IE77629B1 - A process for the production of superfine minerals - Google Patents

Abstract

The invention concerns a method and apparatus for producing fine and superfine minerals. The apparatus is monitored and controlled by a computer-operated data-processing system. The apparatus includes two airswept grinding mills and a classifier. The flow of air through the mills and classifier, and the load amperage on motors driving the mills and classifier, are monitored and controlled by the control means. Likewise the feed rate of raw material to the mills, and the delivery of fine materials from the mills and classifier are automatically controlled by the control means.

Description

A PROCESS FOR THE PRODUCTION OF SUPERFINE MINERALS s Field of the Invention The invention relates to a process for the production of superfine minerals. It also covers the production of fine minerals. The process of the invention is suitable for the production of a wide variety of minerals, such as limestone (calcium carbonate), manganese, china clay, benitoite clay, talc, kaolin, bayerite, silica and the like. The invention is particularly applicable to the production of superfine limestone, which has many uses. It may be used, for example, in the manufacture of human and animal feedstuffs, in the pharmaceutical industry, in the manufacture of plastics and rubber products, in the manufacture of resin-based ceramics, and as an agent for the control of odours and the maintenance of sanitary conditions in livestock holdings.

By the terra superfine mineral is meant a mineral which has been ground to a particulate form and has a particle size which substantially is in the range 5 to 150 microns, and a substantial portion of which is less than 25 microns. By fine mineral is meant a mineral having a particle size in the range 50 to 150 microns and a substantial portion of which is less than 50 microns.

Background of the Invention It is known to produce superfine minerals using a plant which includes inter alia, a feed hopper for the coarse minerals (raw material), a rotary dryer, bulk storage for the dried raw material, mills for grinding the raw material, and means for classifying the ground particulate mineral. Known processes have a very high energy input and require the assistance of a number of operators to control the process. 1630S - 2 77629 Object of the Invention It is an object of the invention to provide an improved process, involving the use of a production process control system, which controls and maximises the energy input, and controls the process parameters involved in the production of superfine minerals. It is also an object of the invention to provide a production control system which is capable of automatically controlling the process without operator assistance.

Summary of the Invention The invention provides a process for the production of superfine minerals comprises feeding raw material consisting of chrushed mineral to a dryer, and drying the raw material to a specified moisture content, electronically monitoring and controlling the feed rate of the raw material to the dryer, passing the dried material to one or more feed bins, conveying the raw material from the feed bins to one or more airswept grinding mills, controlling the volume of material fed to the mill, grinding the raw material in the mill, and subsequently passing the ground material through a classifier attached to the mill, conveying -the classified product to one or more silos depending upon the particle size of product required, wherein the moisture content of the raw material, the feed rate of the raw material to the dryer, the temperature within the dryer, the level of the raw material in the feed bins the volume of raw material fed to the mill, the differential air pressure within the mill, and the speed of the classifier are all monitored and controlled by control means comprising a computer-operated data-processing system.

As an optional step the ground mineral is conveyed from the classifier attached to the mill to an ultra-fine classifier, and wherein both the mill and classifier are fitted with dampers which control the flow of air and air pressure across the mill and the classifier and wherein differential pressure meters are provided at locations on the mill and classifier and feed signals to the control means which in turn controls the automatic operation of the dampers to maintain a predetermined air pressure across the mill and the classifier. - 3 The load amperage on an electrical motor driving the mill is constantly monitored by the control means and the feed rate is adjusted as required to ensure a substantially constant volume feed to the mill, and the load amperage on a motor driving fans for supplying air to the mill and/or classifier is also constantly monitored by the control means, and the feed load automatically adjusted to maintain the load amperage substantially constant.

Brief Description of the Drawings One embodiment of the invention is hereinafter described with reference to the accompanying drawings; wherein: Figure 1 is a schematic flow diagram illustrating the process of the invention; Figure 2 is an elevation of a known mill plant; and Figure 3 is an elevation of a known classifier.

Detailed Description of the Preferred Embodiment Referring to Figure 1 of the drawings, the first stage in the process is that the raw materials are first tested for compliance with a standard specification e.g. moisture content and particle size. In this case, the raw material is crushed limestone having a particle screen size of less than 10mm on average. The raw material is fed from a storage bay, by mechanical shovel to a raw material intake hopper 1.

