Ball Mills

Ball mills are horizontal cylindrical shaped equipment that finds application in grinding hard minerals to the form of fine powder. The grinding operation is achieved here through rotation of cylinder with steel grinding balls/ceramic balls that causes balls to fall back into cylinder and onto material which requires being grinded. These mills are divided into multiple chambers along its length with the chambers further divided by diaphragms.


Working principle :

• Rotation of cylinder is done at relatively speeds that allows balls to cascade through mill base and in turn achieve grinding/dispersing of materials
• Rotation of cylinder is normally between 4 to 24 RPM (depends upon mill diameter where larger diameter decreases the RPM and vice versa)
• In case peripheral speed of mill is very high, it acts like centrifuge where balls do not fall back and stay on perimeter of mill (this point is called "Critical Speed"- ball mills normally operate at 65% to 75% of critical speed)
• With critical speed (rpm) denoted by formula nC = 42.29/vd (d is internal diameter in meters), a mill with diameter of 5 meters will turn at around 14 rpm
• These can be used for grinding material that is 1/4 inch and finer (can grind down to particle size of 20 to 75 microns)
• For achieving desirable efficiency, these are operated in closed circuit system with oversize material being continuously re-circulated back into mill for further reduction
• There is use of different type of classifiers like screens, cyclones and air classifiers for classifying discharge from ball mills
• Here, each chamber allows grinding to particular size with subsequent chamber making particle size finer in terms of diameter
• Mills come cladded with liners in its internal periphery as well as in chambers dividing diaphragms

Design Features :

• Horizontal cylinder of ball mill fabricated using mild steel plates and rotated using side drive system present at one end of mill
• Drive comprises girth gear made from alloy steel casting that are bolted onto mill and driven by its pinion made of alloy steel forging through use of gear reducer and motor (coupled together)
• Two ends of cylinder covered with mill ends (also known as mill headers) with one placed at each end - at inlet and at discharge
• Mill ends made out of alloy steel casting come with central inlet & discharge opening that is used as material entrance and exit point
• Mill headers coming rested on fabricated trunions at both ends on which headers rotate on special self lubricating white metal lining
• Option of external lubrication system provided for keeping mill headers optimally lubricated at all times
• Can be provided with other accessories like set of low pressure & high pressure pumps that support pre-jacking mills during startup
• Grinding Media is generally made of high-chromium steel/manganese steel
• Mill Liners are cladded throughout its periphery of length and are made of high chromium steel/manganese steel

Applications :
These are widely used in grinding of -

• Cement
• Silicates
• Building materials
• Black and non-ferrous metals
• Ceramics
• Mineral ores
• Calcium bauxite
• Other materials that require to be grounded into fine powder form either through wet process/dry process