Modelling and Optimisation of the Nickel-Carbonyl Process
Authors:   , Romero Torrecilla José C. 
Published: June 2022

The increasing demand of nickel in the last decade has motivated nickel refining companies to further increase their efficiency to remain competitive in a global market. VALE Europe Ltd UK desires to make use of advanced process modelling tools to build a model of their nickel refinery plant, based at Clydach (Wales), with three key objectives; namely building a better understanding of the existing process, deploying the model for use in operator training systems and using the model for the testing of new control strategies that lead to a more efficient and economic process while reducing environmental impact. Nickel metal is obtained at its highest purity (99%) by means of the nickel-carbonyl process, which has been successfully used in the metal refining industry for over a hundred years. The nickel-carbonyl process extracts pure nickel from nickel matte in a two-step process. Firstly, nickel contained in the nickel matte is transformed into nickel-carbonyl gas through volatilisation in the Kiln plant. Secondly, this gas is later sent to the Pellet and Powder plants where it is decomposed to produce high purity nickel pellets and powders respectively. This study presents process modelling and steady-state simulation of the volatilisation kiln circuit and the pellet plant circuit in Aspen Plus. Model validation and sensitivity analysis is performed for both models and the resulting simulation was adapted for deployment to operator training systems. Results obtained from the volatilisation kiln simulation show that the kiln can be modelled as a series of plug flow reactors. Simulation results for the objective variables [desired degree of conversion (88%), residence time (~6 days) and carbonyl gas in the product stream (~390 gNi/m3)] agree to a high degree of confidence to industrial data obtained from the companies process historian. Furthermore, results show that lumped models are an effective way to capture the volatilisation kiln behaviour. Similarly, the pellet plant circuit was simulated assuming the nickel decomposer unit as a plug flow reactor with recirculation. Again, the values obtained for product pellet size (6-10 mm) and global conversion (99%) agreed with the available industrial data. Nickel seed feed flowrate and carbonyl concentration in the gas feed were the main control variables of impact in this model. Dynamic simulation of these two processes was attempted. However, the lack of fundamental physical data and the inability of current commercial ‘off the shelf’ simulation software to model this extremely complex phenomena proved challenging. The generic software package Aspen Plus, for example, was of no use for this specific application and attention was moved to gProms, a more rigorous, yet fully programmable, software. Novel code was developed to produce models capable of handling solid-gas reactions in dynamic mode. Overall, as a result of the rational approach taken in this study, the application of modern software and novel coding to the nickel-carbonyl process has led to increased process understanding and an application that is invaluable for operator training.