The paper presents a two-step method for obtaining sodium tripolyphosphate (STPP) with a bulk density of about 0.90 kg/dm(3) after the first step, using sodium phosphates after spray drying and water as the raw materials. STPP with a bulk density of 0.95-1.00 kg/dm(3) was generated in the second stage, using STPP from the first step and water as the raw materials. The paper presents statistical analyses to define the process parameters which significantly affect sodium tripolyphosphate bulk density. The determination of the profile approximation and utility function enabled the optimization of process parameters for obtaining a product with a bulk density of 0.95-1.00 kg/dm(3). Mechanisms of increasing bulk density was indicated by studies on the microstructure of the product and phase transformation during the process. The data were empirically verified and satisfactory results were found. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.