Problems with the Roller Press System
1. Currently, most domestic roller presses adopt a low-pressure, large-circulation control method. The pressure is relatively low, averaging 80–100 MPa. The discharge fineness is about 0.08 mm, with 80–90% residue on the sieve, resulting in a low proportion of fine powder and poor pressing performance. In addition, the operating current of the roller press fluctuates greatly, typically running at only 50–60% of the rated current.
2. There are two approaches to controlling the roller gap: constant-pressure control, which results in large fluctuations in the gap, and constant-gap control, which leads to large pressure fluctuations. However, both methods cause the motor operating current to be low and unstable.
3. The feeding devices of many roller presses have been modified, but the current modifications can only control the opening degree, not effectively control the flow rate, failing to achieve the desired performance.
4. The feeding bin control system of the roller press is unstable, and sometimes a surge of material occurs, resulting in severe leakage at the edges. In addition, the material entering the mill cannot display the flow rate; it is controlled solely by adjusting the speed of the circulating fan and relying on fineness and specific surface area as indicators, which results in poor stability.
Retrofit Measures
1. A high-pressure, small-circulation control method is adopted, applying a pressure of approximately 240–300 MPa to the material. The discharge fineness can reach 0.08 mm with about 70% residue on the sieve, and the specific surface area is greatly improved, achieving the desired grinding effect. In addition, the roller press gap adjustment mechanism is modified to allow the rollers to run more smoothly, making small oscillations within a certain range (±5 mm).
2. A new multi-directional feeding device is used to control the feed and achieve an appropriate material flow rate, enabling smoother roller operation and allowing the roller press to reach normal operating conditions. The operating current of the two main motors reaches 80–90% of the rated current.
3. The PLC control system is improved. Without changing the original foundation, an independent control unit is added for pressure and roller gap control, using a Siemens S7-200 in combination with Siemens drive products and HMI components.
4. An automatic control system is applied to the roller press feeding bin to stabilize the material level and dosing, while also greatly improving the stability of the material entering the mill. An automatic material monitoring system is used to track and regulate parameters such as moisture and temperature, especially in cases of high clinker temperature or a high proportion of fine powder.
Cement Grinding System Retrofit Project
A dual-closed-circuit combined cement grinding system in a cement plant consists of a closed-circuit grinding system with a φ4.2×13 m cement ball mill and a Sepa3000 separator, along with a pre-grinding system comprising a roller press (motor power 900 × 2 kW) and a V-type separator. The system output is 160–180 tons per hour (producing PO42.5 cement), with a specific surface area of 320–330 m²/kg. However, the product quality is unstable, and the roller press circulation elevator is frequently overloaded, sometimes resulting in tripping, which seriously affects normal production.
Existing Problems
(1) A sample of 920 g of cement powder taken from the roller press discharge showed 626 g above 0.9 mm, 781 g above 0.2 mm, and 864 g above 0.08 mm, with a 0.08 mm residue on sieve of 93.4% and only 6.6% passing below 0.08 mm. The feed fineness to the mill has a 0.08 mm residue on sieve of 34–38% (specific surface area 160 m²/kg). This results in a very high circulating load on the roller press, with the circulation elevator current reaching 200–220 A, operating at full load.
(2) The circulating fan speed fluctuates significantly, between 60–90%. The roller press feed bin level fluctuates greatly, as do the roller gap and material flow. Although the feeding device can control the opening degree, its performance is poor and it fails to effectively regulate the flow rate.
(3) The mill operating conditions are unstable. The cement specific surface area is low, at about 320–330 m²/kg. The separator speed is between 1000–1100 rpm, but adjusting the separator speed alone cannot meet production requirements. The elevator current at the mill tail fluctuates greatly, and the internal circulating load of the mill is high.
Retrofit Content
(1) Roller press feeding device (2) Roller gap adjustment device (3) PLC control system (4) Automatic level control system for the roller press feed bin
Retrofit Results
(1) The cement mill output increased from the original 160–180 t/h to 180–200 t/h. The cement specific surface area improved from about 320–330 m²/kg to approximately 350 m²/kg.
(2) The roller press now operates more stably, with the operating current increased to 40–50 A. The circulation elevator current decreased from the original 200–220 A to 170–180 A, eliminating overload tripping incidents.
(3) The stability of central control operations has significantly improved. The elevator current at the mill tail is maintained at a stable 70–80 A. The Sepa separator speed is stabilized at 780–800 rpm. The roller press system’s circulating fan speed remains at 75–77%. The roller press bin level is kept at 10–15 tons. Overall system stability and product quality controllability have been greatly enhanced.
(4) A sampling of cement powder at the roller press discharge shows 15.6% passing below 0.08 mm. There is still room for improvement, as excellent roller press performance can achieve 30–40% passing below 0.08 mm. Since this roller press is an early-generation model with a relatively low motor power configuration (2 × 900 kW), increasing the power capacity would result in even higher efficiency.
Retrofit Cycle
The equipment manufacturing takes about 40 days, and the installation period is about 5 days.
Retrofit Economic Benefits
After the retrofit, the roller press saves 1 kWh of electricity per ton of cement produced. Based on an annual output of 1 million tons of cement and an electricity cost of 0.55 RMB per kWh, the annual savings from electricity alone amount to 550,000 RMB. In addition, the retrofit reduces clinker consumption and increases clinker utilization by more than 1%. Assuming the clinker cost is 190 RMB per ton, this results in a cost savings of 1 RMB for every ton of cement produced, creating an additional annual benefit of 1 million RMB. Overall, the investment in the retrofit can be fully recovered within six months.
