US2678130A

US2678130A - Method and apparatus for demagnetizing magnetic ores having high coercive force

Info

Publication number: US2678130A
Application number: US165076A
Authority: US
Inventors: Nordahl L Onstad; Norman J Foot
Original assignee: University of Minnesota
Current assignee: University of Minnesota
Priority date: 1950-05-29
Filing date: 1950-05-29
Publication date: 1954-05-11
Anticipated expiration: 1971-05-11

Description

May 11, 1954 AL METHOD AND APPARATUS FOR DEMAGNETIZING MAGNETIC ORES HAVING HIGH COERCIVE FORCE Filed May 29, 1950 N. ONSTAD ET 2,678,130

, INVENTOR. NORDAHL Z O/vs-n o NORMA/v J f om MMWW Patented May 11, 1954 UNITED STATES FTENT OFFICE METHOD AND APPARATUS FOR DEMAGNE- TIZING MAGNETIC ORES HAVING HIGH COERCIVE FORCE Application May 29, 1950, Serial No. 165,076

16 Claims.

This invention relates to a method and apparatus for demagnetizing magnetic ores having high coercive force. During the beneficiation of iron ore, the ore, which originally may contain a larger percentage of rock (gangue) than is permissible for industrial application, is passed over a magnetic separator in which the ore, if it is in a condition for being magnetized, is separated from the gangue. Only ferromagnetic iron ores, for example magnetite, are capable of beneficiation in this simple and direct way. Other iron ores, such as hematite, oolite and the like, require a preliminary treatment in order to render them ferromagnetic in character, the treatment usually being a roasting process by which they are converted to the ferromagnetic state. The ore is, of course, crushed either before or after roasting so that the separation during a subsequent stage may take place.

The beneficiation procedures by which the ore is separated from the gangue include not only the magnetic separation, which may be done in several steps, but also includes demagnetization steps with intervening grade or size separation. It is therefore important to be able to demagnetize the ore. For example, finely divided particles of ore which are entirely separated from adherent gangue may, due to their residual magnetism, attract each other and thus form agglomerates of considerably larger size. These agglomerates in some instances will behave just the same as a larger size particle during, for example, a classifier operation for size separation, and therefore travel along with larger size particles, thus defeating a true size separation. To overcome this the ore needs to be demagnetized so that the particles: will no longer adhere together and form larger ag-glomerates. There are also other instances in which demagnetization as a process step is important to beneficiation processes for ores.

As the problem of beneficiating iron ore has become more pressing with depletion of higher grade ores and depletion of naturally occurring magnetite, the art has met with the necessity of demagnetizing artificial magnetite of high coercive force. It has been discovered, as the art develops, that naturally non-magnetic ores which are heat treated in order to make them ferromagnetic (i. e. artificial magnetite) develop a greatly increased coercive force. The exact reason for this increase in coercive force is not known, but it is known that many artificial magnetic ores have a coercive force of 200 to 325 oersteds and even higher. Ores having a coercive force of up to oersteds can be demagnetized successfully utilizing a 60-cycle alternating current magnetic field, but the higher coercive force ores cannot be demagnetized in this way, and up to the present tim inventions available for carrying out such demagnetization have been unsatisfactory for varying reasons. Thus, Patent No. 2,154,399 proposes to demagnetize ores of greater than ordinary coercive force by subjecting them to a magnetic field generated by a damped high frequency discharge which is obtained by repeatedly interrupting a direct current source of magnetic excitation. This method and the auxiliary apparatus is unsatisfactory because of the poor results and because of the difficultyin producing the necessary damped high frequency discharge current. To cite but one objection, it should be remembered that the magnetic field in a large magnet builds up relatively slowly and this factor alone imposes a limitation on the process.

It is an object of the present invention to provide a method and apparatus for demagnetizing iron ores of high coercive force and more particularly to provide a method and apparatus for demagnetizing iron ores having a coercive force of oersteds and higher.

It is a further object of the invention to provide a method of demagnetizing iron ore of high coercive force, for example 150 oersteds and higher, which may be applied commercially at relatively low cost for large scale operations.

