Ores Grinding

The mine ores are invariably pulverized to a size small enough to liberate mineral particles from the barren rock (gangue). This comminution is ordinarily the first step that takes place within the mineral processing plant.

A number of grinding mill types are employed for mines. The classic grinding circuit consisted of a rod mill followed by a ball mill(s) in a two-stage circuit. This arrangement is still found at older installations and some newer ones that were built with used equipment.

A Semi-Autogenous Grinding (SAG) mill followed by a ball (or sometimes a pebble) mill(s) is the common arrangement found in modern plants of medium to large size in North America. Smaller mines often employ an extra stage of crushing to create product small enough to permit single stage grinding with a ball mill(s).

Ball Mills

The ball mill remains the most widely used grinding unit at hard rock mines.

The drum of a primary ball is typically cylindrical and of length equal to or up to 65% longer than its diameter. A number of even longer mills and conical mills have been manufactured in the past on the thesis that these designs better enable classification of the ground ore as it passes through the mill.

The grinding action is obtained by rotating the drum so that forged (or cast) manganese alloy steel balls (or cast iron slugs) are cascaded and tumbled with the ore. The ore is ground between balls and normally between balls and a steel liner. Over a period of time, the balls wear to a smaller diameter so that at any one time there is a gradation in the size. The average gradation is maintained by the regular addition of new (“green”) balls. In the past, steel balls had diameters ranging between two and three inches (depending on the drum diameter). Today, steel balls with four-inch diameters and more may be

employed in larger diameter ball mills. The quantity (charge) of steel balls in the ball mill may range from 35 to 45% of the volume within. A mixture of crushed ore and water fills the space between and around the balls, such that the rotating drum is approximately half full. The pulp (crushed ore and water) in a ball mill is held near 75% solids (by weight).

The ball mill typically operates in closed circuit, meaning that a portion of its output (containing coarse ground ore) is recycled through the drum to be ground down to size. This recycling is a dynamic process in which pulp goes through the ball mill several times (on average). Between 2¼ and 2¾ times (225 - 275% re-circulation) is nominal; however, there are installations where the re-circulation exceeds 500%. Separating the coarse fraction of the ground ore to be returned to the ball mill is normally accomplished in a hydro-cyclone classifier. Rake classifiers and spiral classifiers are virtually obsolete, mostly due to

the space required. Because it has no moving parts, the cyclone classifier requires little maintenance, but it consumes more power because the pulp must be pumped up to it at sufficient velocity to maintain 10 psi (70 kPa) or more of head at the entrance for proper performance.

The nominal product from a ball mill is considered to be 80% -200 mesh. Larger particle size is termed a coarse grind while smaller sized product is referred to as a fine grind.

Autogenous Mills

A few of the larger mines have been successful employing a Full Autogenous Grinding (FAG or AG) mill (the larger chunks of crushed ore act as the grinding medium). This type of mill is very appealing (especially for a remote minesite), since it avoids the cost of purchasing, shipping, and handling grinding balls; however, it is only suitable for very hard ores with cubic cleavage. It is often extremely difficult to determine in advance whether a particular ore will work properly in a FAG mill.

A SAG mill can be described as a FAG mill that did not work properly with ore as the only grinding medium; therefore, steel balls were added. The ball charge is only about one-third of that required for a ball mill (usually 10-15% compared with approximately 40%).

The efficiency of a grinding mill depends on the weight of the grinding medium. This means that FAG and SAG (autogenous) mills are required to be of larger dimensions than a comparable ball mill because steel is 2½ to 3 times as heavy as the ore from a hard rock mine. However, the power consumption is similar, although some efficiency is lost in an autogenous mill because they typically require a grate (diaphragm) discharge to retain the coarse grinding medium while most ball mills have an open (overflow) discharge.

The drum diameter of an autogenous (FAG or SAG) mill manufactured in recent years on this side of the world is typically about equal to twice its length.

For an autogenous mill to be most efficient, an optimum ore feed size related to the diameter of the mill can be determined using the following formula.

F = d80 feed (optimum) = 0.95D2/3

Where d80 = size of opening (inches) through which 80% of the feed will pass.

D = the diameter inside the liners, measured in feet.

Example

1. Find the optimum feed size for a SAG mill 26 feet in diameter.

2. Find the open-side (o/s) setting of an underground crusher to obtain this feed on surface, assuming an attrition of ½ inch

in the transport and storage of ore between the underground crusher and the SAG mill.

