US2805769A

US2805769A - Beneficiation of nonmetallic minerals

Info

Publication number: US2805769A
Application number: US666284A
Authority: US
Inventors: James E Lawver
Original assignee: International Minerals and Chemical Corp
Current assignee: International Minerals and Chemical Corp
Priority date: 1957-06-20
Filing date: 1957-06-20
Publication date: 1957-09-10
Anticipated expiration: 1974-09-10

Description

sept. 1-0, 1957 Filed June 20', 1957 5 Sheets-Sheet 1 ATTORNEYS .Sept 10, 1957 J. E. LAwvER 2,805,769

BENEFICI'ATION oF NONMETALLIC MINERALS A ORNEYS Sept. l0, 1957 J. E. LAWVER 2,805,759

BENEFICIATION oF NONMETALLIC MINERALS Filed'June 20', 1957 3 Sheets-Sheet 5 FEED l l A C@ fag, ffof IN VENTOR lames E'Jawvel' ORNEYS United States ate BENEFICIATION F NoNMETALLIc MINERALS James E. Lawver, Skokie, Ill., assignor to International Minerals & Chemical Corporation, a corporation of New York Application June 20, 1957, Serial No. 666,284

13 Claims. (Cl. 209-11) This invention relatesto therdry beneiiciation of nonmetallic minerals and more particularly relates to a method for the electrostatic beneficiation of non-metallic minerals. y

It lhas long been known to the art that it is possible to attain a mineral beneiiciation of sorts by passing finely divided mineral through an electrostatic field. Despite a prodigious amount of research, however, and prolific patent and literature art reflecting that research, electro static beneficiation of minerals has to date been of little more than laboratory significance except for conductivity separations of lead sulfide from zinc sulfide and the conductivity beneiciation of beach sands. As a result of the problems encountered, both from the standpoint of designing suitable apparatus and from the standpoint of producing the necessary differential charges in the particles lof the feed material, the art turned from electrostatic beneficiation .to flotation procedures and other wet beneiiciation processes despite the added cost of otation reagents and the necessity for ample supplies of water. Thus, in arid regions where many of the available mineral deposits are located, it has been necessary to forego ore beneliciation close to the ore body and, in many instances where cost margins are close, otherwise valuable mineral deposits have gone undeveloped.

Apparatus developed by the prior art for electrostatic beneciation of minerals falls into several categories. One type of apparatus utilizing particle charging by inductive conductance is disclosed in Johnson Patents 2,135,716 and 2,246,253. In this type of apparatus, the electrostatic separation effected depends upon the diiferences in surface conductivity and capacitance of the particles of vcomminuted mineral normally fed in a single layer to a revolving cylindrical electrode. The phenomenon of charging by inductive conductance is a function of the electric field, the time the particle is in contact with a metal in the electric field, particle capacitance, and net surface resistance. `Net surface resistance, in turn, depends on particle history, particle purity, particle contact resistance, and particle moisture content. In apparatus wherein charging is effected by inductive conductance, mineral particles are caused to contact a conductive surface in the presence of an external electric field. The conductive particles become charged by induction much more rapidly than the non-conductive particles. Accordingly, by controlling the time and theeld strength in relation to the surface resistance and capacitance of the particles, it is possible to control particle trajectories in the electrostatic iield and to effect a separation.

f InV the mobile ion type of separator, such as the Sutton or Carp'co machine, the particulate feed is usually delivered .in a single layer to an earthed revolving cylindrical electrode which is spaced adjacent an oppositely charged second electrode. As the material is conveyed around the upper surface of the revolving cylindrical electrode it is in the normal operation, charged by bombardment with ions and/or electrons from the adjacent electrode or from any convenient source. The more yconductive particles rice rapidly share their charge with the earthed electrode, and.

fall into a first receptacle under the inuence of gravity.

The less conductive materials, on the other hand, are heldV to the revolving electrode by their image force until removed by a suitable 'means and allowed to fall into another receptacle. The charging mechanism'used in the mobile ion type machine and the phenomenon of image force are adequately described by Sir James .leans in the Mathematical Theory of 4Electricity and Magnetism, 1933. It is to be noted that in the normal operation of all of the above-described machines, the charging of the particles is always effected in the electrical field. A more detailed discussion of such rapparatus and its mode of operation may be found in Taggarts Handbook of Mineral Dressing (1945), chapter 13, pp. 45-46, and in Gaudin Principles of Mineral Dressing (1939), p. 465.

Because each mineral particle must make metal contact with a metal electrode, the capacity of the Johnson, SuttonA and Carpco type machines is necessarily limited.

Moreover, such machines do lnot operate satisfactorily.

with materials which are substantially non-conductive and hence require 'an unduly long charging and/or discharging time.

A further type of prior art electrostatic beneciation procedure'is disclosed in OBrienU. S. Patent 2,168,681 andniay be'characterized as a-mineralmetal impact or wiping contact electriiicationiprocedure. In this method of beneciation, electritication is effected by impact or wiping contact with a grounded, conductive metal surfacein the absence of an external electric field. In such a procedure the .charged particles enter an external electric field and if the charges on the mineral particle spe-v cies are suiiiciently diife'rent, either in sign or magnitudeY or'both, a species concentration is eiiected as a result of the particle movement in the electric field. Again by' necessity, such procedure is limited to 'a relatively low capacity since yeach particlev must be subjected to the wiping or impact action on the metal surface. The OBrien method, therefore, has not been developed to the point of commercial significance.

