Gold Hammer Mills

Gold Hammer Mills

Gold Hammer Mills

Hammer mills are actually crushers, not mills since the size generated by the process is usually 0.5 to 1 mm. This crusher was devised to work with soft rocks such as limestone, but artisanal miners use them for hard rocks. In a hammer mill, pivoted hammers are mounted on a rotating shaft and crushing is achieved as the rocks break on impact with the hammers and the fixed breaker plates mounted in the mill frame. The speed of the hammer mill can be anywhere from 500 – 1800 rotations/min. Hammer mills thus have little control on particle size and are subject to high wear on harder materials. The discharge of the hammer mills is usually done through a 0.5 to 2 mm manganese steel screen. The price of a hammer mill usually ranges between USS 1000 to 10,000 depending on the size and quality for the machine; good hammermills can be found in Brazil for less than $1500.

The specifications of a hammer mill with a capacity of 1 tonne/h of hard rock or 6 tonnes/h for soft rocks are given as follows:

  • Feed Size: 70mm max
  • Product Size: 1 to 6mm depending on the discharge screen
  • Water: 1500 Liters/hour approx
  • Extent of Mechanization: Fully mechanized
  • Power: lOkW
  • Speed: 900- 1270 rpm
  • Drive: Pulley & V belt

In El Callao, Venezuela, a traditional artisanal gold mining site, the ore is wet-ground to below 1 mm by 25 hp hammer mills. Hammer mills have capacity of 0.2 to 0.4 tonnes/hour depending on the hardness of the ore. For soft material, a pair of cast iron hammers (typically USS 8 – 10) must be changed after grinding 10 tonnes of ore. For some hard ores, hammers are changed after grinding 1.5-2 tonnes. Replacement of hammers requires that miners have an electric welding machine and a supplier of hammers (made of cast iron). Some miners use leaf springs from truck suspensions as hammers.

In South America and China, hammer mills are very popular among artisanal gold miners. One inconvenience is that particles of gold can become retained inside of the mill and at the end of the cycle the mill has to be opened and vigorously cleaned with water.

Gold Stamp mill

Gold Stamp mill

Gold Stamp mill

This is another crusher that has the name of “mill”. Stamp mills operate simply by the crushing action of the cast-metal stamp heads as they fall onto the ore at the bottom of the mortar box. “Normal” stamp mills have 3 or 5 stamps. The stamps are lifted via the cast iron collars by cams driven by a pulley and flywheel. Feed is typically -50mm, loaded into the top of the mortar box by shovel, and discharge is typically -0.5 to 1mm via a fixed screen on the back of the box. Stamp mills can be operated either dry or wet, and can be either manually, electrically, or diesel driven.

During operation, the base (die) of the stamps becomes deformed and quite often miners have to stop the crushing process to grind off the lips formed on the steel base.

The crushing capacity of a 3-stamp mill is around 0.3 to 0.5 tonnes/h depending on the hardness of the ore. The cleaning of a stamp mill is easy but not quick, and usually involves manually digging out all the material accumulated in the mortar box between the stamp dies. Gold, as a heavy mineral, can be accumulated in the mortar box.

In Zimbabwe, stamp mills are an accepted technology because the entire process is VISIBLE. Miners discharge the crushed product through a 0.5 to 1 mm screen directly into a centrifuge or onto an amalgamating copper plate. The lack of gold liberation is well known problem and is the main reason why the miners recover less than 30% of the total gold.

