United States Patent 3658260

A method for determining the extent to which ores should be crushed and ground for optimum beneficiation in which dust is separated from the crushed ore and is then continuously sampled, and concentrated as to the desired mineral component and the concentrate continuously analyzed to determine the content of one of the mineral components thereof. The analysis is then used to determine the extent of grinding to control the composition of the final concentrate. Also the analysis may be used to proportion ore obtained from various sources to assist in controlling the composition of the final concentrate.

Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
241/29, 241/33
International Classes:
B02C25/00; (IPC1-7): B02C25/00; B02C21/00
Field of Search:
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US Patent References:

Other References:

riede, J. R. & Kachel G. C. "Instrumentation and Automatic Control Systems in Modern Processing Plants" Canadian Mining Journal, pages 67- 72, Mar. 1961..
Primary Examiner:
Kelly, Donald G.
I claim

1. In the beneficiation of ores in which an ore is crushed and then ground for separating gangue materials from a wanted mineral, the method of controlling the degree of fineness of grinding to produce the desired concentrate comprising continuously separating dust from the crushed ore, concentrating said separated dust for the content of wanted mineral, continuously analyzing said concentrated dust for one of the mineral components thereof and controlling the degree of grinding in accordance with the analysis for the mineral component whereby the final ground product has the desired concentration of wanted mineral.

2. The method as claimed in claim 1 wherein the concentrated material is analyzed for silica content.

3. The method as claimed in claim 1 and further comprising forming an aqueous slurry of the dust separated from the crushed ore before concentrating said separated material for the content of wanted mineral.

4. The method as claimed in claim 1 and further comprising cutting a continuous series of samples of said dust prior to concentrating said samples of said material.

5. The method as claimed in claim 1 and further comprising initially combining ore from at least two sources, the approximate composition of each of said at least two sources being known, in such proportion as indicated by said analysis that the wanted mineral of the final product will be substantially as desired.

6. The method as claimed in claim 1 wherein the fineness of grinding is increased as the gangue content of the analyzed concentrated dust increases.

7. The method as claimed in claim 1 wherein the fineness of grinding is decreased as the gangue content of the analyzed concentrated dust decreases.

8. The method as claimed in claim 1 wherein said concentrated dust is analyzed for the mineral component by the neutron activation principle.

9. The method as claimed in claim 1 wherein said concentrated dust is analyzed by an on-stream method of analysis which provides substantially instantaneous results.

In beneficiating low grade ores such as taconite, initially containing from about 20 to 55 percent iron mineral bearing material to a concentrate containing from about 85 percent to about 100 percent iron oxide, it is customary to crush the ore in several stages and then grind it in several stages, removing gangue materials between the grinding stages. The removal of non-ferrous materials, including silica, is accomplished by separators such as magnetic separators, hydrosizers, etc. Thus the finer the ore is ground the greater is the extent of liberation and separation of iron bearing minerals from the gangue material. The final iron bearing material is agglomerated and pelletized for easy handling and for shipping to furnace plants for smelting the ore to produce pig iron. For economic reasons it is impractical to grind the crushed ore to a fineness greater than that degree wherein valuable minerals are mechanically liberated from non-valuable minerals at which the final product may contain about 85 to nearly 100 percent iron oxide. This generally is the economically desirable iron concentration for blast furnaces. In order to produce pellets of higher concentration of iron, finer grinding would be required and this in turn increases cost so that it is economically unfeasible.

Furnace operators now generally require pellets having a rather specific iron concentration in order to economically operate the furnaces.

During the mining of taconite ores, such as the ores found on the Mesabi Range in Northern Minnesota, the ores at different locations are found to vary somewhat in iron content so that it is desirable to know the source of the iron ore in order to determine the necessary degree of grinding whereby the final pelletized product will have the desired iron concentration.