It is another object of the invention to provide an improved method and apparatus for demagnetizing magnetic ores having a coercive force of 150 oersteds and higher which involves only an alternating current source of excitation and does not require any interruption of the current flowing to the demagnetizing coils.

It is another object of the invention to provide an improved apparatus for demagnetizing iron ores of high coercive force capable of demagnetizing large tonnages of ore in continuous flow.

Other and further objects of the invention are those inherent in the apparatus herein illustrated, described and claimed.

The method of th invention is illustrated with reference to the drawing which shows an illustrative form of apparatus of the invention, the drawing being partly schematic and partly a longitudinal section through the apparatus, whether arranged horizontally or vertically.

In carrying out the invention it is assumed first that the ore is ground in a previous step of the process so that the particles range in size from about 4 to 200 microns. In carrying out the process of the invention the thus finely ground ore is mixed with water so as to form an aqueous slurry containing approximately 30% to 60%, preferably 50% solids by weight. The slurry is then pumped at a regulated rate of fiow through an apparatus such as that illustrated in th figure of the drawing.

In the apparatus, which is one of several illustrative forms of the apparatus phase of this invention, the slurry of ore is pumped by means of a pump [0, the flow of which is regulated by the valve l l or by varying the speed of the pump, thence along conduit I2 and into and through a section is of the conduit. The section [5 of the conduit is made of non-magnetic material which is also a non-conductor of electrical current. The conduit thus may be made of plastic, fiber, glass or ceramic materials. Around the conduit there is provided a winding for producing in the conduit a sinusoidal alternating magnetic field having a frequency of 200 to 500 cycles per second or even higher frequencies, such as 1000 cycles, and a peak magnetic field strength of from 500 to 1000 oersteds. Thus, in the form of apparatus exemplified in the drawing, there are provided transformer primary and secondary windings around tube IS. The secondary transformer coil generally designated IS has a length L and a mean radius RM, and may be in one, two or more sections connected in parallel as ections IliA and IEB, each of which has a terminal as at ll and I8 for section [6A and terminals [8 and for section IGB. These secondary sections, if more than one is used, are connected in parallel to the secondary circuit composed of lines 2i and. 22 which extend to'a capacitor at 23 which constitutes the entire load on the secondary.

By appropriate selection or adjustment of the size of capacitor relative to the inductance of the transformer (particularly the inductance of the secondary thereof) it is possible to achieve a condition of resonance in the secondary circuit com posed of coil (or coils :6), lines 2% and and capacitor 23, and as a result of the condition of resonance, large circulatory currents oscillate through the coil l5 and condenser 6 at the chosen frequency. This induces the large magnetic flux required while imposing on the primary a load of substantially unity power factor in which the wattage input need only be sufiicient to satisfy the losses of the system, namely IR losses, hysteresis losses and dielectric losses. It is, therefore, a feature of the invention to elect or adjust the values of the inductive and capacitative elements of the system so as, if desired, to achieve a condition of resonance in the secondary at the selected frequency or conversely to select or adjust the frequency within the range stated hereinbefore to the same end.

Around the secondary generally designated l6 of the transformer there is wound a primary winding generally designated 25 which can have a very much smaller number of turns and has

terminals

25 and 21 that are connected by

lines

28 and 29 to an alternator 30. The alternator is designed so as to be capable of delivering an adequate supply of electrical current at a frequency of 200 to 500 cycles per second or even higher frequencies such as 1000 cycles per second and higher, and at a voltage sufficient to energize the primary winding'ffi and through it the secondary winding IE sufficiently to produce in the tube It an alternating magnetic field (sinusoidal wave) having a peak magnetic field strength Value of 500 to 1000 oersteds. The peak field strength occurs once each cycle in a positive direction and once each cycle in a negative direction for each full wave of the alternating excitation current in the circuits. The field strength depends upon the field distribution Within the tube l5, being a maximum at the midpoint of the dimension L of the transformer secondary coil l8 and dropping on" to a small value a considerable distance beyond such dimension, that is to say, beyond each end of coil it. The tube i5 is extended beyond coil it? in the direction of flow of the slurry to a region where the field strength, measured at the peal: of the alternating magnetic field, is not more than 10% of the maximum peak field strength within sec ondary coil 16.

