GB1586822A

GB1586822A - Controlling the corroision of grinding elements in the wet grinding of solid materials

Info

Publication number: GB1586822A
Application number: GB41017/77A
Authority: GB
Original assignee: Erie Development Co
Current assignee: Erie Development Co
Priority date: 1976-10-01
Filing date: 1977-10-03
Publication date: 1981-03-25
Also published as: BR7706477A; AU2927777A; ZA775473B

Description

(54) IMPROVEMENTS IN OR RELATING TO CONTROLLING THE CORROSION OF GRINDING ELEMENTS IN THE WET GRINDING OF SOLID MATERIALS (71) We, ERIE DEVELOPMENT COMPANY, of 1100 Superior Avenue, Cleveland, Ohio 44114, United States of America, a corporation organised and existing under the laws of the State of Delaware, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a method of wet grinding of solid raw materials to form comminuted particulate solids.

Rod and ball grinding mills are commonly used to comminute various raw solid materials. For example, rod and ball grinding mills are widely used in the processing of taconite, copper, cement and many other metallic and non-metallic mineral bearing materials. The grinding of such materials is generally performed in a wet environment in which water is added to the raw materials to form a slurry.

Comminution is commonly performed under atmospheric conditions, i.e. the slurry in the mill is exposed to air.

A major expense incurred in the comminution of the aforementioned ores and other materials is the consumption of grinding media and grinding mill liners.

Typically, in the apparatus currently in use to grind taconite and other ores, the consumption of freely tumbling grinding media may average from about 0.5 to 6.0 pounds of media per ton of crude material processed. Likewise, the consumption of liners in the grinding mill may vary from about 0.02 to 0.80 pounds of liners per ton of crude ore processed.

Grinding media in the form of cylindrical rods and spherical balls, as well as numerous other conceivable shapes, have been used in the ore processing industry throughout the years. Typically, grinding rods are produced from high carbon steel, while grinding balls are fabricated from chilled cast iron or forged steel, all of which may or may not be alloyed.

The typical grinding chamber is usually lined wherever the ore slurry being processed may come into contact with the chamber wall in order to maintain its strength and tightness. The chamber shell and end walls have been lined with materials of variously conceived shapes, sizes and thicknesses over the years. The liners are normally attached to the interior surfaces of the grinding chamber by means of liner bolts. Generally, the liners are of cast construction and may be made from various iron based materials in order to meet the optimum operating economics of an individual operation.

It is important to note that in practice, grinding media, whether they are rods, balls or some other shape, and the grinding chamber liners are made from ironbased materials, and that these materials are constantly in contact with a slurry of ore which contains dissolved and adsorbed oxygen. This causes the grinding media and liners to be consumed. This consumption of the iron-based material, whether it be grinding media or liners, is essentially caused by two factors.

The major factor is abrasion which may be explained as the rubbing action of ore against the grinding media, or the grinding media against themselves, or still yet the dropping action of the cascading mass of slurry and grinding media as the grinding chamber rotates.

A second major factor in iron-based material consumption is corrosion. The corrosion results from a chemical reaction between the iron-based materials within the grinding chamber and the dissolved or adsorbed oxygen in the slurry, derived from contact of the latter with air. It is believed that the chemical corrosion may take place in accordance with the following formula: 2Fe+ O2+2H2OO2Fe(OH)2 (Note: It is not to be taken that the mechanics of corrosion are limited to the above formula, as undoubtedly there may be other complex and unknown reactions.) Since most grinding mills are normally open to the atmosphere, oxygen in the air is dissolved or adsorbed into the liquid used to slurry the raw material during the comminution process.This dissolved and adsorbed oxygen corrosively attacks the iron-based material, that is, the grinding media and chamber liners. Also, since the grinding media are agitated in the liquid during the comminution process, and are even lifted out of the liquid at times, with a thin film of the slurry adhering to each and every piece of the grinding media, that portion of the dissolved and adsorbed oxygen contained within the thin film of slurry, when consumed by the corrosive reaction, is readily and continuously replenished by fresh oxygen from the air within the grinding chamber, so that corrosive consumption of both grinding media and chamber liners is a continuing process.

