Grinding Mills

Grinding in small plants, as in larger installations, has proven to be the most costly of all unit operations from both capital and operating standpoints. Therefore, grinding deserves the most scrutiny of all operations during the design procedure.

A recent survey by a major grinding mill manufacturer reveals that more than 80 autogenous or semi-autogenous mills having between 100 and 1,000 connected horsepower have been sold during the last twenty years. Obviously, this type of grinding approach cannot be arbitrarily excluded from consideration in a small installation.

Usually the grinding system in a modern small concentrator consists of one of the following configurations:

Open Circuit Rod Mill-Closed Circuit Ball Mill

This configuration has proven in the past to be a most reliable system due to the fact that rod mills are not particularly susceptible to variations in feed size. However, the necessity to procure grinding rods may limit the usefulness of this approach in certain locations.primary crusher productivity

Open Circuit Rod Mill-Open Circuit Ball Mill

This configuration is employed when it is desired to produce a final ground product having a very high pulp density. This situation can be useful for various types of leaching circuits but does not normally apply to flotation concentrators.

Single Stage Ball Mill

Single stage ball milling has proven to be very popular in recent years. Both overflow and diaphragm (grate) ball mills have been used as single stage grinding units. If the feed to such a system can be top size limited, for example by the use of closed circuit crushing, this type of installation can prove to be the most efficient from both the capital and operating standpoint.

Double Stage Ball Mills

Occasionally, the liberation requirement for an ore requires very fine primary grinding. Usually, the most expeditious way to accomplish this objective is by double stage ball milling. Both mills are usually operated in closed circuit with separate classifiers.

In smaller plants, grinding mill drives should be as simple as possible. The drive train usually recommended consists of a slow speed motor which drives the mill pinion through an air clutch. This slow speed motor system tends to be slightly more expensive than the alternate drive train consisting of a high speed motor and speed reducer but the ease of maintenance and the simplicity of the slow speed system offset the additional cost.

The mill feed belt should be equipped with a weightometer having cumulative and instantaneous tonnage indicators visible both locally and in the control room. In recent years, load cell weightometers have proven to be very reliable and are relatively simple to calibrate. The digital readouts supplied with load cell weightometers have proven to be rugged and reliable.

The mill dilution water stream should be visible and its control valve should be readily accessable. A simple water proportioning valve is sometimes recommended to give semiautomatic regulation to mill dilution water.

Mill sizing for most grinding installations is usually arrived at through calculations involving required throughput, ore work index estimates, and feed and product size-distribution requirements. This information is then tempered with experience to arrive at actual mill size recommendations. In the case of small concentrators located in remote areas, the method and route of delivery to the plant site must also be considered. There are cases on record where mill diameter has been dictated by railroad tunnel dimensions and mill length by the presence of severe switchbacks and small bridges over precipitous canyons.

Ball mills should be equipped with trommels constructed of punched plate or screen cloth and are usually fabricated with a reverse internal spiral. The trash rejected by the trommel is normally collected in a forklift box.

Ball charging should be arranged for the convenience of the operator. In remote areas, grinding balls are usually received in welded 55 gallon drums each of which weighs between 500 and 600 kilograms. Suitable handling equipment for grinding media must be supplied by the design engineer. It should be noted that the ability to assemble a graded ball charge at the mill site prior to startup may not be possible. An initial properly-graded ball charge should be purchased from capital funds and included in the grinding area unit cost.

Rubber liners represent a significant cost saving for smaller installations. This type of liner, although possibly contributing to somewhat less efficient power utilization, is very simple to install with unskilled personnel as contrasted to conventional cast metal liners.

common grinding configurations used in small plants

Regrinding

The necessity for regrinding is usually determined by laboratory or pilot-scale test work. If a project is to be designed from bench scale work only it is prudent to include a regrind mill in the circuit even though laboratory work does not indicate the necessity for this unit operation. Regrind mills are difficult to size since work index data are not generally available. Therefore, the design engineer should be generous when sizing a regrinding installation.

There is a tendency to employ spent grinding media from the primary grinding installation as media for the regrind mill. It is much better to use small-diameter properly-sized grinding media for regrinding since the utilization of spent media tends to reduce efficiency.

