Development of a process for producing slow released potassium-struvite fertilizer from Vinasse and Grape marc

Economic losses and environmental pollution problems arising from conventional fertilizers having rapid dissolution character has necessitated the use of slow release fertilizers. Moreover, rapidly depletion of natural resources has led to development of new processes utilizing by-products and wastes. In this context, phosphorous fertilizers are important in view of both production of slow release fertilizer and conservation of phosphate reserves. In this study, process conditions of preparation of potassium magnesium phosphate fertilizer and activated carbon, using vinasse generated at distillation step in the alcohol production from molasses together with grape marc, have been investigated. For this purpose, firstly, mixtures of concentrated vinasse and grape marc were subjected to pyrolysis under nitrogen gas atmosphere and potassium was extracted from pyrolysis product by using water. Extraction residues stabilized with acid treatment were characterized in view of the characteristics of an activated carbon. Potassium magnesium phosphate (potassium-struvite, KMgPO4.6H2O), a slow-release fertilizer compound, was obtained by precipitation from the extract containing potassium and process conditions and product characteristics were explored. After dried mixtures of vinasse and grape marc were pyrolyzed at 600-800C, potassium could be extracted from char with water in high yield. High specific surface area of resulting solid residue indicated that the potassium in the mixture caused an activation during the pyrolysis. In the second stage, the products containing potassium-struvite were obtained from extract in high alkalinity by using phosphoric acid and magnesium compounds. However, because the other chemicals used for providing necessary stoichiometric ratio made the medium acidic, alkalinity of extract was not sufficient for providing appropriate pH. For this reason, an extra base such as NaOH is needed to provide the optimal pH level. When only extract was used in potassium-struvite precipitation, excessive amount of extract was needed. In this case, nearly pure potassium-struvite could be obtained, however, resulting potassium recovery yield was found to be as low as about 30 %. When NaOH solution was used to reach optimal pH for the case of stoichiometric mixture (K:Mg:P = 1:1:1), the recovery yield in the base of potassium could be increased to about 50 %. At the conditions of magnesium and phosphate constituents more than stoichiometric ratio and appropriate amount of NaOH solution, potassium recovery yield could be increased but potassium percentage of product decreased. For example, in the K:Mg:P ratio of 1:3:3, about 89 % of potassium could be recovered by precipitation. In this case, product with lower potassium percentage was obtained due to coprecipitation of different magnesium phoshate compounds. The presence of magnesium phosphate lowered the potassium percentage of product compared to that of pure potassium-struvite, however, its presence may not affect the fertilizer value of product because it is also consisted of plant nutrients. In addition to chemical composition, physicochemical properties of products containing potassium-struvite obtained at various conditions have been determined. Additionally in this context, its solubility properties in water and some aqueous solutions containing some substances that are important in plant nutrition, has been explored. Potassium based solubility was found to be about 2 % in water, at the end of long contact time. Besides, in the aqueous solution containing complexing agent, it was more than 10 %. These values showed that the product obtained in this process was a slow release fertilizer material. As a result, by using this process, potassium in the vinasse could be obtained as a slow-released fertilizer, and an activated carbon, which could be used for certain purposes was recovered as a by-product. Key Words: Potassium-struvite, Slow released fertilizer, Activated carbon, Vinasse, Grape marc