OPERATING EXPERIENCE OF VERTICAL ROLLER MILL FOR SLAG GRINDING
Dr. U Marewski &  Mr. P.C. Abraham
 
  
SUMMARY 
 
Vertical Roller Mills are well proven and extensively used for Raw material and coal grinding in Cement industry.  However its use for clinker and slag grinding was limited due apprehensions of the product quality.  To overcome this problem Loesche has introduced a new design of vertical mill with a configuration of master and slave rollers; designated as LM..2+2 C/S.   Such installations are operational for both clinker and slag grinding.  The quality of cement produced by the mills are comparable to that of conventional mills and is well accepted by the market.  The paper discusses the operating experience, and results of such a slag grinding.  Description of the plant and operating data is given in the paper. 
  
  
INTRODUCTION  
 
Vertical Roller Mills have been in use for raw material and coal grinding in cement industry for many years.  Its advantages over other grinding systems such as  higher drying capacity, lower energy consumption, flexibility and reliability in operation, compact layout and ease of maintenance made it a preferred  grinding system.  Despite of these advantages the conventional roller mills were not well accepted for grinding of clinker and slag.   		   
  
There were apprehensions regarding smooth running of mill when grinding to higher fineness and product quality due to narrow particle size distribution.  A lot of research studies and trials were carried out towards overcoming this problem.   

Based on these studies, M/s Loesche has introduced a new design of vertical roller mills of LM..2+2 C/S for grinding of clinker and slag.  The basic design feature of Loesche Vertical Roller mill for raw material grinding with a pair of conical rollers and a flat horizontal grinding table is retained in this design of slag / clinker mills also. The rollers are carried individually by rocker arms.  However, the preparation of the bed and comminution are carried out by separate elements in the mill.   
   
The high specific grinding pressure required for grinding cement and blast furnace slag is achieved by carefully controlled formation of the grinding bed with pairs of rollers consisting  of slave roller and master roller.  The slave roller deareates and prepares the grinding bed and the master roller does the grinding.  Because of this paired mode of operation, such a mill with  4-rollers is  called LM..2+2C (Cement)  or LM..2+2S (Slag).   
 
  
  
 
Fig 1 shows the arrangement of master roller and slave roller on the grinding table and Fig. 2 indicates a general arrangement of the internal parts of the mill. 10 Mills of similar designs has been supplied by Loesche for clinker and slag grinding for application.  Results from an operating plant for clinker grinding is separately presented in another paper.  This paper discusses the operating experience of a similar Loesche mill type LM35.2+2S which is operational since February 1995.
  
 
PLANT DESCRIPTION  

The plant is located in a port city in South France. At this location they have only a slag grinding unit with Loesche vertical mill - neither clinkerisation nor clinker grinding plant is installed at this site. 

   
  
The blast furnace slag is received from the near by steel plant.  The ground slag is transported and blended with ground clinker in other units.  Fig. 3  shows the LM 35.2+2S mill in the plant and Fig. 4 is a view of the slag grinding plant.  
 
Fig. 5 is a  typical flow sheet of the plant with major control loops. The slag with about 12% average moisture  is reclaimed from a stockpile by means of a lateral scraper.  A weigh feeder underneath the feed hopper is used for controlling the feed rate to the mill.   The slag is fed into the mill through the central hollow shaft of the classifier because of its clogging properties. A hydraulically operated triple flap gate is provided at the inlet for air sealing.
 
 
  

  

The material fed through the classifier is ground by the two grinding rollers (Master rollers) which are hydropenumatically actuated against the material bed on the grinding table. 
  
The hydropreumatic spring system ensures uniform loading of the roller by means of double-acting hydraulic cylinders. The two slave rollers are deaerating and compacting the grinding bed to provide a vibration-free and energy-effective grinding. Mechanical buffers limit the downward travel of the rocker arm and roller, and so prevent any metallic contact with the grinding table.  		 The LOESCHE Mill is started with all the four rollers  lifted up which leads to a very low starting torque. Therefore no auxiliary motor is required for the starting of the mill. The hydraulically operated swinging-out device eases the mill maintenance.    
  