  • Three-stamp mill in Zimbabwe discharging crushed material into a centrifuge
  • Miner grinding the base of the stamp mill
  • Discharge screen

Grinding

Grinding is undertaken on already crushed material to achieve adequately fine particle size necessary to liberate the most gold possible. Grinding typically uses some sort of tumbling mill: a round metal barrel driven either manually or mechanically and filled with a grinding media such as steel balls, rods or hard pebbles. Grinding can be undertaken dry or wet. Dry grinding reduces the wear on mill shells, liners and balls, but requires up to 30% more power than wet grinding. Common types of grinding mills are:

Chilean-type (MuHer’Mills)

chilean mill

chilean mill

This type of mill combines crushing and grinding. It utilizes two or three wheels running in a circular trough, driven by one of the wheels or by a central boss gear. The mill can either grind batches (where ground ore is taken out periodically and replaced with fresh feed) or operate continuously (for example where a continuous stream of water washes the ground ore out of the mill onto a sluice). The feed size is usually smaller than 10 cm. Some miners in Ecuador crush the ore in a small jaw crusher to 2 to 5 cm and feed this into the Chilean mill. They use a nylon screen (mesh size as small as 0.1 mm) at the discharge of the mill. The mill operates with water (usually a pulp of 20% solids) and the material discharged through the nylon screen goes to sluice boxes lined with mats. The process is continuous and the output depends on the size of the mill and the grain size of the product. Typically the production is between 0.2 to 0.5 tonne/h for product ground below 0.2 mm. Chilean mills are very efficient, but unfortunately many miners in South America and China add mercury in the mill—the mercury becomes pulverized (floured) and is lost with the tailings.

Ball mills & rod mills

Ball, pebble and rod mills are all cylindrical rotating shells which are mounted on bearings and filled with up to 40% by volume of a grinding medium such as steel balls, rods or hard pebbles. Large models are usually lined with a wear-resistant lining. Mills are typically longer than their diameter, although some ball mills can be shorter in length than diameter. Feed rock size is typically 10-40 mm and discharge sizes can be 0.03 – 0.3 mm. Mills operate at an optimum speed which is typically 70% of the critical speed of the mill – the speed at which the contents will stick to the shell of the mill. The critical speed for a mill N is calculated based on the diameter D (in meters) of the mill: N = 42.3/D ‘ . For a small ball mill with internal diameter of 90 cm or 0.9m, the critical speed is around 44 rpm. The suggested speed is 70% of the critical speed or 31 rpm.

In order to demonstrate the principles and advantages of a ball mill, a simple unlined steel drum with discharge port on the side of the drum can be used instead of a continuous production ball mill. In Indonesia, artisanal miners use sets of 12 to 48 small unlined batch ball mills (048 x 60 cm, 2.5 to 4cm thick walls) to grind primary gold ore. Each mill grinds 40 to 50 kg of material per batch. The grinding time in Indonesia is often too long (3 hours) because miners use excess water and the wrong grinding media (gravels and rods). Similar mills are used in Tanzania but miners do not use water, as they need to carry the ground product in bags to another group of workers who charges for the concentration step. Dry grinding is inefficient, takes more time and spreads dust in the air. With more organization, these Tanzanian miners could improve their milling process. Despite the low production rate of these small ball mills, using many small batch-ball mills instead of large ones is a good concept for artisanal miners with limited capital. Miners and millers can increase their milling capacity in a step-by-step approach by acquiring one mill after another and connecting them with a belt drive. This is not the best solution in terms of energy consumption, but definitely suits the limited financial capacity of the miners, employs more people and is already a fully accepted concept in many artisanal gold mining regions.

Small mills can be electrically or diesel driven. If expansion is required, a larger ball mill or a second unit would b necessary. The specifications of an Indonesian small ball mill with are given below:

  • Size: 00.48 (1.6 ft) x 0.6 m (2ft) long (internal)
  • Lining: unlined (25-40 mm thick steel shell and ends)
  • Critical Speed: Nc = 42.3/D ‘ (in m) = 61 rpm
  • Operating Speed: 70 – 75% of critical speed = 45 rpm
  • Feed Capacity: 40-50 kg/batch
  • Max Feed Size: 12mm
  • Water required:
    – for 70% solids at 40kg load = 17 – 18 L
    – for 70% solids at 50 kg load = 21 – 22 L
  • Product Size: time dependent; typically P80 = 100 mesh (0.150 mm)
  • Ball Load: 40% of the mill volume
  • Ball Load in kg: 350kg
  • 0 max of ball: 44mm
  • Ball Sizes: 50% of 40mm and 50% of 25mm
  • Type of Ball: cast or forged steel (0.9 C, 0.85 Mn, 0.2 Si, 0.5 Cr, 0.1 Mo)
  • Ball Hardness: 63-65 Rockwell
  • Shipping Weight: 280kg
  • Extent of Mechanization: partially mechanized; batch manual discharge
  • Mode of Operation: batch
  • Discharge: lateral door
  • Drive: torque arm gearbox and V-Belt
  • Installed Power: 2.2 kW

