Characterization and reuse of waste from the magnesium nitrate fertilizer industry.

Currently, liquid fertilizers are considered strategic products in the sector, particularly those with nitrogen and magnesium in their composition. During their synthesis, the generated muddy and sticky residue is usually managed as a toxic waste because its properties and feasible valorization methods have not yet been studied. For the first time, this residue has been thoroughly characterized, and, on the results obtained, its possible reuse options have been discussed. This material, with 47 % moisture content, a neutral pH, and a specific density of 0.85, still contains 35 % dry weight of nitromagnesite. These findings, together with a high cation exchange capacity and the presence of iron, aluminium, calcium and silicon as minority components, make its reintroduction into the manufacturing process of fertilizers the most viable option for its valorization, having two alternatives for this purpose. The first is to use it as a feedstock for the production of solid fertilizers by adding 30 % quicklime to the residue to improve its mechanical properties, thus obtaining a fertilizer with 5.7 %, 5.0 % and 24.3 % (dry weight) of magnesium, nitrogen and calcium, respectively. The second option, which focused on obtaining a liquid fertilizer, allowed the recovery of approximately 86 % of the remaining nitromagnesite in the residue by washing it with nitric acid, reducing its initial dry mass by 77 %. Then, the resultant liquid phase, with 16 % magnesium nitrate, could be enriched to the 35 % concentration demanded by liquid fertilizer consumers by a subsequent acid attack of the raw rock.

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.