Regrind mills tend to be rather difficult to reline since they are usually of small diameter. Rubber regrind mill liners are an obvious solution and have found application in many installations. Of course, the rubber composition employed must be compatible with the flotation reagents associated with the regrind mill feed.

  • ball mills with trommel (2)ball-mill-for-sale
    ball mills with trommel (2) ball-mill-for-sale
    china manufacturerIndustrial Ball Mills use horizontal rotating cylinders that contain the grinding media and the particles to be broken. The mass moves up the wall of the cylinder as it rotates and falls back into the “toe” of the mill when the force of gravity exceeds friction and centrifugal forces. Particles are broken in the toe of the mill when caught in the collisions between the grinding media themselves and the grinding media and the mill wall. In ball mills the grinding media and particles acquire potential energy that becomes kinetic energy as the mass falls from the rotating shell. Ball mills are customarily divided into categories that are mainly defined by the size of the feed particles and the type of grinding media. Intermediate and fine size reduction by grinding is frequently achieved in a ball mill in which the length of the cylindrical shell is usually 1 to 1.5 times the shell diameter. Ball mills of greater length are termed “tube mills," and when hard pebbles rather than steel balls are used for the grinding media, the mills are known as "pebble mills.’’ In general, ball mills can be operated either wet or dry and are capable of producing products on the order of 100 um. This duty represents reduction ratios as great as 100.canada manufacturer The ball mill, an intermediate and fine-grinding device, is a tumbling drum with a 40% to 50% filling of balls. The material that is to be ground fills the voids between the balls. The tumbling balls capture the particles in ball/ball or ball/liner events and load them to the point of fracture. Very large tonnages can be ground with these devices because they are very effective material handling devices. The feed can be dry, with less than 3% moisture to minimize ball coating, or a slurry can be used containing 20% to 40% water by weight. Ball mills are employed in either primary or secondary grinding applications. In primary applications, they receive their feed from crushers, and in secondary applications, they receive their feed from rod mills, autogenous mills, or semi-autogenous mills. Regrind mills in mineral processing operations are usually ball mills, because the feed for these applications is typically quite fine. Ball mills are sometimes used in single-stage grinding, receiving crusher product. The circuits of these mills are often closed with classifiers at high-circulating loads. Options:
    • Rubber liners instead of steel
    • Steel mounting frame
    • UL Motor
  • rod mills (13)rod mills (8)
    rod mills (13) rod mills (8)
    A rod mill uses steel rods as the grinding media in tumbling mills is an efficient means to feed ball mills a product finer than could be made in a crusher but coarser than could be made in a ball mill. The principal use for rod mills, which were invented to make ball mill feed in a wet-grinding rod mill-ball mill circuit. Rod mills came into use in the United States beginning in 1900s and eventually became recognized as efficient fine crushers. The 1st ever built rod mill (2 m in diameter by 4 m long) was to be installed at a mine where a coarser feed was required for shaking tables. Crushing rolls and rod mills were eventually used commonly to prepare feed to shaking tables. Later, rod mills of 2.9 m in diameter by 3.9 m long replaced most roll crushers. Rod Mills cannot be to short: With the increase in the diameter of rod mills, problems occurred in the shorter rod mills with rods tangling. When the rods were short relative to the diameter, some were lifted toward a vertical position, and continued tumbling caused tangling. This led to the arbitrary rule that the length of rods should be at least 1.5 times the mill diameter inside new shell liners.
  • ag autoneous grinding mill (8)sag mill manufacturer
    ag autoneous grinding mill (8) sag mill manufacturer
