A System for the Control of the Homogenisation of the Cement Raw Meal

Summary

A system utilizing radioisotope neutron activation and X-ray fluorescence analysers together with a small computer has been developed by Outokumpu Oy in cooperation with a Finnish cement manufacture for the control of the homogenisation of a cement raw meal. The control is accomplished in principle so that the concentrations of the most important components of the output of the raw mill, i.e. CaO, SiO₂, A1₂O₃ and Fe₂O₃, are determined by means of the two analysers. Based on these analyses estimates for the elemental concentrations of the raw materials are calculated and new optimized settings for the raw material feeders are determined.

Control of the homogenity of the ground mix of the raw materials before feeding it into the kiln is a very essential problem in cement manufacture. Earlier, time-consuming and laborious wet chemical methods were the only means for determination of the CaCO₃ content at this stage of production. In recent years X-ray fluorescence technique has, however, achieved ever increasing use as a faster method for the control of the process.

Yet, there are some very serious drawbacks associated with this technique in the special application concerned. Difficulties arise from the strong variation in the composition of the raw materials as well as from mineralogical (matrix) and particle size effects, especially in the analysis of light elements.

In designing the control system presented in this paper one of the underlying basic ideas was to minimize the errors associated with the determination of the Si, Al, Ca and Fe contents by using the neutron activation technique for the lighter elements Si and Al and X-ray fluorescence only for Ca and Fe. A practical way of producing sufficient neutron flux is the use of radioisotope neutron source (usually ²³⁸Pu-Be, ²⁴¹Am-Be).

The control of the process is accomplished by means of a small computer, which is equipped with a 8 K memory, a Teletype writer and interfaces to the analysers and the process. The programming consists of a basic executive part and a set of modular application parts which may be easily modified, added or deleted. The course of an analysis and control cycle can be briefly described as follows: Samples from the mill output material are analysed for Ca, Si, A1 and Fe and the concentrations obtained converted into so called “modules”, which characterize the quality of the raw meal. These are compared with so called “set values”, and based on the differences new control values are calculated through a control algorithm. In the optimization section the control values given by the algorithm are converted into optimized feed ratios for the raw material feeders.