Converting the residue from an infiltration water treatment plant (Wrocław, Poland) into a hematite red pigment-optimising the process with a Doehlert experimental matrix.

Referring to the concept of a circular economy and using optimal experimental design (OED), the research deals with optimising the conversion of industrial residue from the water de-ironing process into a red hematite pigment. The raw, chemically pretreated sludge was thermally converted into hematite under various conditions established in the optimised procedure. The products were thoroughly characterised using XRD, Raman spectroscopy, Diffuse reflectance spectroscopy, and SEM EDS to gain better insight into the process. Because of the unambiguous and quantitative definition of colour and its individual perception, resulting in difficulty in determining the appropriate system response in the optimisation procedure, products' quantitative colour parameters, determined in the CIELab colour space, were compared to those obtained for the commercial red hematite pigment, and the most suitable ones (a*, C*, and L*) were chosen as system responses in the optimising procedure. As a result, the optimal ranges of the process parameters-temperature and holding time-that can ensure a pigment with the desired colour were established by OED methodology. Moreover, it was found that the key factor enabling effective conversion of the studied residue into red pigment is a proper chemical pretreatment in which the process temperature is decreased to below the hematite sintering temperature.