The present invention provides a method of wet grinding of solid raw materials to form comminuted particulate solids, which comprises introducing a slurry of the solid raw materials in a liquid into a grinding chamber of a grinding mill, introducing a plurality of freely tumbling and cascading corrosible grinding elements into the chamber, sealing the chamber against the entry of air and driving the sealed chamber to tumble and cascade the contents thereof to form the comminuted particles.

The method of the invention is conveniently carried out in the grinding mill having a grinding chamber for holding therein a plurality of freely tumbling and cascading corrosible grinding elements mixed with a slurry of solid raw materials, drive means for driving the chamber so as to tumble and cascade such grinding elements and solid raw materials, and means for sealing the chamber against the entry of air therein during the comminution of the solid raw materials whereby replenishment of oxygen in the chamber is greatly diminished.

Consumption of the grinding chamber liners and the freely tumbling grinding media is thus substantially reduced in the comminution processing of raw materials in a grinding mill.

The present invention will be more clearly understood from the following description given in conjunction with the accompanying drawings, in which: Figure 1 is a perspective cutaway view of a ball mill having means for introducing an inert gas; Figure 2 is a perspective cutaway view of a rod mill having a vacuum system on the mill, for use in carrying out the method of the present invention.

Ball and rod grinding mills are well known in the art. As the ball mill 10 of Figure 1 and the rod mill 12 of Figure 2 are of the same general construction, for ease of understanding, like components are identified by like numerals, while new components are identified by new numerals. As is conventional in such grinding mills, a cylindrical grinding chamber 14 is formed from an elongated cylindrical tank 16 which is closed at its ends by end walls 18 and 20. Generally, the inside wall 22 of the cylindrical tank 16 and the end walls 18 and 20 are lined with iron-based mill liners 24, which are fastened to and held in place by a plurality of liner bolts 26.

Doors 28 and 30 are provided in the cylinder 16 for charging the chamber 14 with material to be ground, such as taconite or other types of ores. These doors 28 and 30 also provide for the removal of the ground materials. In addition, grinding media can be placed in the chamber 14 through either of these doors 28 and 30.

Alternatively, if desired, the chamber 14 can be charged with a plurality of freely tumbling and cascading iron-based grinding media through the opening 32. These grinding media can be grinding balls 34, as shown in Figure 1, or grinding rods 36, as shown in Figure 2. It will be understood by those skilled in the art that the material (ore) to be processed is wet milled. Accordingly, a wet pulp or slurry of ore can be fed to the mill or formed in situ by mixing dry ore with an aqueous vehicle.

This wet pulp or slurry serves to transport the material being ground from the feed to the discharge end of the grinding chamber and also acts as a cushion, or coating, on the surfaces of the grinding media, affording some protection against abrasion of the grinding media and the iron-based chamber mill liners.

The final level L of the mixture of raw materials, liquid and grinding media is substantially lower than the top of the cylinder or tank 16. In ball mill grinding, as shown in Figure 1, the total volume of the mixed charge may vary from about 25% of the total chamber volume to about 50%. In rod mill grinding, as shown in Figure 2, the level L may represent a volume which may vary from about 25/n of the total chamber volume to about 40 /0 or 45%. The density of the slurry, i.e., the percentage relationship of solids to liquids, is also an important factor in grinding operations and is dependent among other factors on the desired end product.

Another major factor having an important effect on liner and grinding media consumption is the rotational speed of the grinding chamber. Again, there are certain ranges of speeds for ball and rod grinding mills. All such factors are generally recognized by those experienced in the art and do not relate to the scope of this invention. They are merely mentioned herein in recognition of the bearing they have on the comminution process. In conventional practice, it is important to note that there is a considerable volume, generally occupied by air, above the tumbling and rotating mass of the grinding mill charge.

The cylinder or tank 16 is provided with a suitable rotating mechanism for driving it, such as, but not limited to, a circumferential gear 38, encircling the tank 16, as shown in Figures 1 and 2. This gear 38 is firmly affixed to the tank such as by being bolted to flanges welded on the shell. Other various and suitable means of mounting the gear 38 are known in the art. A suitable driving pinion gear 40 meshes with the gear 38 to rotate the cylinder 16 about its horizontal axis. The pinion gear 40 is driven by a suitable motive force such as an electric motor (not shown) via a pinion shaft 42. A plurality of spaced support rollers 44 may also be provided to support the cylinder 16 during rotation.

