Identification and Control of the Waelz Process Using Infrared Image Processing

Abstract

Today, the recycling of zinc from zinc-bearing residues is mostly done by the pyro-metallurgical Waelz process in a rotary kiln. The optimum recovery rate is only achieved, if the temperature of the slag within the kiln is controlled at optimal values. These optimal temperatures vary and depend on the material mixture. The slag temperature is measured by pyrometers so far. They yield only pointwise measurements that can be erroneous due to cold slag lumps. The control of the slag temperature takes place manually by experienced operators, who adapt the process air to stabilize the process at the desired temperature setpoint. The paper presents a slag temperature measurement system with an infrared camera and an image processing system. It contains an automatic motion based slag detection. Using the camera based measurements, a process model for the Waelz process is derived and its parameters are estimated by a process identification. Using the process model the controller parameters are optimized by minimizing a cost functional. The sensitivity of the cost functional with respect to the controller parameters is analyzed. The optimized automatic closed-loop control is finally demonstrated at an industrial Waelz kiln.