    Starting as small as 13 feet (4 m) in diameter on 330 HP and as large as 24' (14.5 m) in diameter on 3,300 HP, the 911MPE Semi & Fully Autogenous Mills tumble crushed ore without iron or steel grinding media. used sag mill for saleThey are used when the crushed ore pieces are hard enough to perform all the grinding. SAG mills tumble mainly ore but they use up to 15% volume of steel balls to assist grinding. Semi-autogenous mills are more common. The AG Mill or SAG grinds primary crusher product and prepares it for final grinding in a ball mill. Its product is usually passed over a large vibrating screen to separate oversize ‘pebbles’ from correct-size particles. The correct-size material is sent forward to a ball mill for final grinding. The oversize pebbles are recycled through a small eccentric crusher, then back to the SAG or autogenous grinding mill. This procedure maximizes ore throughput and minimizes electrical energy consumption. Assuming your ore is AG-amenable: 7 TPH to 15 TPH is a comfortably production average for a 13' x 4.5' AG Mill. A 13' x 6' SAG Mill has a reasonable range of:
    • 20 to 30 TPH for gold ore SAG & Ball Mill duty
    • 25 to 40 t/h for copper porphyry SAG & Ball Mill duty
    In Single Stage SAG duty, the approximate capacity reduces to only 8 to 13 TPH for gold ore duty. The autogenous mill itself is a coarse-grinding device, consisting of tumbling drum with a 25% to 40% volume filling of ore. Metallic or manufactured grinding media is not used. Autogenous mills are fed run-of-mine ore or primary crusher product that is <25 cm (10"). Inside the mill, large pieces break into smaller pieces a few inches in size. These natural pebbles act as the grinding media the autogenous mill. The main modes of breakage are thought to be impact breakage and abrasion. Many circuit configurations are possible, but essentially the autogenous or SAG mill is operated as a single-stage primary mill, or it can be followed by secondary pebble or ball milling. The semi & autogenous mill is often operated in closed circuit with a trommel screen or external vibrating screen classifying the discharge. Circulating loads are low compared with those in ball mill circuits, because autogenous mills do not benefit from high-circulating loads in the same way ball mills do. Intermediate crushers are sometimes used to crush the largest pieces in the recycle stream. The product size of an AG/SAG mill depends strongly on the nature of the ore being ground so that tests must determine the competency of the ore and its “natural” particle size.
  • bond rod work index test mill
  • ball mill balls (3)cast ball mill grinding media
    ball mill balls (3) cast ball mill grinding media
    china manufacturerThe surface of the cast grinding steel ball is not allowed to have the casting defects such as the crack, and the obvious gas porosity, inclusion, shrinking cavity, cold shut, elephant skin, etc. It is not allowed to have the shrinkage cavity, shrinking porosity, gas porosity, inclusion, and other cavity defects on the section including plane pasting through the pouring gate and the ball centre. Testing Method and Inspection Rules
    1. Impact Test
    2. Rockwell Hardness Test Specimen Preparation
    3. Chemical Composition Inspection
    4. Surface Hardness Inspection
    5. Internal Quality Inspection
  • forged grinding balls
    Our forged steel grinding balls commonly used low-carbon alloy, high manganese steel, high carbon and high manganese alloy steel. On the basis of these material we have developed a series of competitive and efficient patent forged grinding balls.china manufacturer Our products range from 20mm to 150mm, 1 inch to 6 inch, and we can also offer all size of products according to clients’ demand. Features:
    1. forged steel grinding ball has high hardness: surface hardness to 58-65HRC, volume hardness of 57-64HRC
    2. impact toughness high: impact toughness more than 12J / cm2.
    3. broken rate is low: the actual broken rate is not higher than 0.5%.
    4. loss of forged grinding ball round rate is low.
    5. dropping test in 8m high tester for more than 20,000 times.
  • grinding cylpebs
    Our automatic production line for the grinding cylpebs is the unique.china manufacturer With stable quality, high production efficiency, high hardness, wear-resistant, the volumetric hardness of the grinding cylpebs is between 60-63HRC,the breakage is less than 0.5%. The organization of the grinding cylpebs is compact, the hardness is constant from the inner to the surface. Now has extensively used in the cement industry, the wear rate is about 30g-60g per Ton cement. Grinding Cylpebs are slightly tapered cylindrical grinding media with length equalling diameter, and all the edges being radiused. Grinding Cylpebs are made from low-alloy chilled cast iron. The molten metal leaves the furnace at approximately 1500 °C and is transferred to a continuous casting machine where the selected size Cylpebs are created; by changing the moulds the full range of cylindrical media can be manufactured via one simple process. The Cylpebs are demoulded while still red hot and placed