Commissioning a large Slag Grinding Mill
Mr. H. Bhattacharya &  Mr. P.C. Abraham
 

AUTHORS:  
  

 H. Bhattacharya   
   OCL India Limited  
   B-47, Connaught Place  
   New Delhi 110 001 (India)  
   Fax +91-11-3325854, 3731333  		 P.C. Abraham   
   LOESCHE India Pvt. Ltd.  
   E-2, Defense Colony  
   New Delhi 110 024 (India)  
   Fax +91-11-4647674  
INTRODUCTION 
 
Orissa Cement have successfully commissioned vertical roller mill LM 46.2+2 supplied by Loesche GmbH, Germany, for slag grinding in May, 1997.  
 
With this Orissa Cement's cement grinding capacity has been raised from 0.7 to 1 million tonnes per year.  The decision was based on following considerations :  
 
  • Availability of slag  
  • Increasing market size  
  • Customized cement supply 
  Orissa Cement is first in India in using vertical roller mill to finish grind slag and clinker separately and mixing them to produce desired high quality of slag cement. Installation of vertical roller mill to finish grind both the components of cement has lead to possibility of delivering superior quality of blended cements customized to users requirements. Additionally it will lead to saving of costly electrical energy, the cost of which has started spiraling upwards in India and is expected to continue to do so.  
  
 
View of Orissa cement plant
 Reasons for deciding on Vertical Roller Mill  

Major reasons of deciding in favor of VRM technology in finish grinding were:  
 

  • Superior quality of blended cement customized to meet user requirement  
  • Lower overall cost of production due to;  
  • Competitive capital cost  
  • Lower electrical energy consumption  
  • Efficient utilization of waste heat for slag drying  
  • Competitive cost of wear parts  
  • High uptime  
  • Low maintenance  
   
The major factors that weighed in favor of Loesche vertical mill were:  
  • Operating experience of the Loesche mill for raw meal grinding in the existing plant since 1987. Flexibility of grinding either clinker, slag or their mixture with very low change over time from one product to another.     
  • Feed back on performance and quality of cement produced in similar sized installations of Loesche for grinding clinker and slag.  
   
Guaranteed power consumption figures at designed output rate is given in Table-1  
 
Operating Parameters Unit Slag OPC
  Blaine m²/kg 340 340
  Output t/h 92 117
Power Consumption (Guaranteed)      
  Mill drive kWh/t 28,0 20,3
  Mill fan drive kWh/t 6,8 8,1
  Separator drive kWh/t 0,6 1,0
  Auxiliaries, Feed gate, Hydraulics,    
  Lubrication etc.		 kWh/t 0,4 0,4
Total kWh/t 35,8 29,8
 
 Table 1:  Guaranteed maximum power consumption at designed output rate 
  
DESCRIPTION OF PLANT:  
 
MATERIAL TRANSPORT  
 
Clinker obtained from existing five stage precalciner kiln is transported to mill hoppers by Deep Bucket Pan Conveyor supplied by Enexco Teknologies India Limited, New Delhi.  All the moving parts are hardened and conveying elements are dust tight and do not touch one another.  The supporting rollers are mounted independent of chain allowing speedier replacement.  Both the drive and driven sprockets are fitted with replaceable wear segments to reduce maintenance and down time.  
 
The moving elements are designed with minimum dead weight enabling a installation of the deep bucket conveyor with a steep angle of 51°.  
 
Granulated slag received from various steel plants is unloaded and stored in track hoppers and in storage yard.  Gypsum is also unloaded in hoppers.  Material is extracted from the hoppers by means belt conveyors and are conveyed to the mill hoppers through a series of belt conveyors.  From these feed hoppers material is extracted through apron feeders and belt weigh feeders.  
 
