizer, cleaning the surface and permitting freer access of the nickel ions to the cathode sites. The rate of removal of nickel from solution is a linear function of oxygen pressure at constant temperature. At low oxygen potentials, the deposit formed is pure nickel ; at higher oxygen potentials, mixtures of metallic nickel, and nickel and iron oxides form the relative pro¬portion of these depending on the local surface potential (mixed potential). In all cases, the deposits form on the iron surface. Hydrogen gas is liberated by a proton attack on the precipitant at low pHs. Nickel deposited under these con¬ditions is precipitated through hydrogen reduction away from the iron surface rather than by an electrochemical cementation mechanism. At high oxygen potentials, the cementation process is diffusion-controlled, with an activation energy of 3.1 kcal/ mole. An experimental diffusion coefficient for nickel Ion of 3.45 x 10-6 cm2/s was obtained under these conditions. Coarse and porous deposits are produced when the initial nickel ion concentration in solution is small ; however, at high initial cationic concentrations of nickel, the deposits obtained are smooth and dense and result in significantly smaller reaction rate constants. This latter type of deposit is likely to impose a resistance to the counter diffusion of precipitant ions which would explain the decreased values obtained for the reaction rate constant. Cementation onto particulate precipitants was also studied using two deposition reactions: Cu-Zn, and NI-Fe. Under the experimental conditions used, both reactions are mass-transfer controlled, and the rate constants (mass tran¬sfer coefficients) can be described by the Frossllng-type equation: Sh = 2 + 0.75 Rep 1/2 Sc1/3 The particle-Reynolds number is defined in terms of the terminal settling velocity and the arithmetic mean particle diameter. The specific reaction rate Increases with increasing agitation until a condition of full particle suspension is reached. Thereafter, the specific reaction rate constant is essentially independent of increased agitation, and any additional energy input is simply an energy loss.