Through joint effort, Russian and Finnish experts came up with a matte-free leaching process for converter matte processing, which involves precipitation of iron with the help of jarosite process. The new process was implemented at Norilsk Nickel Harjavalta Oy (NNH), and it helped optimize the 1st treatment stage during which iron, arsenic and selenium are removed from nickel-cobalt solution. Compared with the standard process scheme, the new process helped achieve a significant reduction in the amount of valuable components that get lost with the ferrous cake: nickel and cobalt — by 11 times (each); copper — by 23 times; platinum — by 16% (abs.); palladium — by 10% (abs.); rhodium — by 9% (abs.); gold — by 12% (abs.). The jarosite process conducted in a separate cycle benefited the quality of copper cake: thus, the concentration of copper rose from ~35–40 to 57–62%. Due to the new process schemes applied and the jarosite process implemented, the concentration of arsenic in the copper cake decreased from ~1.5–2.0 to 0.15–0.30%. The natrojarosite phase that formed during the 1st (autoclave) treatment stage (when iron is removed from the nickel-cobalt solution) benefited the oxidized slurry thickening and filtering performance. The jarosite solid phase is characterized with higher compactability and filterability. The authors established how the phase composition of the natrojarosite deposit tends to form depending on the process of pressure oxidation deposition of iron ions from sulphate solutions and its thermodynamics. The authors optimized the conditions (such as the composition of the solution, temperature and pressure) for coarse-crystalline natrojarosite to form, which enable reaching the required iron precipitation depth and raising the degree of co-deposition of arsenic and selenium. The authors came up with a mechanism for co-deposition of arsenic ions during an oxide-hydrolytic deposition of iron producing natrojarosite. The authors identified the conditions for and implemented a Fe-cake disposal technique based on the addition of dry neutralizer — i.e. limestone. In this case the sodium jarosite contained in the Fe-cake is not subject to decomposition and becomes stable enough for long-term storage in a wet state in the NNH test field. It is shown that to increase the production of cathode nickel by NNH additional solutions will have to be developed to achieve a reduced concentration of selenium in nickel catholytes, which can also be achieved through a greater depth of its co-deposition with iron hydroxides and its removal with dump ferrous cake. © 2019, "Ore and Metals" Publishing house. All rights reserved.