All exposed surfaces are provided with wear resistant liners. The grinding table is provided with segmental liners and roller with tyres. Both the liners are made of Nihard IV casting and hardfaced with chromium carbide. Wear resistant liners are also provided for the mill body.  
 
  
 
Fig. 6 shows a view of the mill inside. A welding machine is generally 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 rewelding system.  The rewelding of 2 tyres would require about 18 hours and that for table liner about 16 hours.  
 
As against a guaranteed life of 800 hours before the rewelding of liner they have achieved about 1100 hours.  Each liners shall be rewelded about 12 times. It has been observed that there is no wearing of the slave roller. Though the slave rollers were designed with cast tyres for the earlier installations, today they are being supplied with steel fabricated rollers.  
 
The life of the wearing parts is greatly influenced by iron recirculation within the mill.  Due to the higher specific gravity of iron particles they tend to accumulate on the grinding table.  A special dam ring design is provided for the extraction of the iron contamination. These iron particles falling down through the louvre ring is transported along with reject material by the bucket elevator.  A drum magnet installed at the elevator discharge picks up the iron particles and rejects the same.  
 		 A provision for water spray is given in the mill and the material feed chute.  However, neither water   spray nor grinding aid is being used.  
  
A high efficiency separator type LDKS/Z is incorporated in the housing of the vertical roller mill, which provides a wide flexibility concerning particle size distribution and other physical characteristics. The operating principle of the classifier is discussed later.  

An oil fired hot gas generator supply the heat required for drying of feed moisture. The hot gas along with recirculation and fresh air enters the mill through the ring duct.  The louvre ring guides the air into the mill which lifts  up the material to the classifier from where the coarse is returned back on to the table and the fines are carried by the gas to the bag filter. The velocity in the louvre ring is adjusted so that a portion of reject is falling down which is collected by a scrapper and discharged through a flap value to a belt conveyor.  A velocity of about 45 m/s is maintained through the louvre rings. The reject material is in the range of 10-12 tph.  

The basic control loops are given in Fig. 5.  The pressure drop across the mill is maintained at a constant value by variable speed motor of the feed belt.  The outlet temperature is regulated by the oil feed rate to the HAG.  The flow rate at the mill outlet is maintained constant during the normal operation of mill.  The pressure at mill inlet is controlled by the damper in the duct to the stack.
 
  
 
Fig. 7  gives the plant layout. The low noise and low vibration level of the mill allows a simple plant layout.  It makes an outdoor installation possible and increases the plant’s reliability. It could be seen that there is no building for the mill.  The plant was being manned with just one operator during night shifts. This itself is enough indication of the level of confidence of the client. The plant had been commissioned in February 1995.  Since then it has been proving its high operational reliability as well as constantly high product quality  
  
 
CLASSIFIER  

The mill is provided with high efficiency classifier with a central inlet, model LDKS-Z.  Cross sectional view of the classifier given in Fig. 8 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, let 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 are by gravity.  
 		 The revolving rotor blades amplify the rotary flow of the two-phase mixture and bring about dynamic reclassification by the same principle as in the case of the classifier model LKS-(Z).  

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 backflow flaps which serve a seal-back to the middle of the grinding table, where the regrinding is carried out.  
 
  
  
Fig. 9 shows a grain size distribution curve (RRSB) for the fine slag at 3500 blaines.  This classifier gives a flater slope in the curve compared to a rotary classifier and it could be influenced by regulating the static vanes.  
 