Grinding mills thus far described are well known in the prior art. In such prior art, the grinding chamber 14 is allowed to communicate readily with the atmosphere. For example, the chamber charging opening 32 is not provided with an air tight seal. Likewise, the doors 28 and 30 are frequently just axial openings in the end walls 18 and 20 above the level L of the materials charged into the chamber. It has been discovered that the use of such open chambers results in high consumption of the grinding elements 34 and 36 and liners 24 during the grinding operation. This consumption is due to abrasion and to corrosion of all iron-based elements, such as grinding media and chamber liners, by oxygen dissolved and adsorbed in the liquid in the chamber 14 during and throughout the comminution process.Both the abrasion and corrosion consumption have thus far been continuous, since the oxygen dissolved and adsorbed in the liquid which is consumed by corrosion of the elements 34, 36 and 24 is replenished as rapidly as it is consumed by air which freely permeate the grinding chamber. In the grinding method of the present invention, the availability of such corrosible dissolved and adsorbed oxygen is substantially reduced.

In order to carry out the present invention, provision is made for securely sealing the doors 28, 30, the opening 32 and any other opening in the chamber 14 from the ambient atmosphere. Referring to Figures 1 and 2, the doors 28 and 30 may be sealed in any convenient manner. Since means for accomplishing such a sealing effect are well-known, they will not be discussed herein. Likewise, the opening 32, by way of example, includes an outwardly extending and lined collar 46 welded to the tank wall and fitted with a flanged seating 48. A cover 50 is hingedly mounted at 52 to the flange 48 so that the cover 50 can be lined on its inner surface with an iron-based liner material in a conventional manner. The cover 50 is preferably provided with a suitable sealing gasket 54 and dogs 56, so that when the cover 50 is closed and dogged, the opening becomes airtight.

It will be understood that numerous lid or closure constructions may be employed to seal the opening 32, other than the construction shown in the drawings. The details of such closure construction are not deemed to form a material part of the present invention. What is material to the present invention is the concept of providing an airtight seal to the chamber 14 during the comminuting process.

Likewise, it will be appreciated that the doors 28 and 30 may be constructed in a manner similar to the closure means discussed above, the only significant difference being that they are provided with means for controlling the atmosphere in the chamber 14.

By thus sealing the chamber 14, it will be seen that the overall amount of dissolved and adsorbed oxygen within the chamber liquid is substantially reduced during the grinding operation. Only a limited amount of dissolved and adsorbed oxygen in the liquid will be available for corrosion of the grinding media elements and the chamber liners, since the oxygen will not be freely replenished from the atmosphere provided.

The quantity of available corrosible oxygen in the chamber 14 may be still further reduced by charging the chamber 14 with an inert gas prior to or during the grinding operation to remove and replace the air above the liquid level L. As shown in Figure 1, a charging conduit 58 extends axially into the chamber 14 through a swivelled and gas-tight gland 60 on the end wall 18. This conduit 58 is connected to a suitable source (not shown) of an inert gas, such as nitrogen or argon. A swivelled conduit 63 is preferably provided to bleed off air via a valve 62 above the liquid level L as the chamber 14 is being charged with the inert atmosphere. It may be automated for any desired amount of bleed off during operation.If desired, the charging conduit 58 may be mounted so that the inert atmosphere may be introduced through the end wall 20, in which case the bleed-off valve 62 is mounted on the opposite end wall of the chamber 14.

Thereby, it will be seen that during the comminution process, any dissolved or adsorbed oxygen which is consumed from the liquid in the mill charge will not be replenished by additional oxygen, but will actually be replenished (if at all) by an inert non-corrosible gas.

In another embodiment of the invention, the air is not only removed from the chamber 14, but is also removed to some extent from the liquid in the chamber. As shown in Figure 2, a vacuum conduit 64 communicates with the grinding chamber 14, again through a valve 65 and an axial and swivelled packing gland 60, and is connected to a suitable vacuum pump 66 for evacuating the chamber prior to and during operation. A suitable swivelled conduit 63 having a valve 62 is also provided for admitting air to return the chamber 14 to atmospheric pressure when desired, usually upon shutdown of the grinding circuit, since release of the cover 50 may be difficult under the sub-pressure produced in the chamber 14 by the vacuum pump 66. As in the embodiment wherein an inert atmosphere is used to replace oxygen, this embodiment using a negative atmosphere lends itself to simple automation.