The flow of the slurry of finely divided ore particles through pipes l2 and thence through tube It is regulated, either by regulating the speed of operation of the pump [0, or by regulating the valve i l or both so that the slurry moves at a rate such that a particle of ore moving at the average rate of movement of the slurry remains in the region of maximum field strength for not substantially less than 10 COlllplElJG-CYClBS of the applied demagnetizing magnetic field, and thence continues to move as an uninterrupted continuous flow-out of such field (that is to say, the slurry is withdrawn from the influence of maximum field strength) to a region wherein the field strength is not more than 10% of the maximum field strength during an additional time period equal to not substantially less than 10 to additional complete cycles. Thus, the rate of flow is regulated in accordance with the foregoing, dependent upon the particular frequency within the-range of 200 to 500 cycle per second or higher frequency, chosen for exciting the demagnetizer I5l630. When the slurry of ore has passed beyond the field produced by the transformer iii-'25 andto a place where the field is not more than 10% maximum, the ore is sufrlciently demagnetized so as to be suitable for subsequent steps in the beneficiating process.

In the apparatus of the invention the length of the secondary coil is should, for best results, be from two to three times the mean radius of such coil, although different ratios of length to mean radius may be used without departing from the spirit of the invention. In addition, the primary and secondary coils may be connected in autotransformer relationship, if desired, or may be insulated from each other as shown.

In an exemplary form of apparatus made in ac cordance with the present invention the conduit l5 consisted of a fiber tube having an inside diam-- eter of '12 inches. The secondary coil it had a mean radius Rivrof 7 inches and a length of 19 inches. The coil l6 consisted of 320 turns and was divided into two sections. Current in the secondary coil I6 was amperes and was divided evenly between the two sections 16A and NB. The voltage across the secondary, between lines 2 I and 22, was 1575 volts. The capacitor 23 consisted of a bank of units of 280 kva. capacity at 300 cycles per second. The loss in the secondary coil It was 1400 watts and in the capacitor .23 was 1260 watts. The primary consisting of 20 turns was energized at a potential of about volts unity power factor, from alternator 3!] at 300 cycle per second. The primary current was about 23 am peres.

In the apparatus so constructed the slurry, containing approximately 50% solids, moved in tube [5 at a rate of 10 feet per second and was sufficiently demagnetized when it left tube l5 twenty inches below coil 16. The tonnage of solids per hour was approximately 100 tons.

By way of further illustrating the invention, but without limitation thereon, reference is made to the following examples:

Example I Roasted hematitic taconite having a particle size of 4 to 160 microns was preliminarily separated and was then made into an aqueous slurry of approximately 50% solids by weight. The coercive force of the ore was approximately 200 oersteds. The slurry so composed was passed through an apparatus, as illustrated in the drawings hereof, excited at 300 cycles per second and having a maximum peak field strength of 800 oresteds (sinusoidal wave) and was then withdrawn to a region of less than maximum field strength. The rate of flow through the tube i5 was adjusted so that every portion of the slurry remained on an average in the region of maximum peak field strength for 10 cycles or more, and was then carried a a continuous flow to a region beyond coil 16 where the field strength had a peak value of 10% of the maximum peak value, during a time period equal to approximately 10 additiona1 cycles. After being withdrawn the ore was demagnetized to an extent suflicient to permit successful operation of subsequent classification procedures.

Example II OiiIitic type iron ore having a particle size of 4 to 160 microns was made up into a slurry with Water, the slurry containing approximately 50% solids by weight. Utilizing an apparatus as illustrated in the drawing, wherein the primary was energized at a frequency of 600 cycles per second and at a voltage suflicient to produce in tube IS a maximum peak field strength of 900 oersteds, the ore, which had a coercive force of 325 oersteds, was then passed through tube [5, the rate of flow of the slurry being adjusted so that on an average every ore particle remained in the region of maximum peak field strength for a time period equal to 12-15 cycles and was withdrawn to a region wherein the field strength was not in excess of 101% field strength in an additional time period of not substantially less than 25 cycles duration. The ore so treated was demagnetized to an extent sufficient to permit subsequent beneficiation steps.