Facts: 1. D = 26 feet

1. 2. Attrition = ½ inch

3. The product of this crusher is ½ inch less than the open side setting

Solutions: Optimum d80 Feed Size = 0.95 x 262/3 = 8½ inches

Open-side setting (o/s) = 8½ +½ +½ = 9½ inches

Grinding Mills (Autogenous Mills and Ball Mills)

Critical Speed and Optimum Speed

The critical speed, Cs (measured in RPM) is when the centrifugal force on the grinding mill charge is equal to the force ofgravity so that the charge clings to the mill liners and will not tumble as the drum rotates. Cs is calculated using the following formula.

Cs = 76.63√D

Where D = the diameter inside the liners, measured in feet.

Optimum crushing efficiency is obtained when a grinding mill is run at a particular fraction of critical speed. It is often reportedin the literature that the optimum speed is near 75% of critical. This is true of a ball mill that is 10 feet (3m) diameter, but the optimum speed is greater for a smaller diameter ball mill (80% for a 3-foot diameter ball mill). Optimum speed is typically less than 75% for one of larger diameter (as low as 65% for a 20-foot diameter ball mill).

Bond’s Law

During the 1940’s, Fred Bond (largely in association with W. L. Maxon) developed a system for comparing ore grindability in terms of weight passing a specific mesh size per revolution of the grinding mill. Since that time, others have developed similar analyses, but the original system prevails today for grinding mills (and may also be used for crushers).

Bond’s formula is conveniently expressed as follows.

W =Wi (10/√P -10/√F)

W = work (kWh/short ton ore)

P= size in microns (m) through which 80% of the product passes (P80)

Wi = work index

F= size in microns (m) through which 80% of the feed passes (F80)

Bond’s formula contains a mixture of metric and imperial units. To convert to all metric, the denominators (10) are simply changed to 11 to obtain the result in kWh/metric ton (tonne).

W =Wi (11/√P -11/√F)

Some metallurgists add modification factors to the Bond formula in comprehensive calculations to obtain greater accuracy.Table 1 provides typical work indices for some common rocks and minerals. For purposes of designing a proposed grinding mill, the work index of the ore to be treated is obtained from laboratory test reports.

Table 1 Bond Work Index for Rocks and Minerals

Table 2 provides particle sizes in microns (m) required for use in the Bond formula

Table 2 Feed and Product Sizes in Microns (m)

Bond’s Law Example

Calculate the reduction ratio and estimate the power consumption of a ball mill, using the Bond formula.

Facts: 1. F, the feed is from a cone crusher with a 5/8-inch open side setting

2. P, the product desired is 70% passing a Tyler 65 mesh screen (P70 = 65 mesh)

3. Wi of the ore to be ground is 15

Solution:

1. From the Feed and Product Size Table, the feed, F80 = 15,000m for a 5/8 inch open-side setting.

2. From the Feed and Product Size Table, a product, P80 = 210m for 65 mesh.

3. The desired product size, P70 = 210 x (80/70)2 = 274m for 65 mesh.

4. Reduction ratio = F/P =15,000/274 = 55 (55:1).

5. Power, W = Wi (10/√P -10/√F) = 150(1/√274 -1/√15,000) = 7.8 (7.8 kWh/short ton).

Controls

The efficiency of a grinding mill is dependent not only on the optimum RPM of the drum, but also the ball charge and the rate and blend of feed. These multiple variables make it difficult even for seasoned operators to manually maintain optimum efficiency in the grinding circuit. When the efficiency of a dynamic process is dependent on multiple variables, computerized controls and simulation modeling are advantageous. Computers have controlled grinding circuits in some mills for over 20 years. These controls are credited with increasing the efficiency of grinding circuits by 5% and more.

Shutdown and Salvage

A large value of gold may be recovered from a grinding mill that has operated for many years in a mine containing gold in the ore. Ores containing gold often contain minute amounts of mercury, silver chloride, etc. that are released in the milling process. Gold combines with these materials (or remains as elemental gold) and collects as a crude amalgam in every crevice and surface in the grinding mill (not subject to direct abrasion). The amalgam is invisible because it is the same color as steel; however, the amalgam is softer and can be readily identified and removed with a hammer and cold chisel. After

removal, mercury, soda ash, and lead nitrate are added to the amalgam, which is then ground and pressed to remove excess mercury. The compressed material may be then put in a laboratory retort to distill off (and recover) mercury and leave behind a dirty sponge of gold to be washed and refined.

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