JPS6156026B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to the grinding of minerals, more particularly, but not exclusively, to the grinding of calcite marble, intrusive calcite, dolomite and limestone to provide finely divided calcium carbonate suitable for use as pigments or fillers. It relates to the grinding of various hard natural calcium carbonates such as. Generally, hard calcium carbonate is cut out by blasting, and the large rock blocks thus obtained are crushed in a powerful crusher. The final product of the crushing equipment is generally flakes with a maximum particle size of about 20 mm. These flakes are then further reduced in diameter by grinding in a ball mill charged with a suitable grinding medium such as ballstones (other grinding media are commonly used as well) with a diameter of approximately 50-100 mm. be done.
The ball milling step can be carried out wet or dry, but when further milling of the product is required, it is preferable to mill the flakes in the form of an aqueous slurry. Thus, the ball mill normally used in the conventional grinding method of minerals consists of a cylindrical container that rotates around a horizontal axis and grinding balls as a grinding medium housed in the container. The crushing balls are then carried upward and fall, colliding with the material to be crushed in the container to crush it. The falling distance of the crushing ball cannot be greater than the inner diameter of the cylinder of the container. The disadvantage of ball mills is high installation costs. Moreover, the energy applied to the material to be ground is determined by the weight of the grinding balls and the falling distance of the balls, which does not exceed the inner diameter of the cylinder of the mill vessel. For this reason, the grinding efficiency of ball mills, measured by the weight of material that can be reduced to a given particle size per hour, tends to be relatively low. Furthermore, the maintenance costs of conventional ball mills are high. This is because the ball mill is supported at each end by heavy roller or ball bearings which are subject to wear and therefore must be replaced frequently. Also, during the grinding of the feed material, the grinding balls themselves may wear out or break down to some extent, so that the product becomes contaminated with the materials that make up the grinding balls. The aim of the present invention is to eliminate the disadvantages of the traditional method using ball mills, namely to reduce installation costs, increase grinding efficiency, lower maintenance costs and avoid contamination of the product by grinding balls. The purpose of the present invention is to provide a method for crushing minerals. To achieve this objective, according to the invention, (i) large mineral blocks are crushed so that almost all 20
(ii) into a grinding chamber of an attrition mill having a rotatable internal impeller driven by an electric motor;
A method of grinding minerals comprising introducing water, a dispersant, and the product of step (i) to form a slurry; and (iii) agitating the slurry in a grinding chamber. of slurry is continuously taken out from the grinding chamber, the volume of the slurry in the grinding chamber is kept approximately constant,
The power consumed by the electric motor is maintained between the upper and lower limits by controlling the amount of product of step (i) introduced into the grinding chamber, and during stirring of the slurry the power consumed by the grinding chamber is maintained between the upper and lower limits. is free of particulate solid material other than that added as a product of step (i);
The mass flow rates for introducing water, dispersant, and the product of step (i) into the grinding chamber and for removing the slurry of ground minerals from the grinding chamber are such that the slurry of ground minerals removed from the grinding chamber is , containing at least 50% by weight of solids with respect to the slurry. Thus, according to the invention, minerals are ground without the need for grinding media. Thus, according to the invention, minerals are ground in an attrition mill without grinding media. The present invention also provides that a slurry of ground minerals is continuously removed from the grinding chamber, and that the volume of the slurry in the grinding chamber, the power consumed by the electric motor, as well as the water, dispersant, and step (i) The mass flow rates for introducing the product into the grinding chamber and for removing the mineral slurry from the grinding chamber are controlled. As a result, the proportion of fine particles per unit consumption of grinding energy is increased to improve grinding efficiency,
Contamination of minerals by grinding balls that wear out or break as in ball mills can be avoided. Although it is advantageous to carry out milling while wet, it is undesirable to add more water than is required to form a slurry. solids content 50
Less than % by weight reduces the amount of energy dissipated within the suspension per unit weight of dry solids.