As many apparently widely different embodiments of this invention may be made without l departing from the spirit and scope thereof, it is to be understood that we do not limit ourselves to the specific embodiments herein.

What we claim is:

1. The process of demagnetizing finely divided magnetic iron ores having a particle size of about 4 to 200 microns and having a coercive force of more than 150 oersteds which comprises passing an aqueous slurry of said finely divided ore particles as a continuous flow into, through and then out of a continuously excited sinusoidal alternating magnetic field having a frequency of not less than 200 cycles per second and a peak magnetic field strength at the central area of said field in the range of about 500 to 1000 oersteds, said slurry being moved continuously at a rate such that it remains in said central area wherein said peak magnetic field strength is maintained for not substantially less than a time period equal to ten cycles and is withdrawn as a continuous flow during a time period of not substantially less than 10 cycles through areas of diminishing field strength to the fringe of said field where the field strength is not more than about 10% of said maximum field strength.

2. The process of demagnetizing roasted hematitic and oolitic iron ores having a coercive force of not less than 200 oersteds and having a particle size from about 4 to 200 microns which comprises passing a slurry of said ore as a continuous flow into, through and out of a continuously excited substantially sinusoidal alternating magnetic field of not less than 200 cycles per second, said field strength having a central area of maximum peak value in the range of 500 to 1000 oersteds and surrounding areas gradually diminishing in field strength, the rate of movement of the slurry being adjusted so that a particle of ore moving as a continuous flow at the average rate of flow of the slurry, remains in said central area wherein said alternating magnetic field strength reaches said maximum peak value for not substantially less than 10 cycles and is withdrawn as a continuation of said continuous flow through said areas of gradually diminishing field strength to a remote place where the field strength is not more than 10% of said maximum during a time period of not substantially less than 10 cycles.

3. The process of demagnetizing roasted hematitic taconite having a particle size of about 4 to 200 microns and a coercive force of about 200 oersteds which comprises passing an aqueous slurry of said ore as a continuous flow through a non-conductive, non-magnetic tube, simultaneously passing an alternating electric current of not less than 200 cycles per second around said tube, said current being of a strength sufficient when passed around said tube to establish in the central portion of said tube a maximum region of alternating magnetic field of a strength of not substantially less than 600 oersteds peak value, said field strength diminishing gradually along said tube to a region where it is less than 10% of said maximum peak value, regulating the rate of flow of said slurry in said tube so that a particle moving at the average rate of flow of said slurry is subjected to not substantially less than ten complete cycles of said alternating magnetic field wherein the strength of the field reaches said maximum peak value and in a time period of not substantially less than 10 cycles, Withdrawing said slurry as a continuous flow through portions of said tube wherein the field strength gradually diminishes to said region in which the magnetic field is not substantially more than 10% of maximum peak value.

4. The process of demagnetizing roasted oolitic iron ore having a particle size of about 4 to 200 microns and a coercive force of about 300 oersteds which comprises passing an aqueous slurry of said ore as a continuous fiow through a non-conductive, non-magnetic tube, simultaneously passing an alternating electric current of not less than 500 cycles per second around said tube, said current being of a strength sufficient when passed around said tube to establish in a central portion between the ends of said tube a maximum region of alternating magnetic field of a strength of not substantially less than 900 oersteds peak value, said field strength diminishing gradually along said tube to a region where it is less than 10% of said maximum peak value, regulating the rate of flow of said slurry in said tube so that a particle moving at the average rate of flow of said slurry is subjected to not substantially less than ten complete cycles of said alternating magnetic field wherein the strength of the field reaches said maximum peak value and in a time period of not substantially less than 25 cycles withdrawing said slurry as a continuous flow through portions of said tube wherein the field strength gradually diminishes to said region in which the magnetic field is not substantially more than 10% of maximum peak value.

