Continuous improvement of productivity, and the associated demands made on the availability of cement and minerals industry process plants, result in increases in spare-parts stocks, and thus in higher spares procurement costs. Wear in many heavily exposed plant units increases at above-average rates. Jointly with the customer, Teutrine GmbH, of Oelde, Germany, drafts innovative, pragmatic and efficient solution concepts, in order to achieve planning certainty in servicing and maintenance, and to prolong the service lives of heavily loaded machine components. Mobile machining units for direct in-situ use in the customer‘s plant play a particularly important role in this context.
The experts from Teutrine GmbH are at the disposal of customers around the globe with their know-how and machinery, restoring the serviceability of mechanical systems in the shortest possible time. In many cases, it is precisely the preparatory work which is critical, and therefore of exceptional importance for in-situ deployment at the customer’s site. Adaptation of Teutrine’s mobile machining units to the customer’s specific needs must be carefully researched in advance, and then precisely defined, in order to ensure that the work performed on site will achieve workshop standard quality. This scenario is illustrated here, examining the example of a pressure roller mill, the grinding rollers of which are equipped with metal carbide studs.
Pressure roller mills operate with two grinding rollers which are set opposed to each other. One roller is fixed, while the second roller is screwed down by means of hydraulic cylinders, in order to adjust the grinding gap, or to “relax” it for coarse particles. The hydraulic system is also responsible for balancing out one-sided loadings, i.e., the bladder accumulators increase the pressure on the more heavily loaded side. The result of this process is that the rollers, and the metal carbide studs, in particular, are subjected to differing wear effects, which predominate, in this case, primarily at the roller center. The consequence is that the grinding gap becomes too large at the center, the grinding process then fails to achieve the required particle size distribution for the material, and, ultimately, an inferior product is therefore produced. The result is that:
– more energy must be expended for the grinding of slag or clinker, or for the production of slag cements, and
– the cement product is of lower quality.
Investigative work performed in advance serves to clarify the following questions:
– Stud hardness: How, in principle, can the faces of the rollers, bearing the metal studs with their extremely high material hardness (Vickers hardness of above 900 HV‑50), be machined?
– Mobile machining: Is in-situ machining at the customer’s site possible? What methods can be developed for “in-mill” machining, eliminating the necessity for cost-intensive removal and reinstallation of the grinding rollers?
– Cost-effective process/prolongation of service life: In-situ mobile machining should genuinely produce a rationally priced alternative and significant savings potentials for the customer, enabling him to postpone the purchase of expensive spares, in the form of new grinding rollers.
Successful implementation of this project necessitated the exploration of new solution routes. Complex engineering, involving a whole series of extensive fundamental tests, was completed within twelve months. Initial investigations were aimed at determining whether water-jet cutting could be used as the machining technology.
This process involves an extremely high energy input, to generate pressures of 3000 bar. Cut width is around 1.5 mm (consumption: approx. 110 ampere, time: 2´:15´´). The jet of water cuts the extremely hard material, and can be absorbed only in a water bath, but does, however, have a number of disadvantages: the method is extremely dangerous, and it is necessary to control the rebound water. Each stud must, in addition, be machined separately. This procedure has therefore been rejected as unsuitable for cement industry grinding plants.
An alternative method is grinding using conventional grinding agents (Fig. 1). The results of a test using grinding wheels of various abrasive grades were positive; the effective life of the grinding wheels was, however, extremely limited, and grinding wheel consumption thus extremely high. The results were approximately the same, irrespective of whether corundum or ceramically bonded abrasives were used.
4 Successful solution concept
After a series of workshop tests, a special cup grinding wheel was developed for this application (Fig. 2). The machining unit (Fig. 3) was designed in such a way as to permit its use for various face lengths and roller diameters. Great importance was also attached to a compact design which would permit air freight transportation and therefore deployment anywhere in the world. Blasting was used for preliminary cleaning of the rollers (Fig. 4), in order to remove remnants of slag and other fouling which had clogged the interstices between the metal carbide studs during mill operation. The mobile machining unit was then mounted in position, with one machining unit (Fig. 5) installed in front of the fixed roller and one machining unit in front of the movable roller. The alignment tolerance of this operation is 0.01 mm. The rollers were then appropriately measured, and the highest stud marked as a reference (Figs. 6 and 7).
The rollers were turned by the mill‘s existing auxiliary drive and simultaneously machined by the high-precision grinding systems. Frequency control permits selection of the best possible in-feed rate. The special grinding abrasives used (Fig. 8) are cooled with water. This cooling-water is recirculated by means of pumps.
This in-situ machining process can, if necessary, be repeated a number of times, depending on the degree of wear of the rollers.
The benefits for the operator are the following:
– Short mill downtimes
– Elimination of cost-intensive removal and reinstallation of the grinding rollers
– Cost-savings compared to purchase and installation of a new roller
– Cost-savings compared to overhaul of the old roller at the manufacturer‘s works
This innovation in the field of pressure roller mills makes it possible to increase overall roller operating time by a multiple. The grinding gap can again be correctly adjusted, optimum material particle size is again assured, and thus again guarantees superior quality cement production results.