IES950441A2 - A stone milling process - Google Patents

Abstract

The invention concerns a process for milling stone, such as limestone, in which quarried stone is conveyed by belt conveyors through a crushing machine. The crushed stone is passed through a plurality of screens to separate the stone into grades of different sizes. Selected grades of the crushed stone are stored in stockpiles. Graded stone is fed from one or more selected stockpiles to a grinding mill where the stone is ground to a fine or semi-fine product, which is passed to a receiving receptacle. The operation of the conveying means, crushing machine, feeding means and grinding mill is controlled by a computer-operated data-processing system. Also, the load amperage of an electrical motor driving the grinding mill is continuously monitored and the speed of operation of a feeder to the mill is consequently adjusted to determine an optimum volume of stone fed to the mill, and to maintain the amperage load substantially constant. <Fig.3>

Description

A Stone Mil ing Process Field of the Invention Background of the Invention It is known to process quarried stone such as limestone, by first crushing the quarried stone in a crushing machine, then screening the crushed stone into various screen sizes, and finally milling the screened stone to produce the desired milled product. In the case of limestone the raw stone may be crushed, screened, and milled to produce fine and semi-fine mineral products, aggregates for use in the manufacturer of concrete products, and agricultural limestone.

A problem associated with known processes is to provide adequate control and monitoring of the crushing and milling processes to obtain optimum production. In particular, it is not been possible with the known processes to optimise the feed to the grinding mill so as to ensure the maximum power load on the motor driving the mill.

Object of the Invention It is an object of the invention to provide a stone milling process in which there is automatic control and monitoring of the process. It is also an object of the invention to control feeding of stone to the mill in an efficient manner having regard to the electric input capacity of the mill motor, and to achieve optimum output from the mill.

These and other objects will appear to those skilled in the art from the description and the appended claims. 1459S OPEN TO PUBLIC INSPECTION UNDER SECTION 28 AND RULE 23 JNL. No /..........OF ί.'/*/'’6. Ο 4 4 1 ί - 2 Summary of the invention A process for milling stone comprises conveying quarried stone, by conveying means, through a crushing machine and conveying the crushed stone by conveying means through a plurality of screens to separate the stone into grades of different sizes, conveying selecting grades of crushed stone by conveying means to stockpiles, feeding graded stone, by feeding means, from one or more selected stockpiles to a grinding mill where the stone is ground to a fine or semi-fine product, and conveying the ground stone, by conveying means to a receiving receptacle, wherein the operation of the conveying means, crushing machine, feeding means and grinding mill is controlled by a computer-operated data-processing system.

In particular, the load amperage of an electrical motor driving the grinding mill is continuously monitored and the speed of operation of the feeding means is consequently adjusted to determine an optimum volume of stone fed to the mill, and to maintain the amperage load substantially constant.

Brief Description of the Drawings An embodiment of the invention is hereinafter described with reference to the accompanying drawings, wherein: Figure 1 is a schematic diagram of apparatus for carrying out the process of the invention; Figure 2 is a side view of the screens shown in Figure 1; Figure 3 is a schematic diagram of a milling process of the invention; and Figure 4 is a flow diagram illustrating means for controlling the operation of the process.

Referring firstly to Figures 1 and 2, this is a flow chart of a typical arrangement for processing stone in accordance with a first 950441 - 3 embodiment of the invention. Quarried stone, in this case limestone, is taken from a stock pile through a vibrating grizzely feeder screen 1 which is operated by a 25 kw motor. The grizzely feeder is designed to feed the stone into a crusher 3, at a rate determined by the speed of rotation. The base of the grizzely feeder is constructed of a set of bars, fitted so that the apertures measure a nominal 3 inches at the mid point. The objective of the grizzely screen is to remove all material which is already of a sufficiently reduced nature, to avoid crushing and therefore maximises the crusher efficiency and throughput. From the grizzely feeder 1 the stone is fed via a troughed conveyor belt 2 to the crushing machine 3.

The crushing machine 3 is operated by means of a 160kw motor. The crushing machine 3 comprises an impact breaker having a solid steel balanced rotor. The rotor assembly includes three fixed hammers. Rock fed from the grizzely falls directly onto the rotor, and is deflected towards breaking bars, at speeds of from 250 to 1000 rpm, depending on the desired size of the finished stone product. The crusher crushes the raw limestone into aggregate. The crushed stone leaving the crusher 3 is graded and screened into aggregate of predetermine sizes. The crushed stone is fed by means of a conveyor 4 which is driven by a 16 kw motor and shaft through suitable gearing.

The conveyor 4 feeds the crushed stone to a two deck vibratory screen 5, which is operated by an llkw motor. One deck of the screen 5 separates out stone of a screen size of 4-6 inches from the crushed stone, and this is conveyed by means of a conveyor 6, operated by a 7.5 kw motor 7, to a stockpile.