The basic objection to charging principally by wiping or metal donor action lies in the fact that most particles tend to become negatively charged by this mechanism. The negative chargefthat usually results due to mineralmetal'contact in general follows Coehns rule which states that the surface charge 1 density a=15 106 (erf-e2) coulomb/meter2, where 'er and e2 are relative permittivities (.Coehn Handb, der Physik, 13, 332). This rule expressed in the modern theory of solids by Swikker in PhysicalV Properties of .Solid Materials, p. 256 (1954) states that the material with the greater number of energy levels will have the higher permittivity and will be more easily polarized so that itv will give off electrons to the other contact'material. That is, Electron-con-V taining energy bands of the metal may be in equilibrium withthe forbiddenbands of the semiconductor or insulator. By wave mechanics, electrons-pass' from the metal to the boundary and are reflected with exponential return to the `metal thus leaving the dielectric negative upon separation. Y Y

vkIf all of the particles' which are' strongly negatively charged by wiping oirnetalydonor action' are either gangue or desired mineral components, a satisfactory separation may be achieved. lf, as is almost always the case, both gangue particlesy and desired particles are strongly negatively charged, a satisfactory'beneciation is difficult to obtain. charging lie in the excessive apparatus wear engendered by the continuous Wipingkcontact andthe low capacity imposed by the'necessaryscontact ,of eachV particle with the metal donorplate.y fj A Further objections to wiperror ,donor platey With the exception of dielectric separations, differential charging is a surface phenomenon and, consequently, surface treatment of `the mineral particles with chemical agents such as surface active agents, including known flotation collectors; mineral acids, organic acids, and the like, has been suggested.` These reagents, however, are objectionable for a number of-reasons including corrosion of apparatus, cost and deleterious effect on subsequent operations. n n

YIn view of the foregoing problems confronting the prior art with'respect to electrostatic beneciation of minerals, the` primary object of the present invention is to provide an improved processI for beneficiation of non-metallic minerals wherein-electrostatic methods ofV separation are employed to produce commercially attractive yields.

It is a further object of this invention to provide an improved electrostaticbeneciation process wherein sutlicientdifferences -inelectrical charge are maintained between 'thel desired components of the feed` material and the gangue toprovide commercially satisfactory electrostatic separations 1t is an additional object of this invention 'to provide a. method for electrostatic Vbeneiiciation of substantially non-conductive, `non-metallic minerals which produces commerciallyattrac'tive yields and which does not necessitate reagentizing the feed material.

Yet .another object of the invention is to provide an electrostatic process for the separation of substantially non-conductive, non-metallic mineral particles from ganguewhen the..various types of particles` separated are relatively poor eonductors characterized by a conductivity of .substantially the same order of magnitude. In accordance with the. present invention, it has been discovered that eminently satisfactorydry beneficiation of ores and minerals can be achieved electrostatically by means of a series of critical and interdependent process steps. L The process ofthe invention does not require reagentizing of the mineral feed and yet provides a differentelectrification of the particular components in the feed whereby a commercially attractive electrostatic berieficiation and separation of even substantially nonconductive `components is achieved. Moreover, the through-put per lineal foot of electrode obtainable with the process `of this invention is markedly greater than that' possible witlizany other known electrostatic separation procedure.

Generally described, the present .invention is an electrostatic process for beneciating non-metallic minerals which comprises heating themineral to atemperature of` at leastV about 4150I F. while inta state of subdivision sufficient to substantially completely liberate the desired components fromy the 'gangue, inducingthe particulate mineral in` theabsence of an external electric field and whileat a temperature of at least 150 F., to accept differentialelectrical chargesand `then subjecting .the differentially charged particlesas free falling bodies and while. at a .temperature of at least. 150 F. to an electrostatic eld to beneficiate the material `without substantially ,altering the charge on the particles while in the electric field.` i

The mineral processedin accordance with ,the invention mustbe subdivided ,to adegree requisteto liberate the value components. The exact particle size necessary to liberation will vary withthe `crystal size of the different minerals. In general, reduction in particle size toless than about 14 mesh issuicient toliberate the desired components from the gangue whilelreductiou in particle size to less thanZOQ meshr, generally provides a i 4 size. Such additioualcrushing of the individual, liberated particles is often beneficial to the electrostatic separation. The mineral material may be comminuted in any of the conventional apparatus of the art including, without limitation, jaw crushers, cone Crushers, roll crushers, hammer mills, and the like.