Also, the connections for operating the vacuum type of embodiment of the present invention may be located at the opposite end of the mill, in which case the air valve would be opposite to that end of the chamber wherein the vacuum conduit is attached.

In practising the present invention, the grinding chambers 14 of Figures 1 and 2 are first charged with known quantities of iron-based grinding balls 34 or rods 36, as desired, through the doors 28 or 30 or opening 32, as appropriate. Then, predetermined quantities of raw solid materials to be ground and liquid, usually water or some other oxygen-adsorbing liquid, are introduced into the chamber 14 to complete the charging of the mill. The openings or doors are then closed and sealed.

At this point, comminution may commence by driving the pinion shaft 42 and gears 40 and 38 to rotate the tank 16. As the tank rotates, the grinding balls 34 or grinding rods 36, as the case may be, will freely tumble and cascade along with the raw solid materials and liquids. As this action continues, a slurry is formed and the solids are subjected to cotnminution. As dissolved and adsorbed oxygen is depleted from the liquid, this oxygen will only be replaced to the extent that oxygen is available in the air in the chamber 14 prior to the sealing of the tank.

In addition, prior to rotating the tank 16, the valve 62 in Figure 1 may be cracked and an inert gas may be introduced through the conduit 58 to displace the air in the tank. For the duration of the comminution processing of the raw solid materials, automation of the valve 62 and swivelled gland 60 may readily be accomplished.

Also, before rotating the tank 16, a vacuum may be drawn on the chamber 14 by the conduit 64 through the valve 65 by the vacuum pump 66 as shown on Figure 2. As this vacuum is drawn, the valve 62 will be shut, but as comminution proceeds, automation of the valves 62 and 65 may readily be accomplished.

When practising the present invention, it has been established that the consumption of grinding balls, grinding rods and liners in wet grinding is substantially reduced. In one test, as shown in Example 1 below, accurately weighed quantities of iron-based grinding balls were introduced along with water into identical grinding chambers. One chamber was sealed and purged with nitrogen, while the other was left unsealed and in communication with the atmosphere. Both chambers were then rotated for equal periods of time.The test results are summarized as follows: Example I Nitrogen Air Initial Ball Weight (dry), grams 443.3447 444.8115 Final Ball Weight (dry), grams 443.3175 444.3393 Weight Loss, grams 0.0272 0.4722 Annual Corrosion Rate 2.24/ 38.7/ After the test, on examination, the water from the nitrogen chamber was found to be essentially clear and the surfaces of the grinding balls were clean and shiny.

The water from the chamber with the air atmosphere, however, contained a gelatinous type of precipitate of ferrous hydroxide and the surfaces of the grinding balls were found to contain a light rust.

In this test, raw solid materials were not present in the grinding chambers; thus the consumption results reflect essentially only corrosion consumption and that lesser measure of abrasion consumption due to collision of bails with themselves and the chamber walls. Although the ball life would be expected to be somewhat reduced in the presence of raw solid materials (see Example 2 below), the abovenoted test is indicative of the substantial proportional reduction in corrosion consumption realized in the practice of the present invention.

Example 2 Mill Mill Open to Air Closed to Air Initial Ball Weight (dry), grams 6302.900 6440.030 Final Ball Weight (dry), grams 6237.877 6418.417 Weight Loss, grams 65.023 21.613 Annual Corrosion Rate 62.76/ 20.42/ In the above tests, a slurry of 250 g of water and 750 g of taconite was utilized.

The percentage weight loss of the balls over a year was then computed.

From the foregoing, it is apparent that the present invention provides an improved method for grinding wet ore, while minimizing the amount of corrosion experienced by the grinding media.

Although nitrogen and argon are primarily disclosed as the inert atmosphere of the present invention, it will be understood that other inert gases may be employed in practising the present invention. Moreover, it will be understood that grinding balls, rods or grinding elements of other shapes may be readily interchangeably employed in the chambers of Figures 1 and 2. Also, if desired, the vacuum method described in relation to Figure 2 may be employed in conjunction with the inert gas method of Figure 1. For example, oxygen could be removed by the vacuum method before charging the chamber with inert gas.