The raw material intake hopper 1 can store up to 50 tonnes of raw material which is transferred via a belt conveyor 2 to a rotary dryer 3. A belt weigher (not shown) monitors and controls the feed rate to the dryer 3 by increasing or reducing the speed of the belt conveyor 2. The belt weigher, combined with a material flow switch on the belt conveyor 2 activates audio and visual alarms to alert an operator of process completion or raw material requirement. After a preset time this alarm automatically stops the belt conveyor 2 and initiates a complete shutdown sequence of the drying process.

Suitably, the dryer 3 comprises a rotating cylinder having a burner mounted on the feed end. The burner may be fueled by gas oil. The raw material is dried by the heat generated by the burner flame in the dryer. A Parker plant rotary drier of a size 24 x 6 , has been found suitable.

As the dryer rotates a series of lifting plates inside the cylinder help to move the material through the dryer and towards the dryer outlet. The dryer lifting plates are designed to ensure that a layer of material is maintained on the outer wall of the dryer at all times. This ensures minimum heat losses in the dryer.

The dryer is fitted with temperature probes and moisture content probes. These are used to monitor the temperature within the dryer and to adjust the burner flame automatically as required, they also monitor the raw material leaving the dryer to ensure it is of the required moisture content. The fuel consumption of the burner is recorded and automatically compared and correlated with raw material moisture contents in and out of the dryer by the system.

Dried raw material, leaving the dryer 3, is fed directly into mill feed bins 4, 5 -- by means of conveyors 6, 7, respectively, or can be transferred to a dried raw material stock bin 9 by an elevator 8.

The stock bin 9 can hold up to 180 tonnes of dried material, and is used to feed the milling plant for milling the limestone (as hereinafter described) via the mill feed bins 4, 5. This facility helps to reduce maximum demand by obviating the need to run both the dryer 3 and the milling plant simultaneously. It is also an essential part of running the milling plant automatically, for long periods of time.

The raw material stock bin 9 is fitted with high and low level probes which activate audio and visual alarms and initiate call up or shut down of material as required.

The dried raw material from the stock bin 9 is fed to the mill feed bins 4, 5 via belt conveyors 10 and 11, respectively. Low level probes in the mill feed bins 4, 5 automatically activate belt conveyors 10 or 11 as necessary to call up material from the raw material stock bin 9. High level probes terminate the feed from the stock bin 9.

The mill feed bins 4, 5 are used to keep a constant head of raw material over the mills. When the stock bin 9 runs empty this will be indicated by electronic signal from a low level probe activated for preset alarm time. This signal is sent to a PLC controller for the systems which signals that a complete shutdown will take place in a follow through sequence of the process. This will be followed by the low level activated on the mill feed bins 4, 5 and eventually right through the production process.

Raw material from the mill feed bins 4, 5 is fed to one of two mills 12, 13 by rotary feeders 14, 15, respectively. The rate of feed can be automatically controlled by increasing or decreasing the speed of rotation of the feeder. The rate of feed (weight of material) to the mills 12, 13 is monitored by a material flow probe located on the outlet of the rotary feeders.

The mills 12, 13 are airswept grinding mills. Raw limestone is fed by the rotary-'feeders 14, 15 onto a grinding table of the mill where grinding takes place between mill rollers and the table. The ground limestone is entrained in a rising air stream and conveyed to a classifier, as hereinafter described. Oversize limestone is returned to the mill for further grinding. Product ground to the required size is through the classifier to a cyclone. Air is recirculated from the cyclone back to the mill, excess air and fine dust being vented through a dust collector.

Suitably, mill 12 is a Lopulco mill, Model No. LM12, while mill 13 is a Moritz mill, Model No. BG8. The operation of these mills is well known to the person skilled in the art. Ground limestone is drawn from one or both of these mills and is either send directly to silos for bagging or is further classified as hereinafter described, depending upon the intended use of the limestone. In general, the Moritz mill 13 is adapted to produce ground material of a lower mean particle size than mill 12. - 6 The layout of the Moritz mill 13 is now described with reference to Figure 2. Raw material from dryer 3 (see Fig. 1) is fed by means of an elevator 8 to stock bin 9. From here it is fed by conveyor 11 to feed bin. A rotary feeder 15 feeds the material to the grinding mill 13, which includes the grinding rollers and a grinding table (not shown), driven by a geared motor unit 18. The mill 13 is provided with a rotary selector or classifier 19 for classification or grading of the ground limestone produced by the mill. Air flow through the mill is obtained by means of a cyclone 20 which provides for separation out of fine dust [?]. A main fan 21 conveys the ground product from the mill 13, through pipe 22 to the cyclone 20. A dust collector 23 separates out ultra fine material entrained in excess air, before discharge of the air to the atmosphere.