 
HEAT FOR SLAG DRYING  
 
Heat is required for drying granulated blast furnace slag containing moisture as high as 15%.  For this vent air from grate cooler and spare gases from preheater of existing kiln will be used.   As such no hot gas generator is necessary.  However, one hot gas generator has been provided as stand-by in case of necessity. Because of long distance from the existing kiln, a booster fan is used to deliver the hot air/gases to the mill. The system is designed for gases available at 270°C.  
 		 Broad specifications of plant  
 
LOESCHE MILL LM 46.2+2C/S  
 
  Mill Type   LM 46.2+2 C/S
  No. of rollers   2 master + 2 slave
  Table diameter   4,6 m
  Motor [kW/rpm]   3.150/1.000
 
 
Technical design parameters of the grinding plant is given below :  
 
  Material Granulated slag Clinker + Gypsum Mixture: 
45% Clinker,  
  50% Slag,    
  5% 
Gypsum
Capacity   92 t/h   117 t/h Estimated 92 t/h
With spec. Surface   
cm²/g (Blaine)		   3.400   3.400   3.400
Feed size 90% < 25 mm   
10% 25-150 mm		 80% < 80  
mm   
20 % < 150 mm		  
Feed moisture   15% max.   
  12% normal 		   2% max.   7%
Fe content   5%   -   -
Grindability acc. to Zeisel @3.400    
Blaine at closed circuit mill shaft		   45,0 kWh/t   33,5 kWh/t  
Product moisture   < 0,5%   < 0,5%   < 0,5%
  
 
Heat source Clinker cooler gases at approx. 270°C + hot gas generator (oil fired) + preheater gases.  
 
 
Baghouse  
  Flow rate (design)   450.000 m³/h
  Temperature   90-120°C
  Dust concentration at inlet   < 400 g/m³
  Dust load at outlet   < 50 mg/Nm³
  Type of bags   Polyacrylonitrile
  Type of cleaning   Pulse Jet
 
Baghouse Fan  
  Flow rate (design)   480.000 m³/h
  Pressure difference   91 mbar
  Temperature (Design)   150°C
  Motor    2.100 kW
 
MILL DESIGN 
 
This new vertical roller mill for clinker and slag grinding is in principle similar to a standard 4 roller Loesche mill, with two pairs of differently sized rollers. The basic design feature of the Loesche vertical roller mills - pair of conical rollers and a flat horizontal grinding table - is retained for the clinker / slag mill. The rollers are carried in individual rocker arms supported on separate mill stands that are not connected to the mill housing and gas ducts. Therefore, any stress during grinding is guided directly into the foundation.  
 
Due to clogging tendency of slag, as observed in some slag grinding roller mills where slag feeding is from side, material is fed into this mill through a central hollow shaft of classifier. A hydraulically operated triple feed gate is provided at the inlet for air seal.  

One pair of rollers called S-Rollers, de-aerates the material and prepares the bed on the grinding table.  These conical rollers are of simpler and lighter construction than normal grinding rollers.  The position of the S-Roller over the grinding table is adjusted with the individual hydraulic system of the roller.  
 
After the material bed has been prepared by the S-rollers, it is nipped properly underneath the large and heavy pair of the so-called M-Rollers, where the comminution of the material takes place. The M-rollers are also of conical shape. The Hydropreumatic system connected to the pair of M-Rollers, ensures uniform loading of roller by means of double acting hydraulic cylinders. Mechanical buffers limit the downward travel of rocker arm and roller, and so prevent any metallic contact with grinding table.  
 
All exposed surfaces are provided with wear resistant liners.  The grinding table is provided with segmental liners and the rollers with tires. Both the liners are made of Nihard IV casting and hardfaced with chromium carbide. Wear resistant liners are also provided for the mill body.  
   
A welding machine is provided for the slag grinding mills for hardfacing of grinding roller tyres and grinding plate segments inside the Loesche mill.  The system consists of two independent units which will be mounted on two opposite rocker arms of the master rollers through  openings in the mill housing for parallel rewelding of the two tyres. The tyres will be driven by the grinding table which is rotated by a auxiliary drive.   The rewelding of the grinding plate segments will be done separately by one of the units. The complete procedure is carried out fully automatic by the control unit of the welding system.  
 
Roller and tire liners have a guaranteed life of 800 hours before rewelding. Each liners shall be rewelding 12 times.  
 
