Ghana Cement (GHACEM) Plant in Tema is one of Ghana’s largest manufacturing establishments for production of cement with a total capacity of 2.4 million tons per annum.
GHACEM produces cement using three basic raw materials; clinker, limestone and gypsum. Clinker and gypsum are imported whilst limestone is obtained locally. Two products are currently obtained; Portland cement CEM1 class 42.5N (90% clinker, 5% limestone and 5% of gypsum) and Portland limestone cement CEMII/B-L (LL) 32.5 R (70% clinker, 25% limestone and 5% gypsum) (GHACEM plant report, 2009).
Cement manufacturing requires the use of mills that operate with large power consumption. In addition, their capacity and operation must be optimized in order to obtain efficient performance. The performance optimization of such mills will be possible if the technological parameters of the milling process are known.
Grinding systems are either ‘open circuit’ or ‘closed circuit’. In an open circuit system, the feed rate of incoming clinker is adjusted to achieve the desired fineness of the product at the mill exit. In a closed circuit system, coarse particles are separated from the finer product and returned for further grinding. In a closed circuit mill, the total throughput is higher and hence the mill exit material is coarser. Unlike the open circuit mill, material coming from the mill goes to an air separator from where the finer materials are the product and the rejects are returned to the mill.
Cement clinker is usually grounded using a ball mill . This is essentially a large rotating drum (Fig. 2), containing grinding media; normally steel balls (Fig. 3). As the drum rotates, the motion of the balls crushes the clinker. The drum which is divided into two chambers with different sizes of grinding balls rotates at a speed of approximately 16 rpm. As the clinker particles are crushed, smaller balls are used for more effective reduction of particle size. In cement and other mineral processing plants, grinding process requires a considerable amount of power. Grinding of the clinker consumes about 1/3 of the power required to produce 1 ton of cement.
Investigation has been done on the effects of feed rate, feed percent solids, mill speed and discharge trunnion diameter on the slurry hold-up, mean residence time and particle size distribution in a 41.6×64.1 cm pilot mill whose hold-up can be measured while in operation. It was observed that hold-up increases with increasing solids feed rate, solids ratio in the feed and decreasing discharge trunnion diameter (Songfack and Rajamani, 1999). Two equations relating the hold-up in the grinding media and pool zone with flow rate, grate design and mill diameter whose sum gives the total holdup for the data obtained from a laboratory scale 30×15 cm mill at three different grate designs for three different rotational speeds and a variety of flow rates was proposed (Morrell and Stephenson, 1996).
It was found that hold-up is directly proportional to flow rate, mill rotational speed, open area of the grate and inversely proportional to the radial positions of the holes on the grate. All of these valuable studies are very important guidelines for investigating the effects of discharge mechanisms on material hold-up and discharged material. However, these studies that have been carried out in relatively small scale mills need to be validated with industrial scale data as has been done (Morrell and Stephenson, 1996; Rogovin and Hogg, 1988; Zhang, 1992).
Internal view of a two-chamber ball mill
Earlier studies on industrial scale multi-chamber cement mills have shown that diaphragms in multichamber ball mills can be considered as a kind of classifier which is fed by the product of the first grinding chamber and produces a fine product as the feed of the second grinding chamber in conventional cement mills. A coarse product arises from this classification which returns to the first grinding chamber for further size reduction (Benzer, 2000, 2005; Lynch et al., 2000; Benzer et al., 2001b).
Material grinding in a mill depends on many factors including mill geometry, speed, ball size distribution, hold-up, material grindability and granulometry. In addition, partial recirculation of material to the mill inlet introduces a nonlinear positive feedback in the process (Zhang, 1992).
The present radiotracer work was carried out in two mills (mills 3 and 4) both operating in closed circuit regime. The exit material consists of fine cement product (powder of fine grains 0-13 :m) and rejects (coarse grains more than 13 :m) that are returned to the mill. The drum of each mill is made of two chambers.
Basic theory of residence time distribution (RTD): The time of residence of a particle in a system at a particular instance is the age of that particle. The age of a particle which is just leaving the system is known as the residence time of that particle. It is the time required for a fluid element to pass through a system from the entrance to the exit.
The results obtained during the investigations revealed that the residence time of material at the exit of the milling and separator sections of both mills are almost the same. This observation suggests that power consumption of both mills were almost the same during the time of the investigation. However, the estimated efficiency of mill 3 of 42% (with an error of nearly 15- 20%) did not justify its optimal performance. The expected performance of the m ill is >60% for hard clinker (60% fines and 40% returns), and >80% for soft clinker (80% fines and 20% returns). D uring the time of the experiment, both mills were fed with soft clinker, so the expected efficiency should have been >80%.
It could therefore be concluded that, during the time of the investigations, the performance of mill 4 was optimal while mill 3 was operating far below the expected performance level. Mill 3 has to be stopped for maintenance.