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56 GRINDING OPTIMISATION Combining forces Upgrading cement grinding systems offers both process and commercial benefits.

The use of a high-pressure grinding roll in semi-finish grinding mode significantly reduces specific energy consumption and improves production capacity. But this type of grinding system requires two types of air classifiers, which usually ties up additional investment and space. Therefore, Germany-based Maschinenfabrik K?ppern has developed a compact air classifier that enables grinding system upgrades by integrating only one combined machine. n by Florian Kleemann, K?ppern Aufbereitungstechnik GmbH &

Co KG, Germany INTERNATIONAL CEMENT REVIEW JANUARY

2017 High-pressure grinding rolls (HPGR) have been used in cement production for more than

30 years. Since the first industrial application for comminution in 1985, these highly-efficient machines have gained in importance with their main applications in the grinding of brittle materials.1 HPGRs were often added upstream to existing cement plants with a ball mill and an air classifier in closed- circuit grinding. This pre-grinding, which partially moves the grinding load from the ball mill into the HPGR, results in savings of specific energy of around

20 per cent compared to simple closed-circuit ball mill (CCBM) grinding.

2 HPGR in a semi-finish grinding system Further improvements can be made by using the HPGR in semi-finish grinding mode. This efficient grinding system usually includes an HPGR, a static cascade air classifier downstream for deagglomeration and coarse separation, followed by a dynamic air classifier for fine separation in closed circuit with a ball mill (see Figure 1). Compared to an HPGR in pre-grinding mode, material with suitable product fineness after the first comminution stage in the HPGR is directly removed from the grinding circuit and does not have to pass the ball mill. Overgrinding of already fine material is reduced, which leads to better operational performance of the ball mill. Compared with CCBM, these improvements reduce specific energy consumption by 20-40 per cent.2,3 However, a major disadvantage of conventional semi-finish grinding circuits is the necessity of two types of air classifiers (see Figure 1). The dynamic Figure 1: conventional cement grinding with high-pressure grinding roll (HPGR) in semi-finish mode:

1 C HPGR,

2 C static cascade air classifier,

3 C dynamic air classifier and

4 C ball mill Figure 2: cement grinding with HPGR in semi-finish mode with 2-Stage Koesep air classifier:

1 C HPGR,

2 C 2-Stage Koesep and

3 C ball mill

57 GRINDING OPTIMISATION JANUARY

2017 INTERNATIONAL CEMENT REVIEW classifier is usually arranged on top of the static classifier, which requires a larger space. To avoid this, K?ppern developed the 2-Stage Koesep air classifier.4-5 This enables cement producers to use an HPGR in semi-finish grinding mode requiring only one air classifier (see Figure 2). It combines deagglomeration of the HPGR product, coarse separation in a static cascade classifier and fine separation in a high-efficiency air classifier. All classifier functions are installed within one compact housing (see Figure 3). This makes the 2-Stage Koesep classifier well suited to upgrading existing grinding plants with a CCBM or an HPGR in pre-grinding mode to semi-finish grinding systems. In both cases an increase in grinding plant capacity and additional savings of specific energy consumption will be achieved. Set-up and operation of the 2-Stage Koesep air classifier The 2-Stage Koesep classifier consists of two main sections: the static cascade and the dynamic high-efficiency air classifying stage (see Figure 4). The product of the HPGR passes through the inlets (1) into the static stage. It is crossed by the primary separating air (2). The feed falls down and impacts on baffle plates (3) and guiding plates (4) for deagglomeration. The coarse rejects of the static stage move downwards through the outer cone (5) and are discharged at the outlet (6). This material is led back to the HPGR for re-grinding. Fines are carried upwards by the airstream and enter the dynamic section of the 2-Stage Koesep classifier. Here the material passes through guiding vanes (7) before reaching the rotating cage (8) driven by the motor (9). Caused by the different ratio of forces, coarse grains are rejected due to higher centrifugal forces whereas fine particles pass the rotating cage. The rejected material falls through the inner cone (10) and is discharged as middlings at outlet (11). This material is led to the ball mill. The ball mill discharge is directly fed on top of the rotating cage at inlet (12) for dispersion. All material with product Figure 4: set-up of K?ppern 2-Stage Koesep Figure 3: the K?ppern 2-Stage Koesep