The existing trend of finish grinding technology in cement manufacturing shows 50% market share of ball mills, 40% market share of VRMs and 10% of others like roller presses. Finish grinding consumes 35-40% of total electrical energy required for cement production. This is one of the areas where huge energy saving can be done. The main trends concerning grinding processes in the cement industry are higher efficiency, reduction of the power consumption and system simplicity. The technology which has been implemented in the JSW company’s plant in Nandyal, Uttar Pradesh.
HPGR (high pressure grinding rolls) or Roller Press is the finest alternate to ball mill and VRM. There is definitely a shift towards the Roller Press in the recent years. The scope of Roller Press for finish grinding is tremendous due to very less specific power consumption and simple operation. The major equipment in a Roller Press circuit are Roller Press itself, de-agglomerator (V-separator) and dynamic separator and fans along with collection equipment like cyclones and bag filters. All these equipment are simple to operate and easy to maintain. The main benefits are increased strengths and lower power consumption as compared to other systems. The reliability of Roller Press is much higher than VRM circuit. Since the operating cost is less, there is an excellent scope for Roller Press in finish grinding.
Certainly compared to ball mills and VRMs, this is cost-effective equipment. It has less wear and tear, availability is high, easy to maintain, low energy
consumption and longer lifetime. So, JSW cement has installed Roller Press (COMFLEX) with ball mill supplied by m/s. KHD, Germany for its cement grinding. Upgradation has been done in Roller Press operation with finish mode and there is savings of 2 to 3 kwh/ton of cement.
Roller Press Working Principle:
The Roller Press operates under the principle of high pressure – inter particle communication. The bulk material is fed to the gap between two rollers which are driven in counter rotation. The necessary press force is created by a hydraulic – pneumatic system which works like spring.
Roller Press Semi finish mode circuit:
The semi finish mode circuit consists of two Roller Presses (COMFLEX), two V- Separators, single dynamic separator and ball mill.
Roller Press Semi finish mode operation has few disadvantages like higher power consumption due to ball mill operation; water spray required for reducing cement temperature; additional ball mill maintenance and comparatively complex circuit than Roller Press finish mode.
Working principle of finish grinding in KHD COMFLEX Roller Press:
The fresh feed is fed into the Roller Press and adequate grinding pressure is given to grind the clinker. The Roller Press output is then passed through the V-separator. It serves the important functions such as de-agglomeration, separation and drying. It is a simple cascade type separator. Within COMFLEX, the V-Separator takes already up to 50% of the separation job and does the coarse cut. Large feed rates with high air to material loadings are taken by the V-Separator. While coarse cut rejects particles above 1 to 3 mm and they are sent right back to the Roller Press for further grinding, the fines leaving the V-Separator on the top is conveyed pneumatically to the final separator. The cut size is defined by the air velocity in the separating channels. Then the final product is transported to silos through the bag filters or cyclones.
Challenges faced in Roller Press finish mode operation:
- Higher fine recirculation and segregation at the Roller Press prebin cause higher vibration levels.
- Material flushing from prebin and high pressure differences
- Material flushing from V-separator discharge and the belt conveyors got overloaded
- Difficult to achieve required fineness (3500cm2/gm)
- Challenge in achieving the required production rate
- Difficult in mixing dynamic separator reject material with fresh feed
Modifications carried out to overcome the challenges:
- Gap between the prebin discharge chute to Roller Press surface is adjusted. It reduces the material flushing from prebin and increases the operation stability which gives great control over the vibration levels
- Material segregation is avoided by installing the mixing box inside the prebin. This results in homogenous feed distribution to the Roller Press and maintains pressure and gap differences.
- Dynamic separator rejects diverted material to V-separator discharge belt conveyor for better mixing of fines and fresh feed.
- Thus material homogenization is achieved through various modifications and results in improvement in grinding efficiency and reduction of Roller Press vibration.
- From process gas flow measurements, it was found that there was difference in gas flow rate in two V-separators because of blockage in one Roller Press feed chute. After removing the blockage, equal gas flow rate achieved through two V-separators which improves separation efficiency and feed rate.
- Cake breaker beam installed at the Roller Press discharge chute and dynamic separator guide vane angle adjusted to improve separation efficiency.
- Roller Press dedusting system modified to achieve better venting.
- Recirculation bucket elevator and belt conveyor discharge chutes were redesigned to avoid material flushing and overloading.
After modification trials have been carried out with different blaines. The process parameters were assessed.
The Roller Press finish mode operation in cement grinding has been successfully completed by the company at the Nandyal plant and is in continuous operation. Finish mode operation saves electrical energy up to 2-3kwh/ton of OPC. It eliminates water required for cement temperature control. System simplicity provides controlled and stable operation. It is also helpful for reducing the maintenance activities. All these factors put together reduced the operating cost of the cement grinding. Apart from monetary benefits, it also reduces the CO2 emissions, because of less power consumption.