5. An apparatus for demagnetizing finely divided magnetic iron ores having a coercive force value of greater than 150 oersteds comprising a conduit of non-magnetic, non-conductive Ina-- teriai, primary and secondary windings in transformer relationship around a portion of said conduit, a capacitor connected to said secondary to constitute the sole load thereon and a source of alternating current having a frequency of greater than 200 cycles per second connected to said primary winding for continuously energizing said windings and a voltage sufficient to produce in said conduit an alternating magnetic field having a maximum peak value of 500 to 1000 oersteds and means for regulating flow through said conduit.

6. The apparatus of claim 5 further characteriaed in that said primary and secondary windings are electrically insulated from each other.

7. The apparatus of claim 5 further character- Li.

lzed in that said primary and secondary windings are connected to. each other in auto-transformer relationship.

8. The apparatus of claim 5 further character- .ized in that said secondary has a large number of turns as compared with said primary.

9. The apparatus of claim 5 further characterized in that the secondary winding has a length measured along the conduit equal to 2 to 3 times the mean radius of said winding.

10. An apparatus for demagnetizing finely ivided magnetic iron ores comprising a conduit of nonmagnetic material which is nonconduc tive in a direction around the axis of said conduit, primary and secondary windings in transformer relation around a portion of said conduit, said primary being connectable to a source of alternating current having a frequency of about 200 cycles per second or more and a potential sufiicient to continuously energize said I;

windings to a peak magnetic field strength of 500 to 1000 oersteds, a capacitor connected to said secondary constituting substantially the sole connected load thereon said secondary and capacitor forming a resonant circuit at the alternating current frequency applied to said primary and means for regulating flow through said conduit.

11. The apparatus of claim 10 further characterized in that said primary and secondary windings are electrically insulated from each other.

12. The apparatus of claim 10 further characterized in that said primary and secondary windings are connected to each other in autotransformer relationship.

13. The apparatus of claim 10 further characterized in that said secondary has a large number of turns as compared with said primary.

14. The apparatus of claim 10 further char- 8 acterized in that the secondary winding has a length measured along the conduit equal to 2 to 3 times the mean radius of said winding.

15. An apparatus for demagnetizing finely divided magnetic iron ores which is carried in an aqueous slurry comprising a conduit for handling a flow of aqueous slurry of said finely divided ore, said conduit being of non-magnetic, and non-conductive in a direction around the axis of said conduit, primary and secondary windings in transformer relation around a portion of said conduit, a capacitor connected to said secondary to constitute substantially the sole electrically connected load thereon, a source of alternating current having a frequency in the range of about 200 to about 1000 cycles per second and a potential suiiicient to continuously energize said windings for producing a peak magnetic field strength of from 500 to 1000 oersteds, said windings and capacitor forming a resonant circuit and means for regulating flow through said conduit.

16. An apparatus for demagnetizing finely divided magnetic iron ores while the said are in the form. of an aqueous slurry comprising a conduit for handling a flow of said aqueous slurry of said ore, said conduit being of nonmagnetic material, which is non-conductive in a direction around the axis of said conduit, primary and secondary windings in transformer relation around a portion of said conduit, a capacitorconnected to said secondary and constitute substantially the sole electrically connected load thereon, said primary winding being connecti able to a source of alternating current of 200 cycles per second or more and a potential sufficient to energize said windings to produce a field strength of from about 500 to about 1000 oersteds and means for regulating flow through said conduit, said primary and secondary windings being electrically insulated from each other but connected to each other in auto-transformer relationship and said secondary winding having a relatively large number of turns as compared 5 with said primary winding, said secondary winding having a length measured along the conduit equal to two to three times the mean radius of said winding, said capacitor and windings forming a tuned electrical circuit.

References Cited in the file of this patent UNITED STATES PATENTS Begun June 19, 1945' OTHER REFERENCES Scientific Papers of the Bureau of Standards-'- 1 No. 78, The Best Method of Demagnetizing Iron in Magnetic Testing, September 3, 1907.

Spooner, "Properties and Testing of Magnetic Material, published by McGraw-Hill Book 00., New York (pages 288 and 289).