It is essential both to `satisfactory charging of` themineral particles prior to introduction into the electrostatic fieldand `to commercially` acceptable separations in the electrostatic field under normal ,atmospheric conditions, that the particles be maintained thoroughly dry and that the particle surfaces be kept hot. In order to effect thc necessary drying of the mineral either before or after final comminuation, if comminuation is necessary, the mineral should be heated to dryness at a temperature of at least F. and maintained at a temperature of at least 150 F. during the charging and up to the point of introduction as fr ee falling bodies into the electro-` static field. Higher temperatures which do not deleteriously affect the mineral can be employed `'and in many instances are required in order to satisfactorily prepare the particle surfaces for optimum separations. In general temperatures of between about 200 F. andabout 500 F. produce the most desirable results. Obviously, the use of temperatures higher than those resulting in optimum beneficiation cannot be justified economically. Preferred upper temperature` ranges from some of the materials particularly amenable to electrostatic beneficiation are given below:

. F. Phosphatic ore or pebble 550 Sylvinite ore 550 Barite ore` 900 Fluorite ore i 550 Lithium bearing ores, such as lepidolite and spodumene 750 Pegmatite ore '(feldspa`r) 750 The heating and drying of the particles, in accordance with the method of the invention, must be conducted in a drying apparatus such that the surface of the particles do not become deleteriously affected by the mechanical treatment during the drying operation. Rotary kilns, uodriers, hot gas conveying-type driers, indirectly heated rotary driers, and the like, may be employed depending on the requirements of the particular mineral or ore being processed.

Charging, in accordance with the invention, is attained through the .medium of contact electrification. Contact electrification results from the movement of matter in response to s`uch stimuli as differences in escape rate of positiveor negative charges, or transfer of electrons or ions across an interface due to differences in energy levels and the like. It has been determined that real crystals never attain the. st'atic perfection of an ideal crystal lattice andl thata real crystal may have distorted surfaces, displaced ions Yor atoms,interstitial sites and surface sites, `and charge displacement due to separated anion-cation pairs of abnormal ionized atoms and trapped electrons. 1t is postulated that 'these traps are capable of acting as donors andacceptors of electrons and frequently it is these trap's that are probably the controlling iniiuences in contact electrification of minerals.

Particles of dissimilar materials, if `the surfaces thereof do not ,exhibit differential electrification upon subjection to contact electrificat'ion operations,` Asuch as agitation, often rcanbe `causedwto exhibit differential electrification `by thermal, chemical, or..electromagnetic methods. The differential charge ,may he4 acquired, for example, by` rupture of` an.electrical.doublelayen by mechanical means, as, for example', from interparticle contact andseparation or yby. transfer of electrons from a` Basically, `the desired Contact electrilication depends en 1 temperature, impurity content, and mechanical history of-ithezvarious surfaces involved.; kTherefora'it is "neces,- sary to determine the precise conditions requisite to .optie mum selective' charging. Under certain conditions the surfaces of the mineral species are such that it is possible to electrify by mineral-metal contact electriication. For example, if such contact causes high electrication with one mineral species and very low electriication with a second mineral species, a selective separation is possible. By way of specific example, quartz-metal contact electrification will result in a relatively high surface charge density on the quartz compared to the corresponding surface charge density on Florida phosphate after yphosphatemetalV Contact electrication. However, even with these materials it is more desirable to employ quartz-phosphate contact electrication since this charging mechanism results in a surface charge density of opposite polarity on the two mineral species to be separated. A more complete discussion of charging mechanisms may be found in Fraas et al., Industrial and Engineering Chemistry, vol. 32, pp. 601-602 (1940).

Proper charging of the particulate mineral is an especially critical part. of the present invention. It has been discovered that greatly improved dilerential charging of the particles is accomplished in ac-cordance with the invention by essentially particle-to-particle contact while the ydry comminuted material is maintained at a temperature -of atleast 150 F. Ideally, the particles would not contact a metal or grounded metal surface during the charging operation, since as previously indicated, contact with grounded metal surfaces usually causes all particles to accept a negative charge. On the other hand, where the charging of the particles is accomplished essentially by parti-cle-to-particle contact while at a temperature of at least 150 F., the surface charge density found on the mineral species in the ore is equal and opposite in sign. Accordingly, the charged particles move in opposite directions in an electrostatic field. T'hus, in the process of the invention it becomes possible effectively to separate non-conductive particles. The sign of the surface charge density to ybe expected in particle-to-particle contact electrication, depends on the probability of the particle making contact with surface A, B, C, etc. and the relation of the surface energies that control the sign of contact elec-trilication of the particles against A, B, C, etc. For example, if quartz contacts Florida phosphate rock at about 150 F.,cthe resulting surface charge density on the quartz-'will be negative while that on the phosphate rock will 'be equally positive. If quartz is allowed to contact-silica gel, the quartz will become charged positively and the silica gel equally negatively. Accordingly, the charge to be expected on quartz in a mixture of phosphate rock and silica gel depends on -the probability of contacting phosphate and the probability of contacting silica gel.