Usually samples of the ore are taken even before the ore is blasted out of the earth, by obtaining diamond drill core samples, which are physically and chemically analysed. This method has one great disadvantage in that the core samples obtained represent only a very small part of the ore and are taken necessarily only at widely spaced intervals, thus giving a relatively poor representation of sampling, of the order of about 1 part in 8,600,000,000. In addition it is quite difficult to obtain the core samples in the very hard taconite ores so that diamond drilling techniques are usually required which are expensive. The cuttings derived from the jet-piercing method for blast hole drilling, a process required because of the hardness of the ore, are not suitable for sampling the ores because of the high temperatures involved which cause mineralogical changes in the ore particles.

Aside from the fact that the drill core samples do not provide a satisfactory sampling of the ore for mill control purposes there is the additional disadvantage that the chemical analysis of such core samples usually is representative of only a small volume of ore immediately surrounding the ore from which the sample was taken. The mining equipment usually progresses several tens of feet per day along a mining face containing broken ore having previously been blasted, and the diamond drill sample analyses are from holes drilled in the original ore varying distances from the mining face. Such samples are also subject to the serious disadvantage that the chemical analysis of such samples usually requires such a period of time that the ore from which the sample is taken has usually been ground to its ultimate fineness before the test results can be known. Thus the analysis results cannot be used to control grinding of subsequent ores.

The present invention is designed to provide a sampling and analysis process by which a far more representative sample is obtained, i.e. of the order of 100,000 times more representative than the sample obtained by diamond drill core sampling described above. Accordingly, the present method involves cutting samples from all of the stream part of the time whereas the diamond drill core method involves cutting samples from a small part of the stream part of the time. By the present invention a sample of about 15 to 25 pounds is obtained from about 700 to 800 long tons of ore and thus is a very much better sampling of the order of about 1 part in 80,000, and also the time for sampling and analysis is sufficiently short so that the results can be used not only for controlling the ultimate fineness of grind of the ore sampled but also can be used effectively to provide a control, even though somewhat less precise, for proportioning the original ores taken from different mining faces so that the overall mixture when subjected to a relatively uniform fine grinding will result in a concentrate having the desired iron content. It will be understood therefore that the method of the present invention enables one to control the quality of product with much greater ease and certainty than has been possible heretofor.

In the usual process for beneficiating ores, the ore, after being removed from the mining face is crushed in several stages during which time the ore is reduced to particles ranging from a few microns to about 3/4 inch particles. At each stage of the crushing a great deal of dust is produced and is removed from the ore stream by dust collectors, suction means or the like. The crushed ore is subsequently passed to an ore bin and then via a conventional mill line through a rod mill, magnetic separators, ball mill and other separators whereby the ore particles are reduced in size to the extent necessary for removal of most of the undesired silica and other gangue materials. Ore that is not sufficiently ground by the last ball mill may be recycled to the ball mill for regrinding. An example of the overall process and particularly a conventional mill line may be found in "Mining Engineering," May 1963, pages 39 through 54, published by Society of Mining Engineers. It is the final grinding step which determines the ultimate iron concentration in the final pellets and it is this iron concentration it is desired to control.

As mentioned above during the crushing stages, dust is removed. According to the present invention it has been discovered that this dust may be concentrated and the resulting concentrate analyzed for silica content and the analysis used to determine the extent of grinding necessary in the ball mill in the last stages of grinding of the ore. Also this analysis may be used for determining the quantities of ore derived from different mining faces to form the total ore mix to be crushed and delivered to the ore bins prior to being fed into a mill line. In practical operations, such factors are of great advantage, since the desired fineness of grind is established before the ore or ores are subjected to the grinding operations.

The discovery that the dust from the crushing steps may be recovered, concentrated and analyzed to determine the iron or silica content and that this analysis may be directly correlated to the iron or silica content of the final product after the ore is processed into valuable concentrate is the basis for the method of the present invention.

