Author: Shen Xirong
Advantages of the 1 roller press system
As an efficient, energy-saving grinding machine, the roller press is widely used for raw-material and cement grinding. Combined roller-press grinding is a nationally promoted energy-saving technology. Roller presses are extensively used in cement plants and convert a much greater share of input energy into useful grinding than ball mills. A finish-grinding roller-press system can save more than 40% electricity compared with a ball-mill system and substantially reduce metal wear. Conventional ball milling typically consumes nearly 300g/t of metal per unit of product, while combined roller-press systems mostly remain below 40g/t.
The performance comparison of the raw meal grinding system is shown in table 1.
Table 1 Comprehensive comparison of raw meal grinding systems
2 Roller Press System Frequently Asked Questions
There are also some problems during the use of roller presses, such as poor product particle morphology, unreasonable particle gradation, and large water consumption for standard consistency. These problems can be improved by adjusting process equipment, such as roller press pressure, roll gap control, efficient powder separator ventilation, speed control, and cyclic load control. In addition, the roller press system is also prone to problems such as roll gap deviation, pressure deviation, side baffle leakage, material punching at the feed port, and poor unloading of the weighing bin.
The roller press system may also have problems in the design of the feeding device, which may lead to phenomena such as the rod valve not being able to open, the position of the warehouse being unable to be controlled, poor material cake quality, collapse of the warehouse, ash emission, vibration, roll deflection, etc. These phenomena still occur in some factories and affect the efficiency of the roller press. These problems can be solved by using new hyperbolic feeding devices, V-selected distributors and other equipment. The appearance of the hyperbolic feed device is shown in Figure1.
Figure 1 The appearance of the hyperbolic feeding device
Roll skew caused by segregation of the feed along the roll axis is a common and representative problem in roller-press applications, troubling many managers and operators. To address this persistent problem, an in-depth analysis of the root causes of feed segregation and roll skew led to the design and development of a completely new V-type mixer using innovative concepts and solutions. Practical application has demonstrated its many advantages, including an advanced concept, a simple process, and no need for additional power. The following introduces several common misconceptions about roller-press feed distribution and the corresponding solutions for reference by industry colleagues.
3 Common misunderstandings in roller press cloth technology
Differences in particle size, moisture, other physicochemical properties, and dynamic angle of repose cause coarse material to travel forward and downward as material is scattered or rolls down, resulting in segregation of material entering the steady-flow bin. Under either funnel-flow or mass-flow conditions, the segregated coarse and fine materials do not readily move laterally or mix. They therefore remain unmixed as they descend to the roller press feed inlet. This forces open the roll gap at the end receiving coarse material while the end receiving fine material fails to open, directly affecting roller press efficiency. Roll skew generates an axial force component that makes bearings prone to damage. Misalignment of the roll ends reduces the working surface, and side plates that cannot fit tightly allow material leakage.
When feeding a roller press, it is desirable to have the most densely packed materials, with coarse, medium, and fine particles evenly distributed in the axial direction, just like a bucket of broad beans with half a bucket of soybeans plus two bowls of rice and one bowl of flour. Only with fewer gaps between coarse and fine particles and the tightest packing can the dense material bed crushing of the extruded material be better achieved. The segregation of coarse material at one end of the roller and fine material at the other end has a great impact on the efficient operation of the roller press. The extrusion effect will not be good when the rollers are running off-center. Only by mixing the coarse and fine materials evenly can the effective performance of the roller press be improved.
Roll skew affects both output and equipment health. It generates axial force (see Figure 2), causing the shaft system to tilt and shift axially so that the end faces of the movable and fixed rolls become misaligned. This causes material leakage and accelerates side-plate wear. Large axial forces can damage bearings and bearing-housing slideways and shorten hydraulic cylinder life. They also move both bearings and their housings toward the smaller-gap end. The author has encountered various equipment accidents caused by roll skew, including failure of bearing end-cover bolts, outward movement and breakage of the bearing on the smaller-gap side, and inward movement of the bearing on the larger-gap side that split its inner ring.
Figure2 Analysis of axial force component of deflection roller
If a constant roll gap is pursued through one-sided pressure adjustments, for example increasing pressure after coarse material opens the gap, prolonged operation produces uneven roll-surface wear. If constant pressure is pursued, coarse material opens the gap and raises pressure; relieving pressure makes the gap still larger. On the fine-powder side, the gap and pressure are smaller, so applying more pressure is of little value. Inserting a plate inward on the side with the wider coarse-material gap resembles closing a rod gate on one side of a feed pipe: coarse material still will not move to the narrower-gap side. Inserting a plate near the roll end reduces the effective roll surface. Restricting material pressure may barely control roll skew, but makes surface wear even less uniform (Figure 3) and further constrains the material bed. These so-called four-sided adjustment, constant-gap or constant-pressure products, developed without considering process behavior, may work briefly but do not support stable long-term operation.
