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Horizontal Ball Mill

Horizontal Ball Milling

When grinding energy efficiencies are examined over a range of conditions for horizontal ball mill, vertical mill or tower mill, and attrition mill systems, an improved energy efficiency is respectively noted. An approximate average system energy efficiency can be determined by dividing the sum of equipment operating brake horsepower by the processing rate.

For the closed circuit wet ball mill, the majority of the system power is drawn by the main mill motor. Information available from various proposed and actual operating system designs, over a capacity range of 5 to 80 short ton per hour dry limestone processing rate, were evaluated against the normal grind requirement. Design main mill drive brake power requirements averaged around 41 bHP per short ton per hour of dry limestone processed. The secondary power consumers, lubrication systems, tank agitators, and classifier feed pumps, seldom exceed 2 bHP per short ton per hour of dry stone processed. With the additional secondary power consumption, the average wet horizontal closed circuit mill system brake horsepower requirement was estimated at 43 brake horsepower per dry short ton.

When the power consumption is divided by processing rate, an energy dissipated per unit mass of material is given. This is a specific energy value that is indicative of the system grinding energy efficiency. For horizontal ball mills, when designed or adjusted for the normal grind condition, an average specific energy determined over a range of designs examined is about 32 kWh/dry short ton of limestone processed.

From an energy efficiency standpoint, a general rule in comminution is to crush as fine as you can, then impact as fine as you can, and only then grind. The crushing operation from the ¾” x 0” size to 1/8” x 0” size has a significantly greater energy utilization than the tower mill grinding operation from 1/8” x 0” to the 95 minus 325 mesh product. Since it is easier to fracture a larger particle than a smaller one, the crusher produces far more new surface area per unit of energy than the tower mill does. Crusher power requirements vary, however, many operate in the range of 2-4 kWh/dry short ton. When continuing the focus on wet grinding, the tower or vertical mill system shows improved energy utilization and can typically provide the normal grind requirement at an approximate value of 25 kWh/dry short ton. This includes the crusher power and represents an approximate 20% power savings over the mill installation. The increased grinding efficiencies for tower mills can be explained by the stirred media mill characteristics and the pre-crusher utilized to reduce the ¾” stone to a ¼” to 1/8” range for the tower mill feed.

Attrition mills are showing a further improvement in applied energy requirements. The approximate corresponding applied energy requirement for the attrition mill system, inclusive of all pre-crushing activity, is on the order of 15 kWh/dry short ton. This represents a sizable improvement over the vertical ball mill and a dramatic improvement over the horizontal ball mill. For the most part, an approximate 50% reduction in power consumption is possible when utilizing attrition milling over classical horizontal ball milling at the normal grind condition. Since the attrition mill is suited towards finer grind capabilities, the option of fine grind performance with energy levels remaining less than that typically achieved with a standard wet FGD ball mill system is possible.

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