WHAT WE CLAIM IS: 1. A method of wet grinding of solid raw materials to form comminuted particulate solids, which comprises: introducing a slurry of the solid raw materials in a liquid into a grinding chamber of a grinding mill, introducing a plurality of freely tumbling and cascading corrosible grinding elements into the chamber, sealing the chamber against the entry of air and driving the sealed chamber to tumble and cascade the contents thereof to form comminuted particles.

2. A method as claimed in claim I, in which the air is displaced from the chamber by means of an inert gas prior to the driving thereof.

3. A method as claimed in claim 2, in which an inert gas atmosphere is maintained in the chamber during the driving thereof.

4. A method as claimed in claim 2 or 3, in which the inert gas is argon or nitrogen or a mixture thereof.

5. A method as claimed in claim 1, in which air is removed from the chamber by evacuation.

6. A method as claimed in any preceding claim, in which the grinding elements are iron-based balls.

7. A method as claimed in any preceding claim, in which the grinding elements are iron-based rods.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Example I Nitrogen Air Initial Ball Weight (dry), grams 443.3447 444.8115 Final Ball Weight (dry), grams 443.3175 444.3393 Weight Loss, grams 0.0272 0.4722 Annual Corrosion Rate 2.24/ 38.7/ After the test, on examination, the water from the nitrogen chamber was found to be essentially clear and the surfaces of the grinding balls were clean and shiny. The water from the chamber with the air atmosphere, however, contained a gelatinous type of precipitate of ferrous hydroxide and the surfaces of the grinding balls were found to contain a light rust. In this test, raw solid materials were not present in the grinding chambers; thus the consumption results reflect essentially only corrosion consumption and that lesser measure of abrasion consumption due to collision of bails with themselves and the chamber walls. Although the ball life would be expected to be somewhat reduced in the presence of raw solid materials (see Example 2 below), the abovenoted test is indicative of the substantial proportional reduction in corrosion consumption realized in the practice of the present invention. Example 2 Mill Mill Open to Air Closed to Air Initial Ball Weight (dry), grams 6302.900 6440.030 Final Ball Weight (dry), grams 6237.877 6418.417 Weight Loss, grams 65.023 21.613 Annual Corrosion Rate 62.76/ 20.42/ In the above tests, a slurry of 250 g of water and 750 g of taconite was utilized. The percentage weight loss of the balls over a year was then computed. From the foregoing, it is apparent that the present invention provides an improved method for grinding wet ore, while minimizing the amount of corrosion experienced by the grinding media. Although nitrogen and argon are primarily disclosed as the inert atmosphere of the present invention, it will be understood that other inert gases may be employed in practising the present invention. Moreover, it will be understood that grinding balls, rods or grinding elements of other shapes may be readily interchangeably employed in the chambers of Figures 1 and 2. Also, if desired, the vacuum method described in relation to Figure 2 may be employed in conjunction with the inert gas method of Figure 1. For example, oxygen could be removed by the vacuum method before charging the chamber with inert gas. WHAT WE CLAIM IS:

1. A method of wet grinding of solid raw materials to form comminuted particulate solids, which comprises: introducing a slurry of the solid raw materials in a liquid into a grinding chamber of a grinding mill, introducing a plurality of freely tumbling and cascading corrosible grinding elements into the chamber, sealing the chamber against the entry of air and driving the sealed chamber to tumble and cascade the contents thereof to form comminuted particles.

8. A method as claimed in any preceding claim, in which the liquid is an

oxygen-adsorbing liquid.

9. A method as claimed in claim 8, in which the liquid is water.

10. A method as claimed in any preceding claim, wherein the solid raw material is an iron-bearing mineral material.

Il. A method as claimed in claim 10, wherein the mineral material is taconite.

12. A method of wet grinding of solid raw materials according to claim 1, substantially as described with reference to Figure 1 or Figure 2 of the accompanying drawing.

13. A material in particulate solid form, when prepared by a method according to any preceding claim.

14. A material according to claim 13, comprising taconite.