On the other hand, excess water must be removed by costly dewatering processes before the crushed minerals can be sold. In order to keep the volume of the slurry in the grinding chamber approximately constant, for example an inverted U-shaped section is provided in the conduit for removing the ground slurry from the grinding chamber. Inverted U
The height of the highest point of the glyphs determines the liquid level in the grinding chamber, so that the liquid level in the grinding chamber and therefore the volume of slurry in the grinding chamber remains constant. Control of the amount of product of step (i) introduced into the grinding chamber is an on-off control. The product of step (i) is transported to the grinding chamber by, for example, a belt conveyor, and the electric motor for the belt conveyor is turned on and off to control the amount of product introduced into the grinding chamber. The upper limit of current for an attrition mill motor is set at a safe value slightly lower than the motor manufacturer's full load current, for example 70-98% of the full load current. This upper limit aims to protect the motor from damage due to overload and to extend its life. The lower limit is also set at a value of motor current below which the applicant has found that there is little effective grinding of the mineral in the grinding chamber. The amount of crushed mineral introduced into the crushing chamber is controlled on and off as described above, but the supply of water and dispersant (peptizer) is carried out continuously. The volume of the slurry in the grinding chamber is maintained approximately constant, and therefore the liquid level in the grinding chamber is maintained approximately constant ± 5% of the average level.
means the following: The application of the method of the present invention to the grinding of calcium carbonate minerals will be described below. In step (i) of the process, separate batches of calcium carbonate minerals are crushed to different degrees or the particle size distribution of the crushed minerals is varied by sieving to produce a product that is combined in any proportion, which is particularly advantageous. It is possible to form a mixture with a uniform particle size distribution. In step (ii) of the process, the dispersant is a water-soluble salt of polysilicic acid, polyacrylic acid or polymethacrylic acid with an average molecular weight of less than 5000, or a polymer of the type disclosed in British Patent No. 1414964. It can be done. The amount of dispersant used is
Regarding the dry weight of calcium carbonate minerals, generally
It ranges from 0.1 to 0.6% by weight. In step (iii) of the method, the slurry is agitated such that the amount of energy consumed in the slurry is preferably at least 30 horsepower per ton of dry calcium carbonate per hour, but usually 250 horsepower per ton of dry calcium carbonate.
It is carried out under conditions such that the horsepower is less than one hour (80-650kJ/Kg). During continuous removal of the slurry of ground calcium carbonate mineral from the grinding chamber, the ground calcium carbonate mineral is advantageously removed from the attrition mill through a sieve of suitable hole size. The attrition mill is of the type described in Belgian Patent No. 883,516 and is fitted at its outlet with a sieve which retains the coarse particles but allows the fine slurry to pass through. In one embodiment of such an attrition mill, the sieve hole diameter is typically
In the range 0.1 to 0.5 mm (i.e. No. 150 mesh and No. 30 mesh British standard sieves). The mass flow rates at which water, dispersant, and calcium carbonate minerals are introduced into the grinding chamber of the attrition mill, and the mass flow rates at which the ground calcium carbonate slurry is removed from the grinding chamber, are such that the following conditions are met: Preferably. That is, the solids content of the slurry removed from the grinding chamber is at least 60% by weight but not more than 80% by weight, the volume of material in the grinding chamber is kept approximately constant, and the power consumption of the electric motor driving the impeller is kept at a given level. The upper limit is kept between an upper limit and a lower limit, and this upper limit is such that at a given voltage the current drawn by the motor is between 70 and 98% of the full load current, preferably 90
and 97%, and the difference between the upper and lower limits should be 5 to 50% of the upper limit, preferably 20 to 30%. In order to maintain the grinding rate of the feed mineral at a high enough level to allow the attrition mill to operate at or near its maximum capacity or maximum efficiency, the current drawn by the motor must be within the upper and lower limits mentioned above. kept between. An apparatus suitable for carrying out the method of the invention will be described below with reference to the accompanying drawings. The attrition mill includes a grinding chamber 1 with an octagonal cross-section and a width slightly less than its height. At its lower end there is provided an outlet box 2 with a sieve 3 through which the slurry containing sufficiently ground particles passes, but which retains particles that are not sufficiently ground. The supply chute 4 allows solid granular calcium carbonate mineral to be supplied to the grinding chamber 1 . Conduits 5 and 6 with control valves 7 and 8 respectively serve to introduce water and dispersant into the grinding chamber 1 at controlled flow rates, respectively. The calcium carbonate minerals to be ground are placed on an endless belt conveyor 9 at the position indicated by the arrow 10, conveyed to the grinding chamber 1 and discharged into the grinding chamber via the feed chute 4. The grinding chamber 1 consists of an 8
It is equipped with an impeller 11 including a straight rod 12 with a book circular cross section. Electric power is supplied to electric motor 1 through cable 16.