The life of the wearing parts is greatly influenced by iron recirculation within the mill. Due to the higher specific gravity, iron particles tend to accumulate on the grinding table. The iron particles falling  down through the louver ring is transported along with reject material by the bucket elevator. A drum magnet installed at the belt conveyor discharge picks up the iron particles and rejects the same.  
 
As the mill can be started with the rollers in the lifted up position leading to very low starting torque and therefore no auxiliary drive is required for start up of the mill. A hydraulic swinging out device facilitates easy maintenance of the rollers. Provision of water spray is given in the mill body for the temperature control. System of dispensing of grinding aid into the mill is also provided. 
  

View of LM 46.2+2 for slag grinding
 CLASSIFIER  
 
The mill is provided with high efficiency classifier with a central inlet, model LDKS-Z. This classifier is developed as a combination of static air-swept classifier and a dynamic air-swept classifier. It consists of :  
 
  • Components of a rotary classifier.  
  • A vane ring for static preliminary classification, integrated into the classifier upper housing.  The vanes are adjustable.  
  • A grit funnel for feeding back the grit centrally and arranged beneath the vane ring.  
 
The air from the nozzle ring lifts up the ground material through the conical ring chamber between the classifier lower housing and the grit funnel, and is then, after being deflected downward, led to the vane ring.  The deflecting device, in combination with the adjustable vanes at an angle, generates static classification.  In the vane ring, the particles are accelerated tangentially.  A resultant  rotary flow forces coarse particles to the outside, where they impinge on the interior wall of the grit funnel and leave the classification area by gravity.  
 
The revolving rotor blades amplify the rotary flow of the two-phase mixture and bring about dynamic reclassification.  
   
The conical grit funnel ensures an orderly return of grit in the mill. It separates, on the one hand, the rising dust/air mixture from the grit flowing back on its inside and, on the other hand, guides most of the returns - via the back flow flaps which serve as seals - back to the middle of the grinding table, where the regrinding is carried out.  
 
This classifier gives a flatter slope in the curve compared to a rotary classifier and it could be influenced by regulating the static vanes.  
  
Figure 1. Loesche generation 2-
stage high efficiency classifier for 
blast furnace slag
SLAG/CEMENT MIXING  
 
Slag and Clinker with gypsum are ground separately and stored in separate compartments of a four compartment, 8000 t concrete silo. Slag cement is made by mixing desired proportion of ground slag and clinker by a continuous mixer of m-tec make type MD 400 of capacity 200 tph where both materials are thoroughly mixed. The special construction of the centrifugal mixer blades create three-dimensional movement of the components which guarantees a homogeneous mix. The slag cement is delivered either to existing storage facilities or directly to the hoppers of rotary packers for packing and dispatch.  
 
 
ENGINEERING 
 
Engineering was done by Loesche, Germany / Loesche India and Holtec Consulting,  New Delhi.  
 
 
CONTROL PHILOSOPHY  
 
As there are a number of alternatives, control philosophy had to be adapted to the various scenarios of manufacture. Control & Instrumentation system have been supplied by Allen Bradley, a Rockwell group company.  
 
 
ENERGY CONSERVATION/ENVIROMENT  

In the design of the plant utmost importance has been given to environment and energy conservation.  
 

  • High efficiency bag filter has been used for handling the product from the VRM with guaranteed emission below 50 mg/Nm³.  
  • Bag filters have been installed for catching fugitive dust from various discharge points and equipment.  
  • The mill emits very low noise. Sound emission will be restricted to 95 dB.  
  • High efficiency fans (>85%) has been installed for handling gases.  
  • For energy conservation variable speed drives have been used for the Booster Fan (DC Thyristor) and Bag house Fan (slip recovery system).  
  • Energy efficient mechanical conveying has been used instead of pneumatic conveying in most of the areas.  
 		 ERECTION & COMMISSIONING  
 
Orissa Cement set a world record in erection of their LM 46.2+2 C/S in so far as the erection of mill started in first week of November ‘96 and was completed in third  week of March ‘97 i.e., a total of 18 weeks including hold up of work for four weeks due to non-availability of an important component.  
 
 
OPERATING RESULTS 
 
The plant has been  recently commissioned and optimization is in progress.  Meanwhile, operating results till date are quite encouraging.