in a cooling section for several hours to relieve internal stress. Solidification takes place in seconds and is formed from the external surface inward to the centre of the media. It has been claimed that this manufacturing process contributes to the cost effectiveness of the media, by being more efficient and requiring less energy than the conventional forging method. Because of their cylindrical geometry, Cylpebs have greater surface area and higher bulk density compared with balls of similar mass and size. Cylpebs of equal diameter and length have 14.5% greater surface area than balls of the same mass, and 9% higher bulk density than steel balls, or 12% higher than cast balls. As a result, for a given charge volume, about 25% more grinding media surface area is available for size reduction when charged with Cylpebs, but the mill would also draw more power.
  • pilot plant grinding circuitpilot plant rod mill
    pilot plant grinding circuit pilot plant rod mill
    china manufacturer The 911MPEPPGR426 is a small 300 kilo to 3.5 ton per 24 hour day capacity grinding mill acting primarily as a rod mill but can effortlessly be converted to a ball mill. Thanks to the integrated spiral classifier, it forms an easy to operate closed-circuit grinding plant. It includes a fine ore hopper, vibrating feeder and starter rod load. It is the perfect small grinding circuit for pilot plant operations or small micro mining. Under specific conditions, it will grind finely crushed ( -6mm) ore to 75 microns. This is a Dual Media mill (rod or ball).1-year-warranty This continuous grinding mill is offered as a rob mill or ball mill of 420 mm X 600 mm (17" X 24") or as a grate discharge ball mill of 420 mm X 450 mm (17" X 18").
  • high pressure grinding roll (3)high pressure grinding roll (2)
    high pressure grinding roll (3) high pressure grinding roll (2)
    High-pressure grinding rolls (HPGRs) have struggled for acceptance into the hard-rock mining sector. china manufacturer CRITICAL HPGR PARAMETERS HPGR roll diameters typically range from 0.5 m to 2.8 m, depending on the supplies, and roll widths vary from 0.2 m to 1.8 m. The aspect ratio of the rolls also varies as a function of manufacturer. Typical HPGR throughput rates range from 20 to 3,000 tph, with installed motor power as high as 3,000 kW per roll. The roll surface is protected with wear-resistant materials, and it has been these that have traditionally stymied HPGR acceptance, but solutions are now in place. When operating an HPGR, the two most important operating parameters are
    • Operating pressure
    • Roll speed
    The two key operating parameters are inherently linked to the following:
    • Specific throughput
    • Specific pressing force
    • Maximum pressure between the rolls
    • Specific energy input
    Detailed descriptions of the derivation and formulation of the parameters are given in numerous texts, and as such, the following section provides only a precis of the critical formulas, with some examples of actual relationships from testwork.
  • vibratory ring - disc mill
    europe manufacturerThe 911MPELM2000 is a laboratory machine, which is suitable for pulverizing and homogenizing hard and brittle materials in the dry and wet state. The closed milling pulverizing system guarantees complete recovery of the samples. Final fineness of down to 40 μm can be achieved, depending on the milling time and the specific properties of the sample material. The optimum filling of a 911MPELM2000 is around 1-1.3 Liters of sample material.
  • grinding rods
    • Rod mill grinding rods of all sizes and length.
    • china manufacturerHigh hardness, range is 45~55HRC
    • Low bending, less than 1.5mm/m
    • Dense microstructure , Bainite +Pearlite+Carbide
    • Low breakage rate, more than 15000 times/ 3.5m
    • Low unit consumption, 40% reduction compared to unprocessed round steel
  • mini ball mill grinding mill (1)mini ball mill grinding mill (2)
    mini ball mill grinding mill (1) mini ball mill grinding mill (2)
    The 911MPE-GM2024 is a mini but continuous grinding mill (8" diameter X 9 5/8" long) with an approximate capacity of 3 to 10 Kg/Hr (dependent on feed size and hardness).
    • 75 mm diameter x 800 mm Long spiral classifier with tilt regulation from 16 to 22 degree
    • 200 mm Diameter X 240 mm Long Grinding Mill which can be used as Ball Mill or Rod Mill
    • Mill Motor Power of 1.03kW
    • Feed Ore is pre-crushed to 1.5 mm
    This ultra small scale grinding mill can function as a primary ball mill or rod mill for or mini flotation plant or as a regrind mill if your pilot plant is built using our larger primary mill and flotation machines.
I Need Assistance