4, the current flowing through this cable 16, that is, the current taken by the motor, is detected by a measuring device 17, and this measuring device 17 sends a signal proportional to the current taken by the motor 14 to the control device 18, which sends a signal to the endless belt conveyor 9. This control device 18 controls an electric motor 19 that drives the. The current taken by the motor 14 is the first
When a certain value or upper limit of is exceeded, motor 1
9 is deenergized by the control device 18, and the supply of calcium carbonate to the grinding chamber 1 is stopped. Electric motor 14
When the current taken by the current decreases again below a second specific value or lower limit, the control device 18 reenergizes the electric motor 19 and the supply of calcium carbonate mineral is resumed. The control device 18 includes a first preset potentiometer that determines the upper limit of the current taken by the motor 14, that is, the current value at which the motor 19 should be de-energized, and a second preset potentiometer that determines the magnitude of the difference between the upper and lower limits. Equipped with a preset potentiometer. The liquid level in the grinding chamber 1 is kept constant by connecting a conduit 20 with an inverted U-shaped portion 21 to the lowest point of the outlet box 2, the apex of the inverted U defining the desired liquid level. . At this apex, a ventilation hole 22 is provided as a siphon breaker. The milled slurry passes through conduit 20 to product reservoir 23 . The invention is further illustrated by the following examples. Example 1 A large block of Carrara marble is crushed using a conventional crushing device, and the crushed material is
It had the particle size distribution shown in the table. Table 1 Particle size range (mm) Weight % of particle size range >12...6> 61.8 >6...4> 21.9 >4...2> 13.5 >2...1> 1.0 >1 1.8 100.0 Coarsely crushed The material was transported at an average flow rate of 3.72t/h.
It was fed into the grinding chamber of an attrition mill of the type shown in the attached drawing together with water at a flow rate of 2260/h and a sodium polyacrylate dispersant having an average molecular weight of 1650 in a proportion of 0.19% by weight with respect to the weight of the dry solids. . The contents of the grinding chamber were agitated by an impeller into a slurry. No additional grinding media was introduced into the grinding chamber and 65 horsepower per hour of energy per ton of dry calcium carbonate (175 kJ/Kg) was dissipated in the slurry. The total solids content of the grinding chamber during operation was about 86 to 89%. The motor driving the impeller has a full load rating of 375A, and the flow rate of solids supplied to the grinding chamber is determined by the maximum current taken by the motor.
It was controlled so that the minimum current taken by the motor was 280A at 360A. The slurry of finely ground particles was passed through a screen and samples of the slurry were taken at intervals to prepare large samples representing the total production of ground material. The product is
It consists of a defrosted slurry containing 62.2% by weight of ground calcium carbonate, with a particle size distribution of 26
% by weight consists of particles with an equivalent spherical diameter smaller than 2μ, 39% by weight consists of particles with an equivalent spherical diameter greater than 10μ, and 2.3% by weight consists of particles that can be retained on a No. 300 mesh British standard sieve. It was hot. For comparison, the sample of coarsely crushed marble used as feed material in the method described above has approximately the same particle size distribution (i.e., 28% by weight has an equivalent spherical diameter smaller than 2 μ 42% by weight had an equivalent sphere diameter greater than 10μ and 1.3% by weight was retained on a No. 300 mesh British standard sieve). The ball mill was able to crush a batch containing 11 tons of coarsely crushed marble, but it took 6 hours to achieve the desired fineness, including 1 hour for evacuation and recharging.