Manual Crushing and Grinding Methods:

Manual methods of crushing and grinding are important in small scale mining.

Sledge hammer

A common method of pre-sizing rocks before grinding in a stamp mill or ball mill is to place oversize rocks (>2″) on a large flat rock, inside a Hessian or nylon ring which holds the rocks in place while they are hit with a hammer and passed to a second miner/miller who repeats the process to produce typically 1 cm rocks. Some miners use a piece of stone to grind the crushed rocks. In this process, the ground material is screened at 0.5 to 1 mm and the coarse material re-ground.

Mortar & pestle (finer grinding)

Finer grinding can be achieved in a batch fashion with a mortar and pestle. Pre-sized rocks are placed in a steel bowl with a rounded bottom, or in a short length of wide pipe welded onto a steel plate, and pounded with a steel rod such as an automobile axle.

Hand stamp mill (crushing)

Hand stamp mills such as the ones made in Bulawayo, Zimbabwe (Matabeleland Engineering), are useful for grinding small quantities of-50 mm ore to 0.5 -1 mm. The hand-operated mill is a simple construction of stamps and a camshaft supported on a heavy timber frame.

A small one-stamp mill, the “Katanka”, was developed in Zimbabwe by the company Small Mining Supplies to be a portable crusher that can be engine-driven or manually operated. The mill frame is made of flanged pipe construction rather than wooden beams. The discharge screen size is adjusted easily by changing the mesh in the discharge splash box. Typical product size is below 0.6-0.8 mm.

Hand ball mill (fine grinding)

Small ball mills can be made from a simple steel shell such as a gas cylinder or steam pipe, supported on wooden trestles by shafts welded to the ends of the cylinder such as the examples found in Mozambique.

“Quimbaletc”, “Maray”, “See-Saw” or “Rocker” crusher/mills

“Quimbaletes” or rocker mills are have been used extensively in Latin America for crushing and grinding. Quimbaletes can be made of solid stone or scrap steel, and can found in many sizes: Larger units (approximately 300 kg, but sometimes as heavy as 2000 kg) generally accomplish coarser grinding, and smaller ones (perhaps only 50 kg) can used wet for fine grinding. Stone quimbaletes (granite blocks) are sometimes rocked by standing from side-to-side on a wooden plank attached to the top, or by pushing on a lever arm. Steel quimbaletes are made of a sheet metal box mounted on a shallowly curved 1 cm plate and rocked on top of a flat steel base. The box is filled with concrete or stones, and rocked by a handle attached to the end. Quimbaletes represent a significant improvement over hand-operated mortar and pestle mills which grind only about 20 kg per man/day to -0.5mm. A large quimbalete can crush 45 to 90 kg per man/hour, and depending on quimbalete design and length of milling, can grind to less than 0.1 mm.

Guide to selection of crushing and grinding equipment

Stage Ore Feed (cm) Product (cm) Type of equipment
Primary crush Hard 10–50 2- 10 sledge hammer, jaw crusher, gyratory crusher
Soft 10-20 0.5-2 hammer mill
Secondary crush Hard 2- 15 0.5-2 small jaw crusher, gyratory crusher, stamp mill
Soft 2-20 0.3-2 hammer mills, stamp mills
Grinding Hard/soft 0.05 – 0.5 0.005 – 0.05 ball mill
Soft 0.5-2 0.5 – 0.05 mortar & pestle, quimbalete, hand ball mill

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