Engineering & Mining Journal

MAY 2019

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Page 38 of 83

GRINDING CIRCUITS MAY 2019 • E&MJ 37 imum wear. The ground material leaves the table over a dam ring and falls into the process gas (3) heading upward to the classifier (4). Product-sized material passes the classifier and leaves the mill with the gas flow (5) to the bag house filter. Coarse material is rejected by the classifier and led back into the center of the grinding table by the grit cone (6) for further size reduction. A small amount of very coarse material — so called reject — falls through the gas into the ring channel (7) where it is removed by scrapers and is taken back to the mill feed by an external reject conveying system. Operating parameters such as filling rate, classifier rotation speed, table rota- tion speed, working pressure and process gas temperature can be accessed and con- trolled online, ensuring that any changes to the target parameters — throughput, product fineness and product moisture content — can be implemented quickly. As a result, Loesche VRMs can respond directly to changes in the grinding charac- teristics of the material and compensate for any resulting fluctuations. EuroChem's Resource The Kok-Djon deposit lies in the Karatau basin in southern Kazakhstan. The opera- tion is located near Zhanatas, about 120 km north of Shymkent. The region has a continental climate with temperatures up to 50°C in the summer and minus 40°C in winter, with short but intense rainfall. The main phosphate mineral in the deposit is apatite, with an average P 2 O 5 content of 26% to 29%. The main gangue minerals are calcite and quartz. The rock is relatively hard and mod- erately abrasive. It is considered to have medium to poor grindability. Considering the process- and prod- uct-related constraints in combination with the difficult climatic conditions, the pro- duction of 80 mt/h of dry phosphate meal with a particle size (P90) of 500 µm and a water content of less than 1% is complex. The deposit is heterogeneous, leading to variations in the material properties and its grindability. Segregation effects, especially during material handling prior to the crushing stage, result in frequent changes in the feed size distribution to the crushing plant and subsequently to the grinding plant. The feed moisture fluctuates strong- ly depending on the weather, with high moisture content making the material hard to transport. Regardless of the fluc- tuations in the feed, product particle size and product moisture need to remain con- stant to assure optimum feed character- istics for the beneficiation plant. To avoid the formation of dust and problems in the downstream processes, the amount of the -150 µm fraction was to be minimized. Basically, the characteristics of the mill feed vary widely while the product properties remain constant for the sake of the downstream operations. VRM tech- nology can compensate for the fluctu- ations that occur. Conversely, trying to address the same task using conventional technology would result in a clearly more complex processing route (See Figure 2). Taking into account commonly accepted limits for the feed and the product size of the conventional equipment, the size reduction from 80 mm (P90) to 500 µm (P90) would need at least one more Transferring Technology Based on the experience of more than a century of supplying VRMs to various industries, Loesche is now transferring the technology to the mining industry. To meet the demanding requirements and to counter prevailing prejudices, the company has adapted the design of its grinding technology over recent years. Headed by the department for Ore and Minerals Technology, the company undertook an in-depth review of all the components of its grind- ing plants, working with internal and external experts. The review covered equipment-, process- and plant-design, as well as process control. This approach was applied not only to Loesche equipment but included also the equipment provided by sub-suppliers. Special attention was given to handling the wide variety of potential product sizes and material densities for different ore applications, and to the utilization of the VRM technology to sta- bilize throughput and product size by using parameters such as the classifier speed, working pressure and table rotation speed. The major aspect during this process was the higher abrasive- ness found in ore-grinding applications compared to materials from standard applications, such as limestone, clinker, blast fur- nace slags and coal. Under the premise of achieving a lifetime of at least one year with a minimum availability of 93% also for highly abrasive materials, Loesche focused on the enhancement of the lifetime of the various plant components. The occurrence of wear was minimized as far as possible by means of design. In addition, components still exposed to wear were protected by in- dividually selected, exchangeable high-quality wear parts. Easy access and easy handling of the parts to be exchanged were an important factor throughout all considerations. In consequence of the review, Loesche has introduced the new M type series of its mill, dynamic classifier and rotary star feeder for use in mining and industrial minerals applications. In addition to other features, this series incorporates an increased number of exchangeable components, more massive wear protection, several wear material qualities, improved access and faster exchange. For the main wear parts of the mill (the grinding tires and grind- ing table segments), the latest high-quality metal matrix composites (MMC) are used, combining extremely wear-resistant ceramic inlays with a shock-proof cast-iron body. These composites result in very durable grinding elements that offer a significantly increased life- time compared to standard high-chrome elements and hardfacing. Since the phosphate rock from the Kok-Djon deposit is one of the harder, more abrasive materials handled by the wide range of Loesche installations, the project provided an ideal opportunity to implement the solutions that had been developed. Figure 2—Block diagram for the VRM process and a conventional process.

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