Basic Layout
The attrition mill, due to its inherent design, will more ef- fectively grind a larger percentage of fine particles, compared to the horizontal ball mill. This finer grind will result in a smaller recirculation load when designing hydroclones for the final clas- sification. The recirculation load is defined as the ratio of feed to the hydroclone to the overflow of the hydroclone. A typical recirculation load for an attrition ball mill processing limestone with a bond work index of 10 and a feed particle size of 1/4”x 0”is between 140 to 150%. The result is smaller hydroclones than for a typical horizontal ball mill system of similar capac- ity.
There are two basic recirculation loop methodologies for an attrition ball mill.
- Single Recirculation Loop
- Double Recirculation Loop
The single recirculation loop is typical for attrition mills as well as conventional horizontal ball mills. This layout relies strictly on the hydroclone to perform the classification function. In this layout, the recirculation load can range from 200 to 250%.
The double recirculation loop(Fig. 2)utilizes a primary re- circulation loop to re-grind coarse particles and a secondary re- circulation loop for the final classification. In this layout, the recirculation load is in the previously stated range of 140 to 150%.
Overall the attrition mill system is area and foundation size efficient. In a study performed by Chemco on a wet FGD sys- tem that required two 17. 5 ton/hr vertical ball mills versus two attrition mills, the building size and height were reduced. This resulted in substantial savings on the metal building costs as well as the foundations.
Redundancy
Multiple attrition mill arrangement
Many wet FGD system owners have required 100%redun- dancy for the limestone grinding system. This is typically achieved by doubling of the equipment, such as feeders, crush- ers(if incorporated in the design), ball mills, mill product tanks and accessories, such as hydroclone feed pumps and the hydroclone clusters.
Not only does this approach increase the capital expendi- tures for the process equipment, but it enlarges the building size and foundations required to house and support the entire 100% redundant limestone grinding system.
An alternate approach to system redundancy is to evaluate the use of multiple mills of a smaller capacity. For example, a 60 tph capacity system could be achieved with the use of four 15 tph capacity attrition mills. One, two or three additional mills could be added to satisfy redundancy needed for the degree of availability sought. In this manner, the mill sizes are reduced to the demonstrated capability range of the attrition mill, thus al- lowing the significant energy savings possible with attrition grinding technology.
A total system installation can be designed in less floor area and with much less foundation work due to the significantly lower weights and loads of the attrition mill as compared to the horizontal ball mill. This multi-mill redundant system can re- sult in the following advantages.
- Less overall floor space
- Less connected power
- Less operating power costs
- Smaller electrical loads and motor control centers
- Smaller foundations
- Less process equipment costs
- Increased operational flexibility with the capability of bringing mill capacity on line as needed to suit the limestone demand
Fig. 3 depicts one possible multi-mill arrangement utilizing typical bin activated silo designs. Alternate storage and feed- ing arrangements can produce variations in reliability that trade against spare capacity. However, even when the storage and feed equipment is designed for extremely high reliability, the total installed cost appears less than the typical 100%redun- dant ball mill system. The large installed motor and the mill foundation costs of massive horizontal ball mills are two of their major drawbacks. Compared to the vertical ball mill these two items are improved, but remain substantially larger than that required for the attrition mill system.
Energy Efficiencies
When grinding energy efficiencies are examined over a range of conditions for horizontal ball mill, vertical ball or tower mill, and attrition mill systems, an improved energy efficiency is re- spectively noted.
When testing for material grindability or mill performance, the 80%passing particle size for the mill system feed(F 80 )and mill system product(P8 0)is often used. A typical wet FGD grind requirement is to comminute a 3/4”x 0”feed size(F 80 @12000 microns)to a 95%-325 mesh(P 80 @25 microns)product slurry at a limestone bond work index of 10 kWh/short ton. In the context of this paper, this is referred to as the‘normal grind requirement’.
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 horizontal ball mill, the majority of the system power is drawn by the main mill motor. Informa- tion available from various proposed and actual operating sys- tem designs, over a capacity range of 5 to 80 short ton per hour dry limestone processing rate, were evaluated against the nor- mal 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, lu- brication systems, tank agitators, and classifier feed pumps, seldom exceed 2 bHP per short ton per hour of dry stone pro- cessed. With the additional secondary power consumption, the average wet horizontal closed circuit mill system brake horse- power 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 com- minution is to crush as fine as you can, then impact as fine as you can, and only then grind. The crushing operation from the 3/4″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 95-325 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 ball mill system shows improved energy utilization and can typically provide the normal grind requirement at an ap- proximate value of 25 kWh/dry short ton. This includes the crusher power and represents an approximate 20%power sav- ings over the horizontal ball 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 3/4″stone to a 1/4″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 con- sumption is possible when utilizing attrition milling over clas- sical horizontal ball milling at the normal grind condition. Since the attrition mill is suited towards finer grind capa- bilities, 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.