It was found, for example, that a direct relationship exists between the magnetically concentrated dust from the crusher and the concentrate of a diamond drill core sample Davis tube test. Various correlations were discovered including the fact that the iron and silica analyses of each screen size fraction closely approximate that of the ore concentrate after processing. It was further found that the silica analysis of the dust concentrate compared to the silica analysis of various mill products, such as the rod mill feed tube test concentrate, finisher concentrate tube test heads and finisher concentrate tube test concentrate, showed a much higher degree of correlation than was found between the diamond drill tube tests compared to the mill results.

The present invention is therefore a practical application of the discovery of this high degree of correlation, together with the continuous supply of dust for sampling as well as the much more representative nature of the samples compared to diamond drill tube tests.

The method of the present invention will now be described in greater detail with reference to the accompanying drawing, in which

FIG. 1 shows a flow sheet of the overall beneficiating process and the location of the silica analyzer of crusher dust therein and

FIG. 2 shows diagrammatically the steps followed for sampling the dust and the controls derived therefrom.

According to FIG. 1, ore from different mining faces 1, 2 and 3 or more is delivered to a series of crushers, of which two are illustrated at 4 and 5. Dust from one or both of the crushers is slurried with water and then passed to a concentrator 6 and the iron rich concentrate is continuously analyzed for silica as at 7. In the meantime the crushed ore is moved to an ore bin 8 from whence it is passed via a conventional mill line to grinders 9 and 10 and eventually to an agglomerating and pelletizing plant 11. As indicated by dotted lines the results of the silica analysis at 7 may be used to control the one or more grinding operations at 9 and/or 10 and/or may be used to control the relative quantities of ore supplied from one or the other of mining faces 1, 2 and 3.

In FIG. 2, the dust from one of the crushers is introduced into a rotoclone 12 and is fed to a sample cutter 13 after having been slurried with water introduced at 14. The series of samples cut out from the slurried dust are continuously fed over a screen 15, such as a sieve bend, to remove the small amount of larger tramp particles. The slurried samples are then fed to a magnetic separator 16, which may be a 3 drum laboratory type of magnetic separator to form a concentrate. The waste material or gangue, i.e. non magnetic material is removed at 17. The samples of concentrate are then passed to a silica analyser 18 which may be of any type, giving practically instantaneous results, e.g. an on-stream silica analyser of the x-ray type. It has been found however that an on-stream analyser based on the neutron activation principle is particularly suitable for this analysis. Analysers of this latter type are described in the technical paper entitled "Nuclear Techniques in On-Stream Analysis of Ores and Coal" (ORO2980--18) dated Sept. 26, 1968, by J.R..Rhodes, et al., Texas Nuclear Corporation, Austin,Texas, published by the U.S.Atomic Energy Commission, U.S.Dept. of Commerce, Bureau of Standards, and reproduced and distributed by Clearinghouse of Springfield,Va. 22151. An additional description appears in a technical paper entitled "Neutron Activation Analysis for Industrial Process Control" by P.F.Berry and J.B. Ashe, Nuclear-Chicago Corporation, Texas Nuclear Division, Austin, Texas, prepared for 4th Annual Symposium of the Instrument Society of America (Lake Superior Section), Duluth, Minn, June 19--29, 1969.

As used herein and in the appended claims the term "on-stream" as applied to the analysis of the concentrated dust samples is intended to mean an analysis the results of which are available for use substantially immediately after the representative samples are taken from the concentrated dust in a continuous or rapidly repetitive manner.