Figure3 Uneven wear on the roller surface
At present, some equipment companies in the industry use electromechanical and hydraulic control, such as constant roll gap flexibility control and other technologies to solve the problem of roll deviation. This can achieve certain results in the short term, but it cannot solve the fundamental problem of material thickness and fine segregation that causes roll deviation, and may even have negative impacts in long-term operation.
4 Working principle of V-type mixer
In order to solve the problem of segregated deflection rollers, the cement industry has conducted many years of research. Many units have concentrated and extended the discharge pipes and added buffer plates in the flow stabilization silo, which has a certain effect. However, the deflection rollers cannot be stably improved due to changes in the position, flow rate, and physical and chemical properties of the material.
Common feed-distribution methods for correcting roll skew each have advantages and disadvantages. An early design was the multilayer staggered fan-shaped distributor developed by the HF institute. It worked well but was complex and prone to blockage. More recently, bidirectional slide plates have been introduced to change the direction of material flow in the feed pipe, redirecting the drop point in the bin to reduce roll skew. Operation is cumbersome, wear occurs readily and there are few successful examples. Powered rotary distributors spread material uniformly and address segregation well, but are complex, consume power and cost more.
The use of V-shaped mixer can effectively solve the common problem of roller separation and deviation in roller presses. The appearance of the V-type mixer is shown in Figure4.
Figure4 Appearance of V-type mixer
The V-type mixer has a simple process and operates without power. It uses a combination of multiple trays to imitate the mixing action to achieve uniform distribution of thickness. In conjunction with the hyperbolic feeding device and V-selective distributor that have been popularized in the industry in recent years, it can solve common abnormal working conditions of roller presses and achieve better production, energy saving and stable operation effects.
5 Case Study
The V-shaped mixer mixes coarse and fine materials evenly, making the feeding of the roller press more dense and even, effectively improving the efficiency of the roller press and ensuring the safe operation of the equipment. It has a simple structure, does not require power, is not prone to clogging and wear, and is relatively reliable in operation. Some units have achieved good results in use.
5.1 Case 1
The XCHL grinding system uses a G180160 roller press with a unit-hour output of 280t/h. Problems such as roller deviation, side leakage, and overloading of the circulating bucket often occur during operation. The operation of the roller press is extremely unstable, affecting output and quality.
In May 2022, after the plant installed a hyperbolic feeding device in the steady-flow bin, roller-press efficiency improved and the proportion of acceptable fine powder in the pressed cake increased significantly. At the same output, the circulating bucket elevator current fell by more than 20 A, but roll skew limited the increase in production. Further optimization in July included installing an SG180VH2 mixer in the steady-flow bin, completely resolving the roll-skew bottleneck. The roll-gap difference was effectively reduced by 10 mm and output increased by more than 10 t/h. The roller-press current increased, while the circulating load decreased and powder-separation efficiency improved.
It is planned to continue optimizing the V-selection distributor in the next step to increase system production and reduce consumption by improving powder selection efficiency.
Table 2 XCHL Comparison of process parameters before and after adopting V-type mixer technical modification
5.2 Case 2
The LLHS grinding system has multiple sets of HFCG150100 roller presses, which used hyperbolic feed devices earlier. Due to the market downturn, they have been producing in low cycle load energy-saving mode. The process design of fresh materials and recycled materials enter the stable flow bin in two ways. The particle size of the raw materials differs greatly, and the roller deviation is serious, which even affects the operation of the equipment. The plant has not achieved significant results in treating material segregation for many years. The tapered journal and bearing seat slide groove have been repaired by online surfacing and turning for many times.
The material pipe entering the steady flow warehouse was shrunk to 400mm × 400mm, and the buffer support plate was added, which still could not solve the problem of roller deflection. Generally, the roller gap deviation reached more than 18mm. During the peak production season at the end of 2021, major equipment accidents occurred, including wear of the tapered journal of the roller press and cracking of the inner ring of the bearing. After the bearing was replaced and production was resumed for an emergency, the problem of bearing inner circle occurred again in early 2022, and production was stopped again for emergency glue repair.
Figure 5 The inner ring of the bearing of the shaft press is cracked
In August 2022, the plant installed an SG150VH2 mixer in its steady-flow bin, completely resolving roller-press skew and increasing output. Table 3 compares process parameters before and after installation of the V-type mixer.
Table 3 LLHS Comparison of process parameters before and after using V-type mixer
6 Conclusion
The V-type mixer, combined with the hyperbolic feed device and V-selective distributor that have been gradually popularized in the industry in recent years, can solve the problems of roller deflection and silo collapse from the perspective of process principles, improve the density and restriction of the material bed, accurately control the cycle load, and improve the effective performance of the roller press and system stability.
Source: World Building Materials Network
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