*Article* **A Study of Treatment of Industrial Acidic Wastewaters with Stainless Steel Slags Using Pilot Trials**

**Mattia De Colle \*, Rahul Puthucode, Andrey Karasev and Pär G. Jönsson**

Department of Materials Science and Engineering, School of Industrial Engineering and Management, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden; rahulpu@kth.se (R.P.); karasev@kth.se (A.K.); parj@kth.se (P.G.J.)

**\*** Correspondence: mattiadc@kth.se; Tel.: +46-765768409

**Abstract:** Different stainless steel slags have been successfully employed in previous experiments, for the treatment of industrial acidic wastewaters. Although, before this technology can be implemented on an industrial scale, upscaled pilot experiments need to be performed. In this study, the parameters of the upscale trials, such as the volume and mixing speeds, are firstly tested by dispersing a NaCl tracer in a water bath. Mixing time trials are used to maintain constant mixing conditions when the volumes are increased to 70, 80 and 90 L, compared to the 1 L laboratory trials. Subsequently, the parameters obtained are used in pH buffering trials, where stainless steel slags are used as reactants, replicating the methodology of previous studies. Compared to laboratory trials, the study found only a minor loss of efficiency. Specifically, in previous studies, 39 g/L of slag was needed to buffer the pH of the acidic wastewaters. To reach similar pH values within the same time span, upscaled trials found a ratio of 43 g/L and 44 g/L when 70 and 90 L are used, respectively. Therefore, when the kinetic conditions are controlled, the technology appears to be scalable to higher volumes. This is an important finding that hopefully promotes further investments in this technology.

**Keywords:** stainless steel slag; recycling; acidic wastewater treatment; upscale trials; mixing time