The results of the analysis determine the amount of silica in the samples and these data can be directly correlated with the original ore from which the sample was taken and therefore with the final iron content of the concentrate which is to be agglomerated and pelletized. For example, when a sample shows a high silica content it indicates the need for giving the ore from which the sample was taken a finer grind than samples showing a lower silica content. Furthermore, when the sample analysis shows such a high silica content that an excessive amount of grinding would normally be required, it then becomes desirable and even necessary to select another source of the ore from those mining faces where the iron content and physical characteristics of the composite ore are sufficiently favorable to eliminate the necessity for finer grinding. Table I below gives a representative analysis of a concentrate so produced from a dust sample and a representative analysis of a mill line concentrate. It is readily apparent that the iron contents as well as the percent weights are quite comparable for the concentrate from the dust sample and the mill concentrate. Some variance in iron content can be noted in the plus 200M fraction; however, this is to be expected because of the wide variance in size possible in that coarse fraction. ##SPC1##

In the practice of beneficiating ores, it has become a well known principle in the art of ore dressing that a direct relationship exists between requisite fineness of grind and desired grade of concentrates produced therefrom, regardless of the concentration process that is applied to treatment of the ores.

The process described heretofore applies specifically to treatment of magnetic taconite ores. Said process is in universal commercial use throughout the world.

Treatment of other than magnetic taconite ores has similar principles in that the ores must be crushed, ground and concentrated by some process such as various gravity processes and the well known flotation process used to produce the bulk of the world's supply of lead, zinc, copper and others. Most metallic and non-metallic minerals, contained in ores are treated by similar principles of the art of ore dressing.

In the treatment of all ores there exists, as is well known to those who practice the art, certain common problems. One problem is the requisite fineness of grind to produce a finished concentrate of specified grade. It will be understood that too coarse a grind can produce a concentrate that will chemically analyze low in desired valuable mineral content due to incomplete liberation of valuable and non-valuable mineral constituents. Contrarily, ores may be ground too fine, wherein the grade of produced concentrate may be obtained at the expense and high cost of overgrinding. Also, extreme overgrinding can readily reduce the desired valuable mineral constituents to so fine a size that the mineral particles will not respond to the principle of the concentration process in use, and is thus lost and passes out with the rejected non-valuable constituent of the ore.

A second problem common to treatment of all ores is that of blending. Most concentration processes are generally based on treating ores of uniform or average values, both as to the valuable and non-valuable mineral constituents. Uniformity is generally considered both from physical and chemical viewpoints. However, nature did not produce ore depositions of uniform stylized types. Thus, in mining of ore deposits, considerable efforts are devoted and much monies expended to produce ores from various mining faces at any single mining venture that tend to provide the mill or concentrator at that mining property with a blended ore having more or less uniform character.

Thus the subject of this invention is generally directed to the treatment of any and all ores requiring ore dressing treatment for production of concentrates. For instance, the invention could readily be used at a copper porphyry mining operation by merely substituting a flotation machine in place of the magnetic separator and analyzing the flotation concentrate thus produced for its copper content. Likewise, in treating certain types of hematitic ores, gravity concentration units such as shaking tables or jigs could be used as the concentrating device and the resultant concentrate analyzed for either its iron or siliceous content.

The subject of this invention is further directed to the treatment of ores and like materials by means of dry processing, such as for example, treatment of ores by means of dry electrostatic concentration methods which processes are commonly applied in the treatment of specular hematitic ores, ilmenite and rutile ores, et cetera. All dry concentration processes are included in the broader aspects of the present invention.

In such processes where dry concentration methods are used, the invention is used in a like manner to that employed where wet concentration methods are used. Thus the dust is collected, sampled and fed to a dry concentration unit and the dry concentrate is subjected to analysis in a dry form. The dry form may consist of the concentrated solids suspended in an air stream, or they may be briquetted or prepared in a suitable disc form, or in any other manner in common use.

It is, therefore, to be understood that the invention as described in detail herein, may be applied not only to the treatment of slurries in the wet manner but also to treatment of dry materials in a dry manner, or, in any combination thereof.

Although the process has been described with particular reference to the silica analysis of dust from crushers employed in beneficiating taconite ores, it will be understood that the process is equally applicable to other magnetic ores and even to non-magnetic ores where other means are used for concentrating same, such as flotation and gravity concentration processes. Thus, it is applicable to non-ferrous ores as well.