Reaction mechanism

The reaction principle is that without chelating phase, nickel, manganese and cobalt ions react with hydroxide ions in a low quantity forming undesired Ni(OH)2, Mn(OH)2 and Co(OH)2-precipitates. Although the formation rate of individual precipitates are defined to be very small in the initial process environment due to chelating phase principle suggesting

the formation of individual hydroxides can be avoided, they are being taken account of in the reaction equilibria phase. Logic in the chelating step is that present metal ions are chelated separately into ammonia chelate complex, and in the precipitation phase the metal ions shift into a multi metal precipitate compound of which mixture ratio is defined by the ratio between rates of chelated metal ions. This is deduced from the equilibrium reactions provided by Van Bommel & Dahn (2009), chelating reaction pathway is suggested by Lee et al. (2004) and in this work MATLAB-script has been written to solve the reaction equilibria.

According to Barai et al, (2019) it is reasonable to assume, that desired metal hydroxide precipitation occurs only between ammonia complexes and hydroxide ion. Ammonia is then released in the formation reaction of mixture hydroxide, which leads to conclusion, that it functions as a catalyst towards the desired precipitation phase, as it enables the formation of wanted substance without itself elapsing. Reasoning explaining the vital part of ammonia in the formation of NMC-cathode precursor is proposed to be following: as the individual hydroxide precipitates occur if the chelating agent is not applied, it can be deduced that ammonia prevents phase separation into several different precipitates and results into homogenous precipitate with molar ratio of chelated metals (Lee et al. 2004). Alternatively, Van Bommel & Dahn (2009) have proposed, that metal ammonia complex interacts with metal hydroxides thus increasing the solubility of the hydroxide precipitates, leading to decreased rate of precipitated individual precipitates. This is a consequence of slowed reaction rate between metal ions and hydroxide ions (Lee et al. 2004).

As both of proposed principles for chelating phase are credible, pathway proposed by Lee at el. (2004) is applied in the simulations of the work environment as it seems to be logical and it’s successful modeling is realistic, especially in view of the aspect that this is the first time for such system to be modeled so thoroughly. Simulating the chelating principle proposed by Van Bommel & Dahn (2009) is more problematic to model as the principle of increased solubility of hydroxide precipitates due to increased volume of metal ammonia complex in the solution is less straight-forward: the solubility changes are directly proportional to the rate of fed ammonia, and all information and parameters regarding the kinetics of reached proposed equilibrium are unknown.

In the initial Aspen Plus simulation individual hydroxides are not defined as solids due to their insignificant concentration, which is caused by ammonia’s assumed effect of

preventing the undesired solids formation. Simplified mechanism of precipitation process with chelating phase based on the Lee et al. (2004) reasoning is shown in the Figure 35. The ammonia molecules depicted as pink balls are depicted to isolate free metal ions from the rest of the solution forming a chelate cloud consisting of several different chelate compounds such as Ni(NH3)²⁺, Mn(NH3)22+, Co(NH3)42+ etc. After formation of chelate cloud, hydroxide ions depicted as black balls precipitate the metal ions attached to ammonia molecules, leaving ammonia into the solution.

Figure 35 Pictorial of simplified reaction mechanism

After reasoning of reaction logic, proper defining of reactions present in the reactor can be done. Figures 4 and 5 in the Appendix (IV) contain reaction series for chelating and precipitation phase in the constructed Aspen Plus model. The mentioned reaction sets are not activated in the simulation due to Aspens quality of demanding a reaction set to be defined in the simulation section, and not just in properties.

Following premises are taken account when the reactions are defined; all involved reactions are equilibrium reactions, this includes also the formation of unwanted individual precipitates with a significantly low concentrations providing theoretical presence of mentioned precipitates in the chelating phase. Equilibrium constants for formation of chelate ligands are used in a manner where the reaction rate constant for opposite reaction is set as unity, and reaction rate constant for formation of the complex is calculated based on constant.

Chelate hydroxide precipitation reactions are relatively slow, so the reaction rate constants are estimated to be significantly lower than the ones used for the chelate formation equilibria.

Dynamic CSTR model of Aspen plus assumes all involved phases to be always in an equilibrium. Initial guess for equilibrium constant for mentioned reactions is valued to be 10, as mentioned in the Table 5, due to the lack of information about activation energy and pre-exponential factor for mentioned reactions. Assumed value is also significantly higher than ones for the precipitation of individual precipitates due to favoring the NMC-precipitation in simulated process containing ammonia. The selection of reaction rate constants is discussed further in the Chapter 9.1.