PLANT DATA  
 
Feed material   

Material: blast furnace slag
Moisture average.: 12 %
Moisture max.: 16 %
Iron content: 5,0 to 8,0 %
Grain size: 80% passing 5 mm
Density of slag: 3,0 t/m³ approx.
LOESCHE load factor (Lf): 0,3 at 3.500 blaines
Grindability: 42 kWh/t at 3.500 blaine according to Zeisel test
 
 
 Fig. 10 gives the test results of Zeisel test 
 
 
Vertical Roller Mill  
  
Mill type: LM 35.2+2S
Rated capacity: 48 t/h at 3.500 blaine 
32 t/h at 4.500 blaine
Table diameter: 3,50 m
Number of master rollers: 2 nos.
Number of slae rollers: 2 nos
Mill motor: 1.600 kW
Classifier type: LDKS
Classifier motor type Variable speed
Drive: 2 x 75 kW
 		 Aux. Equipment  
 
Fan  
Operating flow rate: 150.000 m³/h
Temperature: 92 °C
Design flow rate: 200.000 m³/h
Static pressure developed: 70 mbar
Motor type: Variable speed
Drive: 500 kW
Bucket Elevator  
Capacity: 25 t/h
Hot gas generator  
Capacity: 6,5 x 10^6 kcal/h
 
 
OPERATING RESULTS  

The plant has been in operation since February 1995 for producing ground slag fineness ranging from 3.500 to 4.500 blaines.Typical operating parameters of the mill during the performance test is given below :  
 

 Guaranteed  Achieve 
  
A comparison of guaranteed Vs Achieved operating results is given below.  Performance guarantees were given for 2 cases one with a product fineness of 3.500 blaines and other with 4.500 blaines.  
 
Feed moisture : 11,10 %
Product moisture: 0,15 %
Capacity: 51,1 t/h
Product fineness: 3.500 cm²/g
Dust concentration at mill outlet: 330-350 g/m³
Flow rate at filter outlet: 155.000 m³/h
Fan power: 314 kW
Temperature at mill inlet: 237 °C
Temperature at HAG outlet: 496 °C
Temperature at mill outlet: 92 °C
Pressure at mill inlet: -4,8 mbar
Pressure drop across mill: 34,3 mbar
Pressure drop across bag filter: 15,0 mbar
Pressure before fan: -55,2 mbar
Mill vibration: 2,4 mm/s
 
Case 1: Guarantee Achieved
Product fineness (acc. Blaine) 3.500 cm²/g 3.513 cm²/g
Product rate (d.b.): 48,0 t/h 51,1 t/h
Specific power consumption: 
(measured at meter for mill drive and classifier drive)		 24,70 kWh/t 23,92 kWh/t
Diff. pressure mill: < 35,0 mbar 34,0 mbar
Case 2:    
Product fineness (acc. Blaine): 4.500 cm²/g 4.577 cm²/g
Product rate (d.b.): 32,0 t/h 35,6 t/h
Specific power consumption: 
(measured at meter for mill drive and classifier drive)		 33,1 kWh/t 30,9 kWh/t
Diff. pressure mill: < 35,0 mbar 34,8 mbar
 
 
 As against the product moisture of max. 0,3% the ground slag could be dried to less than 0,1%.   
 
CLINKER GRINDING   

After the successful commissioning of the plant several trial tests have been carried out with clinker in order to test the possibility of clinker grinding in the slag mill. In a continuous operation of 5h approximately 250t of cement could be produced in an excellent quality. During this test runs no mechanical adjustments had to be done with the mill.  
  
  
CONCLUSION  

The new Loesche design vertical roller mill for slag grinding with master and slave roller has been successfully commissioned and in operated since February 1995.  The paper describes the plant and discusses the operating parameters.  A specific power consumption 24,7 kWh/t at 3.500 blaine has been achieved for the mill and classifier for slag grinding. 

   
  
In comparison to conventional grinding plants equipped with ball mills more than 30% energy savings may be achieved with the LOESCHE Mill LM...2+2S. 

The quality of the product was in line with that of the product from a ball mill.  The simple and compact layout, case of operation and control and overall the reliability makes it a potential solution for the slag grinding needs of the industry. The trial runs also indicate the same mill could be used for clinker grinding as well.