The air suction through the mills 12, 13 is critical to the efficiency of the process.

Air suction is monitored using differential pressure meters 17 located on the mill inlet, mill outlet and a dust/air bleed to the dust filter 23. Consistency of suction/pressure at these points ensures maximum conveyability of product through the mill. Pressure/suction reading at these points are preset to the optimum setting; any diversion from the preset activates automatic adjustment of the critical air dampers in the system. To achieve this, the meters 17 each have a digital output signal to the computer-operated control means, which in turn is adapted to automatically operate the dampers.

As indicated above the mill 13 has a classifier 19. Likewise the mill 12 has a classifier or so-called whizzer. The classifier/whizzer in each case consists of a rotating disc fitted with blades separated by a definite gap. The whizzer separator is rotated by a variable speed motor. The faster the speed of rotation the smaller the particles will have to be in order to pass through the blades. Any material unable to pass through the blades is returned to the mill for further grinding.

The required whizzer speed is selected in advance of production and is monitored and controlled by a shaft rotational switch. Any variation outside of a preset tolerance sends a signal to the control means which - 7 activates an immediate audio and visual alarm and causes system shutdown.

Material under process during alarm situation is automatically diverted to a down graded product bulk storage silo.

The mill motors 18 drive the mill tables (via a gear box). A motor 16 drives the fans for supplying air flow to the system. Sensors on the motors 18 and 16 measure the amp loading on the motors and send information to the control systems. Maximum amp loadings are preset and monitored by controlling the feed rate of raw material and the air suction/pressure in the mill system, under control of the control system.

On line particle size analysis takes place on the mill outlet and any variation in product analysis initiates an automatic adjustment in process parameters and diversion of product to a down graded silo if required.

Milled finished product is pneumatically conveyed by the fan air from the mill to the mill cyclone 20. The mill cyclone 20 is fitted with a rotary airlock which discharges to material to storage silos, or for further processing via pneumatic conveyors, mechanical conveyor, or gravity as required.

The product free air is recycled through the milling system, dust is removed by a reverse jet air dust filter. Each mill has its own individual dust filter system.

In the arrangement shown in Figure 1 there are eight silos for the storage of bulk finished products, but the number may be varied depending upon the products desired. The capacities of the silos range 35 tonne to 150 tonnes. Each silo is fitted with high and low level probes which alarm and shut down production process or divert finished product to a second designated product silo as programmed on the control system in advance. Each silo is also fitted with set or weight monitoring probes, which are used to evaluate production rates from the mills. - 8 Thus, product produced by mill 12 may be conveyed, via pneumatic conveyors, to one or more of six silos 25, depending on the quality and particle size of the product. Thus, each silo may be used to store product having particular characteristics for different end uses. The product characteristics and quality are monitored automatically by the control means at the mill output and if product intended for a particular silo is found not to meet the particular product parameters for that silo, it may automatically be diverted to another silo.

Likewise product produced by the mill 13 is pneumatically conveyed to one or more of four silos 26.

From the silos 25, 26 product is conveyed to selected bagging machines 27 where the product is bagged for distribution.

The milled limestone can be classified into a superfine product, the finest grade which can be produced on the mill table is 99% passing 100 micron (0.1 mm). However, it is frequently desired to produce a superfine product, e.g. for use as minerals filler as fine as 100% passing 5 micron (0.005mm).

To achieve such superfine grades, milled material from either or both of the mills 12, 13 is fed to an ultra fine classifier 29. A typical ultra-fine classifier is that shown in Figure 3, which is an Alpine Turboflex classifier, Model 750 ATP.

The classifier 29 comprises a horizontally mounted classifying wheel 31 with fine material and classifying air outlet 32. The classifying air 33 injected into the machine base, flows inwards through the classifying wheel 31 and discharges the fine material, whereas the coarse particles, being rejected by the classifying wheel, leave the classifier through the coarse material outlet 34. Clean coarse material is obtained by intensively rinsing this coarse material with air before it leaves the classifier. Further separation of fine material is thus achieved. (The air guiding system is schematically shown in the drawings). Product feed is made from above by a rotary valve 35, which also acts as an air seal. Normally, the fineness is adjusted simply by varying the classifying wheel speed. - 9 The speed of the classifier 29 is controlled by a frequency converter (not shown) for stepless speed control of the squirrel cage motor which drives the classifier. The efficiency of the classifier is monitored r and controlled by an electronic shaft rotational switch combined with a monitor of the motor load amps.