It was time. The average production was therefore 1.57 t/h of dry ground calcium carbonate compared to 3.72 t/h obtained by the method of the invention. One attrition mill used according to the method of the invention therefore corresponds to two or three ball mills used according to the conventional method. Example 2 A large block of white limestone was crushed by conventional crushing equipment and the crushed material was further crushed by a hammer mill to produce a material having the particle size distribution shown in Table 2 below. Table 2 Particle size range (mm) Weight % of particle size range >12 7.9 >12...<6 35.1 >6...<4 11.6 >4...<2 19.9 >2...<1 10.3 >1 15.2 100.0 The material ground in the hammer mill was transferred to the grinding chamber of the attrition mill shown in the accompanying drawing with water at a flow rate of 948/h and the sodium polyacrylate dispersion used in Example 1 in a proportion of 0.45% by weight with respect to the dry solid weight. was fed with the agent to form a slurry containing about 85-89% solids by weight. The average output of crushed material was 1.84 t/h, but the belt conveyor 9 was operated at such a speed that the feed rate of crushed material to the feed chute 4 was greater than 1.84 t/h. The current drawn by the electric motor 14 could be maintained at about 80% of the full load rating by energizing and deenergizing the belt conveyor 19 using the control device 18. The electric motor driving the impeller had a full load rating of 375A, and the rate of milled material delivery was controlled such that the maximum current drawn by the motor was 340A and the minimum current drawn by the motor was 260A. The contents of the grinding chamber 1 were agitated by an impeller 11. No additional grinding media was introduced and 135 horsepower per hour of energy per ton of dry calcium carbonate (357 kJ/Kg) was consumed in the slurry. Samples of the fine slurry passed through a sieve 3 with a hole size of 0.250 mm were taken at intervals and a continuum (bulk sample) representing the total production of ground material was prepared. The product was 65.7% by weight. The particle size distribution consists of 39% by weight of particles with an equivalent spherical diameter of less than 2μ and 15% by weight of particles with an equivalent spherical diameter of greater than 10μ. 0.91% by weight consisted of particles that would be retained on a No. 300 mesh British standard sieve. The product was further processed to make ultrafine calcium carbonate by the method described in Example 1, except that a sufficient amount of dispersant was added in the first attrition grinding step so that no further addition was necessary. It was suitable for crushing. Large blocks of Carrara marble were crushed using conventional crushing equipment and the crushed material was further crushed in a hammer mill to produce a material with the particle size distribution shown in Table 3 below. Table 3 Particle size range (mm) Weight % of particle size range 12 or more 18.1 6-12 43.9 4-6 8.6 2-4 11.5 1-2 3.9 1 or less 14.0 100.0 The material ground in the hammer mill is divided into three parts:
That is, it was divided into a portion A according to the present invention and portions B and C not according to the present invention for comparison. Part A according to the invention is first fed with water to the grinding chamber of an attrition mill similar to the attrition mill shown in the drawing; 0.45% by weight (based on the weight of the dry marble) of the acid dispersant is dissolved in the marble, which causes approx.
A slurry was formed containing 85 to 89 weight percent solids. The endless belt conveyor 9 delivers the material ground in the hammer mill to the supply chute 4 in a slightly larger amount than the average output of ground material, so that the current taken by the motor 15 is controlled by the control device 18 to energize the conveyor motor 19. and was operated at a speed such that it could be maintained at approximately 85 to 90% of the full load rating of 375 A by deenergization. The amount of crushed material delivered in the hammer mill was controlled such that the maximum current taken by the motor was 340A (91% of full load rating) and the minimum current taken by the motor was 270A. Therefore, the difference between the upper and lower limits is approximately the same as the upper limit.
It was 21%. The contents of the grinding chamber 1 were stirred by an impeller and the amount of energy consumed in the slurry per unit weight of dry marble was calculated from the number of kWh of electricity consumed by the electric motor during the grinding period. Samples of the slurry of finely ground particles passing through sieve 3 with hole size 0.250 mm were taken at intervals (every 10 minutes) to prepare a continuum (bulk sample) representing the total output of the ground material. 300
The weight of the sample held on a No. 3 mesh British standard sieve (nominal hole diameter 53μ), the weight consisting of particles with an equivalent spherical diameter smaller than 2μ, and the weight percent of all dry solids in the slurry were determined. The current drawn by the motor is almost constant and for most of the grinding period
It turned out that it was in the range of 330 to 340A. The grinding conditions were almost stable. Part B, as a comparative example not according to the invention, was processed in the same way as part A, with the following exceptions, i.e. in this case the attrition mill corresponds to the aforementioned Belgian Patent Specification No. 883,516. Controlled by the method described in British Patent Specification No. 1469028. In this control method, the discharge through the sieve 3 is not continuous as in part A, but is controlled by an automatic on-off valve, which is opened when the motor current drops below a predetermined level, and when the current drops below another predetermined level. Closed as it rose more. Marble crushed in a hammer mill and water containing a dispersant are continuously fed at a constant rate to an attrition mill, thereby forming a slurry containing approximately 82% solids by weight, and passing the resulting slurry through a sieve. The discharge amount became slightly larger than the water supply amount when the on-off valve was opened. In this case, therefore, the volume of material in the grinding chamber was not kept constant. When the discharge valve was closed, the volume of material in the grinding chamber steadily increased, and at the same time the slurry was gradually diluted, as a result of which the current drawn by the motor decreased. When the current drops to its minimum level,
The discharge valve opened, the volume of material in the grinding chamber decreased, the concentration of the slurry and the current drawn by the motor increased.