From another deck of the screen 5, stone of less than 4 inches is fed by means of a conveyor belt 8 to a three deck screen 9. The two deck screens are constructed of steel frames, fitted with perforated plates or wire mesh screens (screen decks) or specific aperture sizes. Stone is fed onto the top deck and particular grades are separated by vibrating the frame of the screen, the separated grades are directed via chutes onto conveyors for stockpiling.

Stone of a size less than 4 inches, i.e., less than 2 inches but 0 4 4 1* - 4 greater than f inch, is fed by means of a 30 inch troughed by-pass conveyor belt 10 to a three deck screen 11, the operation of which will be described hereinafter.

The stone conveyed to the two deck screen 9 is screened into 3/4 inch chips which are conveyed by means of a conveyor belt 12, operated by a 7kw motor 13, to a stockpile. Fines of less than J inch size from the second deck of screen 9 are conveyed by a conveyor 14 to a heated and deck screen 15. The heated deck screen 15 is constructed of a steel frame with stainless steel wire mesh screens, which are heated using low voltage electricity. The heating prevents the material blinding the screen and helps to optimise the throughput.

From the three deck screen 11, 20mm stone chips are screened off and are conveyed by a conveyor 16 to a wet mix for use in the manufacture of wet-mix macadam. Wet-mix macadam consists of continuously graded crushed rock containing a specified quantity of water. The function of the water is to prevent segregation of the aggregate during transportation and laying and to lubricate and facilitate compaction. Wet-mix macadam is used in the construction and strengthening of roads.

The remainder of the screened material is conveyed by conveyor 17 to a main stockpile 20.

The stone milling operation is illustrated in Figure 3. Stone, of a size of 2 to 4 inches, from the main stockpile 20 is conveyed through a vibratory feeder 21 to a troughed conveyor belt 22 which feeds a surge hopper 23. The conveyor 22 is fitted with a metal detector 29 which detects any unwanted metal contained in the stone. The surge hooper 23 is located over a mill 24. From the surge hooper a vibrator feeder 25 feeds crushed stone through the mill.

The milling process is for manufacturing a ground limestone for pH correction in agriculture. The mill 24 includes a plurality of hammers which are used for the milling process. The hammers are attached to a rotating shaft rotor. The rotor has a series of disc plates, which space the hammers apart. 0 4 4 - 5 Stone entering the mill 24 is immediately impacted by the hammers. Rotor speeds can vary between 1400 and 1800 RPM. The stone is pulverised quickly. The speed of the rotor creates an air stream to help blow pulverised product through a series of steel grates or grid bars on the mill outlet. These grid bars have a specified aperture, so as to control the grading of the pulverised stone i.e. maximum size of milled material.

The feed rate and the speed of the rotor can be used to control the particle size distribution of the product.

Milled stone from the mill 24 is fed by a conveyor belt 26 and a screw conveyor 27 to a finished product bulk store 28.

The surge hopper 23 is fitted with load amp speed sensors 30, the purposes of which will be described below.

In accordance with the invention a computer-operated dataprocessing system is used to control and monitor the stone milling process. Figure 4 is a flow chart of a part of the program for controlling and monitoring the process.

The computer system is designed to meet the requirement of an MMI (Man-Machine Interface), SCADA (supervisory Control and Data Acquisition), and MIS (Plant Wide Monitoring and Information System). In doing so it maximises energy inputs and throughput capacity of the process.

The volume of stone in storage hoppers is controlled by: a) High level indicators; which trigger alarms to advise drivers when it is suitable to discharge loads. b) Low level indicators, which trigger alarms to stop the crusher 3 after a pre-set time interval.

In order to maximise energy input, load monitoring takes place at each critical point in the process to ensure that; a) No unnecessary blockage/spi1lage of stone can occur. 50 4 4 1: - 6 b) Optimum throughput can be achieved. c) Downtime is eliminated.

The system is designed to trigger alarms or to cause shutdown in the event of a blockage or spillage of stone occuring.

In relation to the milling process shown in Figure 3 the control programme operates as follows: On command from a preset real time clock (R.T.C.), or manual request, the plant is started in sequence. First screw conveyor 27 is started. When this is up to speed, belt conveyor 26 is started. The grinding mill 24 is then started. Only when the mill is up to speed and a rotor shorting contactor of the mill 24 is energised is the vibrating feeder 21 switched on. Load amp speed sensors 31, 32, fitted to belt conveyor 22 and screw conveyor 27 are continuously monitored, as are the load amps and temperature of the mill 24. The combination of these serve to dictate the optimum load feed to the mill 24. The load to the mill 24 is automatical ly adjusted, upwardly if it is too low, and downwardly if too high. The computer program continuously scans all associated items of plant under its control.

The milled product, e.g. ground limestone, is discharged from the mill by belt conveyor 26, which, as stated above, feeds to screw conveyor 27, in a finished product store 28. The weight of material in surge hopper 23 is monitored to maintain a sufficient head of material on the mill feeder 25. In conjunction with this weight, and the load on conveyor 22, the speed of the vibratory feeder 21 is adjusted.

As mentioned above, belt conveyor 22 has a tramp metal detector 29 in its control system which when it operates, stops conveyor 22 and operates a cleaning system to discharge the metal. When this is completed conveyor belt 22 returns to normal operation.

The processing system continues until one or other of an excessively high level of product in the store 28, a low level of feed stone in the stock pile 20 is detected, or alternatively the real time clock (R.T.C.), or manual shut-down request is activated. This shut 0 44 1; - 7 down is done in reverse sequence to the starting operation described above, to prevent any blockages.

Each conveyor in the system is monitored for load, tail drum rotation (belt slipage), and alignment. In the event of a problem arising either an alarm is sounded or the conveyor is shut down.

Critical bearings e.g. those on the crusher 3, mills 24 and screens, are fitted with; a) Temperature monitors - (which trigger alarms and automatic shutdown of the system). b) Automatic lubrication monitors which ensure specified lubrication takes place. Each stone stockpile is monitored for level control . If a predetermined maximum or minimum stockpile volume is exceeded the system gives an alarm and the process is shut down after a pre-set time.

The tolerance for moisture content of each process/milling operation is monitored by infra-red sensors. Again, an alarm signal is provided or the process is shutdown if required.

On-line particle size analysis is carried out on milled fines. There is an alarm and shutdown if required.

Each stone feeder is fitted with low level indicators which activate an alarm, shutdown, or change over to secondary feeder as required.

Each feed hopper is fitted with a tramp metal detection system which will initiate an automatic cleaning system. The system provides for an alarm or automatic rectification.

Vibration monitoring takes place to ensure that all milling and crusher plant is balanced and controlled. Due to the high speeds of rotation and the large size of the crusher, mills and screens, this equipment tends to create a lot of vibration. If due to a component 950441 - 8 failure or wear, the machine becomes off-balance nuisance vibration can occur, which if ignored can lead to structural damage, and/or damage to the machine itself. The control and monitoring of vibration maximises the optimum life expectancy of bearings.

Dust emission control is monitored to ensure maximum efficiency. An alarm is provided if dust emission exceeds a predetermined level.

The control process of the invention offers a number of advantages. It provides remote control of the whole process from crushing, through grinding, to storage of the finished product. It enables the process to be run at night when electricity charges are low, with a minimum of human supervision. It enables crushing and milling of the stone to be effected in an efficient manner with maximum conservation of energy and without overloading the crusher or grinding mi 11.

The control system accurately synchronises the extent of vibration of the vibrating feeders to the grinding mill with the amperage loading of the motor driving the mill.

Although the process of the invention has been described with reference to the crushing of stone, such as limestone, it also applicable to other stones, and to the treatment of ores in the mining industry. 0 44 1;

Claims (5)

1. A process for milling stone comprises conveying quarried stone, by conveying means, through a crushing machine and conveying the crushed stone by conveying means through a plurality of screens to separate the stone into grades of different sizes, conveying selecting grades of crushed stone by conveying means to stockpiles, feeding graded stone, by feeding means, from one or more selected stockpiles to a grinding mill where the stone is ground to a fine or semi-fine product, and conveying the ground stone, by conveying means to a receiving receptacle, wherein the operation of the conveying means, crushing machine, feeding means and grinding mill is controlled by a computer-operated data-processing system.

2. A process as claimed in Claim 1, wherein the load amperage of an electrical motor driving the grinding mill is continuously monitored and the speed of operation of the feeding means is consequently adjusted to determine an optimum volume of stone fed to the mill, and to maintain the amperage load substantially constant.

3. A process as claimed in Claim 2, wherein the feeding means comprises a vibrating feeder and the data-processing system continuously adjusts the vibration cycle of the vibrating feeder to synchronise the extent of vibration of the vibration feeder feeding the mill with the load amperage of the electric motor driving the mill.

4. A process as claimed in any preceding claim, wherein the conveying means, stockpiles, and feeding means, and also hoppers associated with the conveying and feeding means, all include monitors which are constantly scanned by the data-processing system to detect if a predetermined maximum or minimum volume of stone is carried by said means in which case the data-processing system issues a command to means associated with said system to give an alarm and/or to shut down the process, and/or to re-direct stone to secondary conveying means or feeding means, as required.

5. Apparatus for carrying out the process as claimed in any of the preceding claims substantially as hereinbefore described with .9 5 0 4 4 1' - 10 reference to and as illustrated in the accompanying drawings.