The required speed of the classifier is preset by the plant operator when setting up the production parameters. Any variation in the speed during production is detected by the control means which initiates an alarm and automatic shutdown of the classifier with product diversion to a downgraded finished product silo.

The differential pressure across the classifier wheel is monitored using a differential pressure meter and again any variation from the norm is detected by the control means which activates an alarm (shutdown) and product diversion.

The feed of material into the classifier is split into two grades superfine product and oversize. The superfine product combined with the classifier air is transferred to the classifier cyclone. The cyclone separates the product from the air, and the systems air is recycled in. the classification system. Dust generated during the classification process is recovered by a reverse dust filter. The cyclone and dust filter are not shown but they are similar to those described above in relation to the mill illustrated in Figure 2, and similar differential pressure meters 17 and air dampers are used; and the control means is as described in relation to Figure 2.

The superfine product is fed via a rotary air seal to a holding hopper 36. The holding hopper 36 is fitted with high and low level probes. Reaching the high level activates a slide valve on the outlet of the hopper. The contents of the hopper are then gravity discharged into a pneumatic transfer vessel. The low level probe on the holding hopper activates the slide valve and initiates the air compressor which pressurises the transfer pot and the contents are pneumatically transferred via 3 air pipes to designated silos 37.

Superfine material from the transfer vessel can be transferred to any - 10 one of four designated bulk storage silos 37 from which it may be conveyed to bagging machines 38.

Course particles rejected by the classifier wheel during classification oversize leave the classifier via the course material outlet which is fitted with a rotary air seal. This material is gravity fed into a holding hopper 40. The principle of operation and transfer from here to bulk storage silos is as per the superfine material, described above.

The system allows for the option to recycle coarse material/oversize into the milling process for regrinding if required.

The process takes place by pumping the material from a pneumatic transfer pot into the mill feed hopper. The oversize material is combined with the dried raw material feed and reground in the table mill. Each of the silo groups may be connected to a bulk transfer pot 39 which may be used to transfer material for recycling.

Alternatively, if required the system also allows for the coarse material/oversize to be recycled through the classifier system for reclassification.

Claims (6)

1. A process for the production of fine and superfine minerals » comprises feeding raw material consisting of chrushed mineral to a dryer, and drying the raw material to a specified moisture content, <, electronically monitoring and controlling the feed rate of the raw material to the dryer, passing the dried material to one or more feed bins, conveying the raw material from the feed bins to one or more airswept grinding mills, controlling the volume of material fed to the mill, grinding the raw material in the mill, and subsequently passing the ground material through a classifier, conveying the classified product to one or more silos depending upon the particle size of product required, wherein the moisture content of the raw material, the feed rate of the raw material to the dryer, the temperature within the dryer, the level of the raw material in the feed bins the volume of raw material fed to the mill, the differential air pressure within the mill, and the speed of the classifier are monitored and controlled by control means comprising a computer-operated data-processing system.

2. A process as claimed in Claim 1, wherein as an optional step the ground mineral is conveyed from the classifier attached to the mill to an ultra-fine classifier, and wherein both the mill and classifier are fitted with dampers which control the flow of air and air pressure across the mill and the classifier and wherein differential pressure meters are provided at locations on the mill and classifier and feed signals to the control means which in turn controls the automatic operation of the dampers to maintain a predetermined air pressure across the mill and the classifier.

3. A process as claimed in Claim 1 or Claim 2, wherein the load amperage on an electrical motor driving the mill is constantly monitored by the control means and the feed rate is adjusted as required to ensure a substantially constant volume feed to the mill, and the load amperage on a motor driving fans for supplying air to the mill and/or classifier is also constantly monitored by the control means, and the feed load automatically adjusted to maintain the load amperage substantially constant. 12

4. A process as claimed in any of the preceding claims, wherein raw product from the mill or mills is conveyed to one or more silos and wherein the control means monitors the level of product in the silos, and also monitors the product parameters, and causes the product to be 5. Diverted to a different silo if the original silo is full, or if the product does not meet the required parameters.

5. A process for the production of superfine minerals, substantially as hereinbefore described with reference to, and as illustrated in, the

6. 10 accompanying drawings.