When the current rose to the maximum level, the discharge valve was closed. In this case, the current drawn by the motor oscillates over a wide range, so that the difference between the maximum and minimum currents sometimes exceeds 50% of the maximum current, and the current remains constant for periods during which the motor may become overloaded and stop rotating. There was a tendency for the level to be maintained above the set maximum level. That is, because marble crushed in a hammer mill tends to vary in hardness and particle size distribution, it has been difficult to maintain the current supplied to the motor 14 within a desired range. It has therefore been found that even when the method of GB 1469028 is applied to an attrition mill, certain monitoring is required. Finally, part C as a comparative example not according to the present invention
is treated the same as part A except that:
That is, the amounts of water and crushed marble added were chosen such that the slurry formed in the grinding chamber contained about 65 to 70% solids by weight. The control system was the same as that used in Part A except for the maximum current of the motor. That is, the maximum current of the motor is set to avoid overloading the motor, but in the case of part C, it is difficult to set the maximum current to a current greater than 300A, i.e. 80% of the full load rating. found. The minimum current of the motor is
Since it was set to 240A, the difference between the upper limit (300A) and the lower limit (240A) was 20% of the upper limit. The results are shown in Table 4 below.

Table: Although Parts A and B were milled to products of similar particle size, the relatively better controllability of the method of milling Part A compared to the method of milling Part B resulted in It has been found that the production of polished marble can be increased and the amount of energy consumed during the crushing period can be saved. For Part C, the weight percent solids in the resulting slurry was lower than for Parts A and B. The yield of crushed marble was significantly reduced and it was not possible to obtain a finely divided product. As can be seen from these results, the method of the present invention is suitable for grinding hard calcium carbonate materials such as lime, although the productivity is lower and the energy consumption is higher than in the case of marble. In the present invention, it is not the volume of the solid material in the grinding chamber that is maintained substantially constant, but the volume of the slurry (the solid material and the water medium in which it is suspended).

[Brief explanation of the drawing]

The figure is a block diagram of an apparatus for carrying out the method of the present invention. DESCRIPTION OF SYMBOLS 1... Grinding chamber, 2... Outlet chamber, 3... Sieve, 4... Supply hopper, 5... Water introduction conduit, 6... Dispersant introduction conduit, 9... Belt conveyor, 11... Impeller, 1
4, 19...Electric motor, 17...Current measuring device, 18...
Control device.

Claims (1)

[Claims] 1. (i) crushing a large mineral mass to obtain a product consisting almost entirely of small pieces of 20 mm or less; and (ii) an attritor having a rotatable internal impeller driven by an electric motor. Inside the grinding chamber of the Tushion mill,
introducing water, a dispersant, and the product of step (i) to form a slurry; and (iii) agitating the slurry with an impeller in a grinding chamber to grind the product of step (i). a method of grinding minerals having the steps of: a slurry of ground mineral particles substantially all of which are smaller than a predetermined particle size is continuously removed from a grinding chamber; the volume of the slurry in the grinding chamber being held substantially constant and the power consumed by the electric motor controlling the amount of product of step (i) introduced into the grinding chamber;
maintained between the upper and lower limits during stirring of the slurry;
There are no particulate solid materials in the grinding chamber other than those added as the product of step (i), and water, dispersant, and the mass flow rate introducing the product of step (i) into the grinding chamber and the ground The mass flow rate for removing the slurry of ground mineral particles from the grinding chamber is such that the slurry of ground mineral particles removed from the grinding chamber contains at least 50% by weight of solids with respect to the slurry. How to grind. 2. The method according to claim 1, wherein the mineral is a calcium carbonate mineral. 3. In step (iii), the slurry is stirred under conditions that the amount of energy consumed in the slurry is at least 30 horsepower per hour per ton of dry calcium carbonate and not more than 250 horsepower per hour per ton of dry calcium carbonate. A method according to claim 2, characterized in that the method is carried out. 4. The mass flow rate of water, dispersant and calcium carbonate mineral introduced into the grinding chamber of the attrition mill in step (ii) and the mass flow rate of the slurry of ground calcium carbonate removed from the grinding chamber are Solids content in the slurry removed is at least 60
% by weight and not more than 80% by weight
The method described in section. 5. The upper limit of power consumption is selected such that the current drawn by the motor at a given voltage is 70 to 98% of the full load current, and the difference between the upper and lower limits of power consumption is 5 to 50% of the upper limit. A method according to claim 1, characterized in that: