The design of an industrial OSLO crystallizer is based on experiments in the lab, in asmall scale pilot plant, in a semi industrial plant and in an industrial plant that has been operating for 20 years. The objective was to produce spherical particle of salt with a size of about 5 mm; the known state of the art allowed only to produce particle size of about 2mm maximum. A development was made for 2 years in lab and small pilot scale; an industrial plant was started in 1987 This paper describes the main problems encountered at all stages of development and how they were solved: choice of the main operating parameters, validation on all the steps, resolution of hydrodynamics problems and practical controls of nucleation, control of scaling Most of the solutions adopted for production of large particle size of sodium chloride are also valid for other salts production and the OSLO technology can be applied for other salt production ; in some cases, very large particle size obtained can compete with usual crystallization + drying + size enlargement by compaction technology.