Highly energy intensive unit operation of size reduction in cement industry is intended to provide a homogeneous and super fine (3000-4000 Blain) cement. Grinding operation is monitored for following parameters to ensure objectivity and economy of operation.
Grinding Operation Objectives and KPIs.
Chemical analysis of cement, generally on hourly basis.
Product fineness, Blain surface and 45-micron residue.
Cement SO3, %
Grinding aid usage, grams/tonne
Cement moisture, %
Production rate, tonnes/hour
Operating hours as run factor in %
Specific power consumption (SPC) kWh/tonne.
Specific Wear rate, g/t (checked during maintenance of mills)
Grindability and Power Consumption. Among various theories of comminution, most commonly accepted which is relevant to ball /tube mills is Bond's theory, which states that power input in comminution process is proportional to the surface generated in the process and the grindability of the material. To measure grindability, Bond developed 'Bond's Work Index' (BWI), a 'test mill' and a testing procedure for WI. With the help of this we can work out power required to grind a material from a given feed size to a product of given fineness.
Water Spray in Cement Mills. Water spray installed generally in second compartment of ball mill to control cement temperature. Cement discharge temperature should be kept below about 110oC but, the same time should allow some 60% dehydration of gypsum to optimize cement strength without excessive false set. Water spray is controlled with mill outlet gas or material temperature. A hardware interlock is recommended with mill main drive to avoid accidental spray in mill.
Grinding aids are generally added to the ball mill to reduce electrostatic agglomeration of fine particles and to reduce coating formation on grinding media which reduces grinding efficiency. The optimum addition rate should be determined carefully to enhance grinding efficiency. Grinding aids also serve to reduce coating problems in cement storage and enhances cement strengths.
GRINDING MILLS
Mills will be selected according to type most suitable for required capacity and duty and for overall power consumption. Cement grinding operation may be performed in one of the following mill setups.
- Ball and tube mills.
- Vertical Roller Mills (VRM).
- Roller press with Ball mill.
Ball Mill. Ball mills with high efficiency separators have been used for cement grinding in cement plants all these years. Ball mill is a cylinder rotating at about 70-80% of critical speed on two trunnions in white metal bearings or slide shoe bearings for large capacity mills. Closed circuit ball mill with two compartments for coarse and fine grinding are generally found in cement plants for cement grinding. Compartments (filled with grinding media) are divided by a double diaphragm with flow control to utilize maximum mill length for effective grinding.
Grinding media contain balls of different sizes in designed proportions with large sizes in feed end and small sizes in discharge end. About 27 to 35 % volume of mill is filled with grinding media. Equilibrium charge is that charge where compensation for wear can be done by balls of one size only usually the largest size in the compartment. Grinding media could be made of forged steel, cast steel or even cast iron. To economize grinding media consumption, presently grinding media used are high chrome steel balls.
Mill shell is lined with lining plates to protect it from wear, high chrome steel liners are now commonly preferred to give longer life. Lifting liners are used to enhance impact in first compartment, where coarse grinding is dominated by impact. In second compartment which is longer in size (L>1.5D), classifying liners are used to ensure media classification along the length of mill with large size balls near mid partition and smaller balls at mill discharge end.
Ball mills are of 'bucket elevator' type for cement grinding, material is taken by conveyors to a separator where coarse was returned to the mill and fine sent to cyclone separator or bag filter for collection.
Different drive arrangement for ball mills are in existence. Commonly existing arrangement is mill drives with a girth gear and a pinion driven by motor with a gear box. Larger mills have a twin drives of half the ratings on either side of same girth gear. In central drive arrangement both the girth gear and pinion are avoided by connecting gear box output shaft directly to mill.
Closed circuit ball mills are existing with all types of separators () grit, mechanical and high efficiency in cement industries. Presently high efficiency separators are common to achieve maximum energy optimization. Brief description of separators is presented at the end.
Primary Ball mill controls are:
Mill drive power or mill differential pressure to control mill feed rate.
Mill Sound level to control filling level inside mill with feed rate.
Mill outlet gas temperature.
Mill outlet material temperature.
Cement temperature.
Outlet gas flow determined from mill inlet and outlet drafts or flow meters installed.
Vertical Roller Mills. In Vertical Roller mill 2 - 4 rollers (lined with replaceable liners) turning on their axles press on a rotating grinding table (lined with replaceable liners) mounted on the yoke of a gear box. Pressure is exerted hydraulically. This mill also has a built in high efficiency separator above the rollers to reduce circulation loads and consequently reducing differential pressure across the mill.
The mill is started either with the rollers in lifted-up position, or with the hydro-pneumatic system at low pressure. In grinding mode, actual metal to metal contact should be prevented by limit switches or a mechanical stop and by consistent feed. In VRMs the material cycle time is usually less than a minute against several minutes for a ball mill or tube mill. Thus, control response should be accordingly faster. In case mill feed fails action should be taken within no more than 45 seconds or excessive vibration will cause mill shut-down. Moreover, the vertical mills are subject to vibrations if material is too dry to form a stable bed. Therefore, provision is made for controlled spray water inside the mill During mill operation magnetic separator and metal detector should be always functional to ensure to exclude tramp metal which can damage the grinding surfaces.
Primary roller mill controls are:
Mill drive power or mill differential pressure to control mill feed rate.
Outlet gas temperature.
Cement temperature.
Outlet gas flow.
Few countable salient Features of Vertical Mills
- Space needed for vertical mill is much less than a closed circuit ball mill of same capacity.
- As separator is integral part of the mill itself, the number of auxiliary equipments are less.
- Total power consumption for grinding circuit as a whole is also less by about 30% compared to closed circuit ball mill in spite of higher fan power.
- Feed size can be as large as 75 to 100 mm. Feed size can be as large as 5% of roller diameter.
Roller Press. Roller press consists of two rollers lined with wear resistant material. One roller is fixed and the other one is movable to exert pressure, applied hydraulically. A roller press looks similar to a roll crusher. However, the pressure exerted between rollers is very high - of the order of 400 kg/cm2 as compared to roll crushers. Feed is fed over the total width of the rollers by a central chute. About 30 % material gets pulverized to the required product fineness. Roller press output pre-ground material is fed to a ball mill operating in closed circuit. Ball mill required is smaller in size and larger grinding balls are no more required.
Separators
Several types of separator are employed in mill circuits and there are numerous variations of each type:
Mechanical separators Mill discharge material is fed onto a rotating dispersion plate whence it is spread off into a rising air stream. Coarse particles either fall directly from the dispersion plate or are rejected between the auxiliary fan blades and the control valve. Fine dust is taken along with main fan flow and is detrained as the gas flows downwards loses momentum (velocity) and diversion through the return vanes. Controlling parameters are the number of auxiliary blades, the clearance between auxiliary blades and control valve, and the radial position of the main fan blades
High efficiency separators, 3rd generation separators were introduced to improve the mechanical separator's fines recovery efficiency. Examples of these separators are O-Sepa (FLSmidth), Sepol (ThyssenKrupp) A simplified process flow these separators is as follows. Material is fed onto a rotating dispersion plate via air slide, whence it is dispersed off into the classifying air stream. Separator loading is recommended to be up to about 2.5kg feed/M3 air flow. Vortex is formed by the rotor which classifies particles between centrifugal force and the inward air flow. The fine fraction exits upwards/downwards with the air sucked by ID fan passes through cyclone separators or a bag filter for product collection, while the coarse fraction falls and is discharged from the bottom and send back to mill for regrinding. Fineness is controlled by rotor speed (increasing speed increases fineness).