- 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). This continuous grinding mill is offered as a rod 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").
- 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
- 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. They 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
- 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.
- The 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.
- Industrial 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. 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
Grinding Mills: Ball Mill & Rod Mill Design & Parts
Common types of grinding mills include Ball Mills and Rod Mills. This includes all rotating mills with heavy grinding media loads. This article focuses on ball and rod mills excluding SAG and AG mills. Although their concepts are very similar, they are not discussed here.
Photographs of a glass ended laboratory ball mill show action of ball mass within the mill. The action of other grinding media is similar.
As the mill revolves, lifters assist in picking up the grinding charge and elevate it to an angle at which gravity overcomes friction and centrifugal force. The charge then cascades downward, effectively grinding particles of material within the mill by continuous, repeated impact and attrition action.
Grinding Mill RPM – Rotating Speed
Grinding Mill speed is one of the factors affecting the character of the cascading charge. As shown in the illustrations, the lower the percentage of critical speed, the smoother the flow of balls from top of charge to bottom. Higher percentage of critical speed is used for impact grinding of large feed. Lower percentage of critical speed is used for attrition grinding when a fine product is desired. The graph below will be helpful in determining percentage of critical speed when internal mill diameter and RPM are known.
A Grinding Mill is a revolving cylinder loaded to approximately one-half its volume with steel rods, balls or pebbles.
Grinding mills reduce particle size by impact, rolling and sliding. Of the many types in use, the cylindrical mill, which employs a cascading mass of balls or rods, is universally used for the size reduction of hard, moderate to highly abrasive materials, such as minerals, ores, stone, and chemicals. ,
A cylindrical mill, when operating under uniform conditions, will produce a uniform product. Wear on grinding surfaces has little effect on capacity or product size. Very little maintenance is required with these mills, downtime being a negligible factor in their operation. For continuity of operation, the cylindrical mill has no equal.
Grinding mills of this type will give you dependable, trouble-free operation year after year, with “planned” periods of stoppage for renewal of parts. Initial cost is distributed over a long operating period. Many grinding mills are still in service after more than 40 years of almost continuous operation. Ton for ton of material handled, the cylindrical type mill has proved to be the most economical investment for reducing moderate to extremely abrasive materials.
911Metallurgist sources manufacturers of all the proven mill designs in a “small” range of sizes — your assurance of getting the most suitable mill for your purpose. Best grinding efficiency and economy can be obtained only when the type and size of your mill is matched with your grinding job. Mills can also be furnished with modifications to suit any special application.
Difference Between Wet or a Dry Grinding Mill
The choice between wet or dry grinding is dependent upon the use of the product or the subsequent process. It is imperative to dry grind many materials because of physical or chemical changes which occur if water or a solution are added. Wet grinding with water (or with a concentrated solution of the soluble salts being ground) is generally preferred, because of the overall economies of this operation.
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.
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.
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.