The desired particle-to-particle charging may be effected in numerous ways, such as by tumbling the particles down an elongated chute in such quantity that contact between the particles and the chute is at a minimum. Alternatively, the comminuted mineral, while maintained at the proper temperature, may be delivered from they drying apparatus to the electrostatic separator by means of a vibrating trough. At high throughput, the great preponderance of charging is engendered by particle-to-particle contact rather than *by contact of the particles with the trough. Suitable charging also may be obtained by air agitation of the hot, comminuted mineral. y

The beneiiciation of the mineral feed and `the separation of the mineral components is effected by passing the comminuted mineral as lfreely falling 'bodies through an external electrostatic field. It is essential to the satisfactory operation yof the process that lthe particulate mineral, when delivered to and dropped into the electrostatic field, be dry and hot in order 'to achieve commercially acceptable benecati-on. It is further essential that the charge on the mineral 'be lmaterially unaltered as ,it is delivered to and passes through the external electpostatic eld. Any corona discharge causing bombardment of the feed with ions orv electrons or any contact which materially will effect alteration of 4the charge on the individual particle as it is introduced into or passes through the external electrostatic lield, must essentially lbe avoided. Otherwise, selectivity of charge on the respectiveparticles resulting from the combination of steps preceding exposure to the external electric field will be deleteriously affected an-d the degree of Ybeneficiation of the material will be correspondingly reduced.

ln practicing the process of the invention, therefore, it is necessary to employ an apparatus which minimizes the possibility of altering the' previously acquired charge with corona discharge or of charging Iby inductive conduction such as is employed in the roll-type conductive electrostatic separa-tors such as the Johnson, Sutton and Carpco machines. It is desirable to employ either lla-t plates or relatively large rolls or cylinders as electrodes, which are specifically designed to minimize corona and to avoid metal contact in the presence of the external electrical field which results in inductive conduction and/or alteration of the charge on the particles.

The surfaces of the oppositely charged electrodes of the electrostatic separator desirably will be positioned or formed at an angle to the normal path of flow of the material if undiverted by the electrostatic forces. Such arrangement of electrodes is provided to make the angle of divergency as great as possible, thus permitting the separation of materials with dividers to be more readily accomplished. Although a variety of electrostatic appara- -tus may be employed in the process of the invention, it is preferred that lthe electrostatic field be create-d by one or more pairs rof spaced, oppositely charged electrodes, the lower portions of which curve outwardly from the perpendicular. The paired electrodes are desirably secured in place by members with smooth, convex surfaces.` Although the lield gradient may vary considerably, it has been found that field gradient of from about 6,000 to about 15,000 volts per inch is suicient for -most separations.

The process of the invention may be employed to beneliciate a wide variety of minerals and ores heretofore deemed unsuitable for commercially acceptable electrostatic separations. The process of the invention is especially adapted to beneficiation of ores or minerals, the liberated particles of which are substantially non-conductive. Such materials include, without limitati-on, phosphatic ore, pebblev phosphate, sylvinite, fluorspar, feldspar, barite, lepidolite, spodumene, kyanite, beryl, brucitemagnesite, wollastonite and cinnabar.

The beneficiation process of this invention is characterized by materially greater capacitythan the electrostatic separation procedures of the prior art. Previously, the highest capacities have been achieved with Carpco type machines in the separation of conductors from non-conductors, viz, feed rates of about 1 to about 1.5 tons per hour per lineal foot of electrode. With the process of this invention, however, an operating capacity up to about 10 tons per hour, per lineal foot of electrode may be obtained depending upon the particular material being treated. l

Since electrostatic separations depend on surface properties, it is imperative that the surfaces of the mineral species being differentially electrified be discrete. Accordingly, it is sometimes necessary to remove undesirable surface lms by known desliming operations or other known physical methods for preparing surfaces for treatment.

Having generally described the process of `the invention, more specific and detailed description will 'be given with reference to the accompanying drawings in which Figures 1 and 2, taken together, are llowA sheets illustrating the general application of the process ofthe' invention and in which Figure 3 shows a preferred type of electrode ar'.-

rangement for electrstatic separators' usable in the invention.u Y

Figsi 1 and 2, taken together, are flow-sheets Aof a complete process for beneficiatinlg silica-containing phosphatic ore. They are intended merely as one illustration and not as a limitation of the instant invention. Modification of the process illustrated by these dow-sheets, while employing the principles of they instant invention, willbe apparent to those familiar with electrostatic beneciation processes in general.

Referring to Fig. l, phosphate ore 11, as mined, 1s slurried with water and pumped into a washer 12 where phosphate pebble concentrate 13 of +14 mesh is separatedv from washer fines 14 of -14 mesh. The 14 mesh fines 14 are processed as shown in Fig. 2; line B of Fig. 1 being the `same as line B of Fig. 2. If the pebble is of highgrade, it need not be further processed and may be shipped as a product concentrate 15. If the pebble concentrate comprises essentialy low-grade pebble as shown at 16, it is subjected to a grinding process at 17 to substantially liberate the phosphatic values from the gangue comprising essentially quartz, feldspar and chert. The ground material is subjected to a screening operation `at 18, to separateparticlcs of` ,+24 mesh and the` +24 mesh material 19 is recycled to "the grinding process 17.` The -24 mesh fines 20 are subjected to an air classification at 21 to remove 200 meshl material 22 which may be discarded.` The coarse material 23 is again subjected to classification at 24, and the resulting fines (not shown) also may be combined with the previously obtained fines 22 and sent to discard. The coarse material 25 of 24 +200 mesh is placed in `storage or surge bin 26. (Although preferably 4removed and discarded, the 200 mesh material may be retained or may be subjected separately to electrostatic separation if desired.) From the storage bin, the material is passed into heater 27 which may take the form of a vertical type heat exchanger, a rotary kiln, a tunnel kiln, a multiple hearth furnace or other vsuitable heating device. It is preferred, though not essential, to avoid an` open Vfiameas a direct source of heat for the comminuted or'e to preclude an environment conducive to a concentration of mobile ions or electrons. The particles at a temperature lof atleast 150 F. andv preferably from 300 to 350 F., are passed from the heater 27 lin to charging unit 28. The 'charging unit may be heated to Asupply heat to the comminuted ore therein. However, it is preferred first to heat the ore as in heater 27 and then pass the hot ore into the charging unit. The ore must be at a temperature of at least 150 Fi during charging to i insure a satisfactory separation. The optimum charging temperature for commercial operations lies between about 200 `and about 225 F.

` The `charged ore is i then introduced as free falling 4bodies into a suitable `electrostatic separator 29 while at a` temperature of at leastuabout 150 F. and prefearbly slightly below 'the temperature to which the material was heated where the lower operable temperature limit will permit.` ,l

j AsaA result of the passage of theV material through the electrostatic field, a phosphatic concentrate 30 is separated which is saleable as product 31. A middling fraction 32 and a tailing fraction' 33 also are obtained. The middling fraction 32 is recycled tothe heater 27 yas shown by line 34, or alternatively to the grinding step at `17 as shown I by the dotted line 3S. The amount of middling fraction, a` continuously circulating load, is dependent upon the grade of feed material and the amount of. middling` that the operator desires to carry. Since the middling recycle circuit is closed, in starting the operation the middling circuit must be filled before equilibrium is reached between the amount of feed material introduced and theamount of product and tails removed. If the 'middling fraction is found to be within the required temperature range, and

-if the particles surfacesha've been maintained iii `a'fdry 8 condition, itmaj7 be resubjected to electrostatic separation at- 29 without repassage through the heater 27 orthe grinding step at 17, as shown by dotted line 36.

rlfhe tailing fraction 33 is subjected to a scavenger electrostatic separation 37 in whichtailings 38 and a phosphatic concentrate 39 are produced. The concentrate 39 is `conveyed tolheater 27 as shown by line 40 or alternatively to the grinding step at 17. The concentrate 39 is again passed through the charging unit 28, and again passed as free falling bodies through the electrostatic separator 29. The final tails 38 are sent to waste 41.

The `tailing concentrate 39 produced as shown in Figure 1 is normally produced of lower B. P. L. content than the primary concentrate 30. If it is desired further to upgrade the tail concentrate 39, it may be recycled to washer 2 to cleanse the surfaces and/ or to pulverizing or grinding operation 17. Alternatively, the tail concentrate 39 may be passed through a separate grinding stage and thereafter subjected to further electrostatic beneficiation.

Referring to Fig. 2, the washery fines 14,` whichare -14 mesh, are deslimed at 43 and material of` +325 mesh 44 is separated from material of 14 +325 mesh 45. The -14 +325 mesh material`45 is sent to a dryer 46 and then to the dryer storage bin 47. From the bin 47 the material is then passed through a heater 48, a charging unit 49 and dropped as free falling bodies into the elec trostatic separator 50. The fractions produced include a tail fraction 51, middling fraction 52, and concentrate 53 which constitutes a product 54 of the process. This product may be sent to a sizing operation 55 where a product 56 of +20 mesh size is separated from a product 57 of 20 mesh size. Each product is separately stored in

bins

58 and 59, respectively.

Alternatively, as shown by dotted line 60, the -14 +325 mesh material may be sized into +20 mesh material 61 and -20 +325 mesh material 62 and stored in coarse and

fine storage bins

63 and 64, respectively. Materials from bins 63and 64 may be separately processed by 'separately passing them through line 65, as described.

The middling fraction 52 is returned to heater 48 by means of line 6 6, as in the case of the process shown in Figure 1. Similarly, if the temperature of the middling fraction is within the desired range and the particle surfaces are dry, it may be directly passed as free falling bodies through the electrostatic separator as shown by dotted line 67.

`The tails 51 are subjected `to a scavenger electrostatic separation at 68 and the concentrate 69 is returned to the heater 48, by means of line 70, with the nal tails 71 going to waste 72.

The -325 mesh material 44, instead of being sent to waste 73, may, as shown by dotted line 73, be sent to a spray dryer 77 and then to storage 75 from which it may be conveyed,` as shown by dotted line 76, to heater 48, and raised to the proper temperature for electrostatic beneficiation, charged by particleto-particle contact, and dropped through the electrostatic field as in the case of the coarser sized material. However, the further processing of the -325 mesh material is not commercially attractive and it is preferred to discard this fraction and prevent any'deleterious effect from the presence of such finely divided and dusty material in the system.

The processes as shown in Figs. 1 and 2 may be modilied in a number of ways depending upon the character of the feed and the` B. P. L. content desired in each of the products. Feed material, for example, may be introduced `into the system at the heater 27, as shown in F1g. 1, and after heating,.be subjected to the grinding operation shown at 17 and sizing operations shown at 18, 21 and 24 of Fig. 1. Two stages of electrostatic separation are shown in Figs. 1 nand 2. A thirdy stage may be added 'to afford means for further removal of the high silica `fraction to thus produce` a lower B. P. L; tail having a B. P."L.Acvontent in the range of l5 to 30%. Thus, the aforedescribed process'il'lustratesone-method by which phosphatic ore maybe completely processed dry without the use of costly or corrosive reagents.

-In the modified form of electrostatic separator shown in Fig. 3,

electrodes

100 and 101 are disposed in spaced relationship on opposite sides of electrode 102. Electrode 102 is a straight member formed by a flat, smoothsurfaced platel and is preferably of the permeable type, i.-e., an electrode composed of spaced, vertical elements at .the same potential and having spaced openings therebetween. The member 102 thus furnishes a common, oppositely charged electrode for

electrodes

100 and 101 and provides two parallel electrostatic fields at a considerable saving in space.

Electrodes 100 and '101 are parallel fto electrode 102 .throughout most of ltheir length but are curved smoothly outwardly from the central electrode 102 adjacent the lower extremities. In the preferred ten foot electrode, the curvature preferably begins at a point about four feet from the lower ends of the electrodes .and sweeps smoothly outwardly on a radius of between about ten and about fifteen feet. v

Feed hopper 103 is centrally disposed above

electrodes

100 and 102 and 4delivers differentially charged feed material to the electrostatic field therebetween. Feed hopper 104 is similarly 4disposed above

electrodes

101 and 102 for delivery of ldifferentially charged feed material to the second electrostatic field between electrodes 101 and-102.

Hoppers

105, 106 `and 107 are ldisposed beneath the

electrodes

100, 101 and 102 for collection of separated fractions. Adjustable-angle `

dividers

108 and 109 are provide-d between

hoppers

105 and 106 and 106 and 107, 'as shown.

In the apparatus as shown, one fraction is collected in

hoppers

105 and 107 while a second fraction is collected in hopper 106. may be employed if desired to reco-vera middling fraction under each electrostatic field.

The rfollowing examples are given to illustrate specific applications of the instant novel process and are not to be construed .as limiting the invention thereto.

EXAMPLE 4I Enough low-grade pebble of a B. P. L. content of between about 59.5 and 63.5% B. P. L., was comminuted and air sized to produce 1400 pounds of an electrostatic feed material of -35 +325 mesh. This material washeated to a temperature of between about 225 and about 250 F. About 200 pounds of the hot material was placed in a grounded galvanized iron hopper. By means of a vibrating feed trough, the heated feed material was differentially -charged essentially by particle-toparticle contact and fed from the hopper to fall freely through an electrostatic field created by a series `of eight pairs of stationary, vertically arranged electrodes having smooth, curved surfaces opposed and providing an electrical field having an essentially uniform intensity of about 10,000 volts per inch.

The feed material was dropped through the electrostatic eldat `a rate of about 200 pounds per hour. The hopper was continuously filled with material from the heater. Three fractions were separated; a phosphate concentrate, a middling fraction, and a tailing fraction. The middling fraction was continuously recycled to the heater by means of a bucket elevator and chute, and the phosphate andtailing fractions ywere separately collected. The process wasV run forv about five hours until vthegconcentrate and tailings, totaling about 1000 pounds, wereob-V tained. T-he results are indicated in Table A.

Table .4

Feed material Low-grade phosphate pebble. Rate of; feeding 200 pounds per hour. Time ofV operation 5 hours. 'i Pounds feed material 1,000.

It is apparent that additional hoppersY Pounds -of concentrate 833."y 1 Pounds of .tails 165. f B. P. L. feed 62.5%. y B.V P. L. concentrate 72.2% (3.0% insoluble). B. P. L. tails. 15.0%. B. P. L. recovery 96.0%.

EXAMPLE I-I p lAbout 1400 pounds of unreagentized deslimed Florida pebble phosphate ore Iwasher debris of 14+35 mesh -was subjected `to an electrostatic eld created by the same ap paratus as indicated in Example I and under substantially the same conditions as indicated in Example I. The results are indicated in Table B.

Table B Feed material 14+35 phosphate ore. v Rate of feeding 200 pounds per h-our. Time of operation 5 hours. Feed material 1,000. Pounds of concentrate 722. Pounds of tails 278. B. P. L. feed -.7 57.2%. Y B. P. L. concentrate 77.0% (2.9% insoluble). B. P, L. tails 5.8%. B. P. L. recovery 97.3%.

EXAMPLE III Florida pebble phosphate of -/s +1 lmm. size, and havin-g a B. P. L. content of approximately 70.3%, was comminuted and sized to produce feed for electrostatic separation of +14 +35 mesh. This feed material is the undersized material for treatment in accordance with the iiow-sheet illustrated in Figure 1.' The +14 mesh material was recycled to the hammer mill. The undersized material was heated to a temperature of approximately 350 F. in a heat exchanger of the cascade type. Hot material from the heat exchanger was delivered to a vibrating trough feeder which delivered the material .as a comparatively thin ribbon between .the electrodes of a first electrostatic separation stage. At the time of drop through the electrodes, the temperature of the material was in -the range between about 300 F. and about 325 F. The material was fed between the electrodes at -a rate of approximately one ton per hour per lineal foot of electrode length. The electrodes were of the type described in Figure 3 yand were approximately 10l in vertical dimension and about 10 long and had impressed thereon, a potential difference of approximately 40,000 volts giving a field gradient of approximately 4,000 volts per inch of distance separating Vthe electrodes. Y

In this first electrostatic separation, a number 1 concentrate was produced of 75.3% 'B. P. L., 2.5% insolubles, and the concentrate constituted approximately k37% by weight of the feed. The bala-nce of .the feed was fed directly to a second stage electrostatic separation of substantially identical design `and field strength, but adapted to recover three products. The midfdling fraction recovered from the second separation stage had approximately 57.9% B. P. L., 22.9% insolubles, and constituted approximately 17.3% by weight of the original feed. This middling fraction was recycled to the comminution operation. The tail fraction recovered assayed about 30% B. P. L., about 60.2% insolubles, and constituted approximately 10.76% by weight of the original feed. The concentrate fraction had 74% B. P. L., 4.2% insolubles, and about 35.1% of the weight of the original feed. The combined concentrates constituted a recovery of 80.7% of `the total B. P. L., the middling accounted for approximately 14.9% of the B. P. L., and ythe tail .fraction accounted for approximately 4.5% of the total B. P. L. l

EXAMPLE IV 4Fluorite-quartz ore analyzing `approximately 53.4%

11 CaFz was ground and sizedto recover a 120 +150 mesh standard screen size fraction.

This sized material was heated to approximately 350 F. in an electrically heated oven. The heated material was transferred to the hopper of an ironrchut'e vibrated by an electrical vibrator. Material ran through the chute to give a depth of about one-eighth inch at the discharge lip. p

The contacted materialwhen passing into the electrostatic held was at a temperature ofapproximately 160 F. The `feed Vrat'e` ivas approximately 1.1 tons/ hour/ linear foot of electrode width. The field gradient of the electrostatic field` Was 9,000volts per inch of` distance separating the electrodes. The -electrodes were l feet longt The results obtained utilizing `a single pass through the electrostatic Vfield `are indicated in Table C.

Table C Isereent CBF;

Percent Weight Material EXAMPLE v A barite-fluorite ore upon analysis showed 38% Abarite, 58%` fluorite, 4% quartz, 7-8% clay andothergangue material. The ore was crushed and screened to recover a `l4 +200 fraction. This fraction was deslirned by slurrying in water at 75% solids content.

The. deslimed and washed ore fraction was dried and heated to approximately 500V F. in an electrically heated oven. The heated material was transferred to `the hopper of a Syntron feeder grounded to the earth by an electrical conductor. Material ran through this `chute to give a depth of about 1A inch at the discharge lip. t

The contacted material while within the temperature range of between about 250 F. and about 300 F. was fed through the electrostatic field at a rate of approximately l ton/hour/linear foot of` electrode width, the field gradient being 9g000 volts per inch of distance separating the electrodes.

Table Percent BaS O4 Percent Weight Material iee d Miaaiing. 9 Concentrate 42.

EXAMPLE v1 range of between 500 F. and 400 F. was fed to the electrostatic eld `at a rate of approximately one ton 'per hour per linear foot of electrode width, the yfield gradient being 9,000 volts per inch of distance separating the electrodes.

electrostatic held are indicated in Table i he results, obtained utilizing a single pass throughthc L HK5 is substantially beneficiated with respect to LizO by this process.

EXAMPLE VII `Lepid'olite ore containing about 45%` feldspar, 30% vquartz and 24% micas and assaying approximately 1.02% Liz'O was cone crushed and sized on a Sweco screen to recover a -f35 +325 mesh feed material. This sized material was heated to approximately 500 F. in an electrically heated oven. The heated material was trans# ferred to' the hopper of a Syntron feeder having a cast iron chute grounded to the earth by an electrical con- 'duct'on 1 Material ran through this chute to give a depth of about 1A; `inch at the discharge lip.

The contacted sample while within the temperature range ofbetween 500 F. and 400 F. was fed `to the electrostatic field ata rate of approximately one ton/ hour/ linear foot of electrode width, the field gradient being 9,000 volts per inch of distance separating the electrodes.

The results obtained utilizing a single pass `through the electrostatic field are indicated in Table F.

Inspection of Table F shows clearly that the lepidolite is substantially benefici-ated with respect to Liz() by this process.

EXAMPLE VIII Lepidolite ore containing about 30% feidspars, 45% quartz, about 20% mica and assaying approximately 1.42% `LizO was cone crushed and sized on a Sweco screen to recover a -35 +325 mesh feed material. This sized material was heated to approximately 500"` F. in a'n electrically heated oven. The heated material was transferred to' the hopper of a Syntron feeder having a cast iron chute to' give a depth of about 1A; inch at the discharge lip.

The contacted sample while within the temperature range of between 500 F. and` 400 F. was fed to the electrostatic field at a rate of approximately one ton/ hour/linear foot of electrode width, the field gradient being 9,000 volts per inch of distance separating the electrodes.

The results obtained utilizing a single pass through the electrostatic field are indicated in Table G.

Y, Baritefluorite analyzing 21.9% F, 35.4% CaO, and 24.7% BaO was wet screened on a Sweco screen to` re-` cover a -35 +325r mesh feed material. This sized material was heated to approximately 500 F. in an elec-u trically heated oven. The heated material was trans 13 ferred to the hopper of Syntron feeder having a'cast. iron chute grounded to the earth by an electrical conductor. Material ran through this chute to give a depth of about As inch at the discharge lip.

The contacted sample while within the temperature range of between 500 F. and 400 F. was fed through the electrostatic field at a rate of approximately one ton/ hour/ linear foot of electrode width, the eld gradient being 9,000 volts per inch of'distance separating the electrodes. i

Table H Material Percent Percent Percent Weight BaS O4 CaFZ ooncentrauon se. 6 7s. 4 7. 4 MiddllJlgS. 7. v 50. 6 35. 6 Tails- 55. 9 4. 7 74. 0

Inspection of Table H shows clearly that the bariteuorite is'substantially beneficiated with respect to BaSO4 by this process. f

l EXAMPLE X..

A barite-'bearing ore from South Carolina upon analysis showed a composition of approximately 17% barite, 51% quartz and about 30% vsericite. The ore was crushedand screened to produce a feed 100% of which passed 30 mesh standardY screen. This minus 30 mesh materialjwas deslimed by slurrying in water at 75% solids content and a feedY produced whose particles were of a size -30 +200 mesh.

The 3.0- +200 mesh was dried and heated to approximately 5 00 F. in an electrically heated oven. The heated material was transferred to the hopper of a Syntron feeder grounded to the earth by an electrical conductor.l Material ran through this chute to give a depth of about 1A; inchy at the discharge lip.

The contacted material while within the temperature range of between about 250 F. and 300 F. was fed through the foot of electrode width, the field gradient being 9,000 volts per inch of distance separating the electrodes.

Fractions obtained in `a rougher and a cleaner pass through the electrostatic eld were as follows:

Table I Percent BaSO4 Percent Material Weight A barite-tluorite ore upon analysis showed 38% barite, 58% uorite, 4% quartz, 7-8% clay and other gangue material. The ore was crushed and screened to recover a 14 |200 mesh. This fraction was deslimed by slurrying in water at 75 solids content.

The deslimed and washed ore fraction was dried and heated to approximately 500 F. in an electrically heated oven. The heated material was transferred to the hopper of a Syntron feeder grounded to the earth by an electrical conductor. Material ran through this chute to give a depth of about Ms inch at the discharge lip.

The contacted material while within the temperature range of between about 250 F. and about 300 F. was fed through the electrostatic field at a rate of approximately one ton/hour/ linear foot of electrode width, the field gradient being 9,000 volts per inch of distance separating the electrodes.

The results obtained utilizing a single pass through the electrostatic field are indicated in Table J.

The generic concept of this invention was first disclosed and exemplified in Lawver application Serial No. 142,982, filed February 8, V1950, which presented examples illustrating the process of the invention. This disclosure was' perpetuated'in Lawver application Serial No. 312,730,

iiled October 2, 1952,*as a continuation-in-part of application Serial No. 142,982, subsequently abandoned. The disclosure of the invention was amplified and further illustrated in application Serial No. 646,189, filed yMarch 12, y1957, as a continuation-in-part of now abandoned application Serial No. 312,730. This application is a continuation-in-part of application Serial No. 646,189, filed March 12, 1957, and, inter alia, amplifies the disclosure of that application by the exemplication of additional species of the invention and by more detailed elucidation of the technical phenomena involved.

What is claimed is: f

1. An electrostatic process for beneciating a nonmetallic mineral which comprises heating the mineral to a temperature'of atleast 150 F. while in a state of subdivision suficient to substantially completely liberate the desired'components from the gangue; causing the mineral particles without chemical pretreatment, in the absence of an external electrical field and while at a temperature of at least F., to accept differential charges; and thereafter subjecting the differentially charged particles as freel falling bodies and while at a temperature of at least 150 F. to an electrostatic field to beneficiate the mineral without substantially altering the charge on the particles while in the electric field. i

2. An electrostatic process for beneficiating 'a nonmetallic mineral which comprises heating the mineral to a temperature of at least 150 F. while in a state of subdivision sufficient to substantially completely liberate the desired components from the gangue; causingthe mineral particles without chemical pretreatment, in the absence of an external electrical field and while at a temperaturepof at least 150 F., to accept differential charges essentially by particle-to-particle contact; and thereafter subjecting the differentially charged particles as free falling bodies and while at a temperature of at least 150 F. to an electrostatic field to beneficiato the minerai Without substantially altering the charge on the particles while in the electric field.

3. An electrostatic process for beneficiating a nonmetallic mineral which comprises heating the mineral to a temperature of at least 150 F. while in a particle size of between about 14 and about 200 mesh; causing the mineral particles without chemical pretreatment, in the absence of an external electrical field and While at a temperature of at least 150 F., to accept differential charges; and thereafter subjecting the differentially charged particles as free falling bodies and while at a temperature of at least 150 F. to an electrostatic field to beneficiate the mineral without substantially altering the charge on the particles while in the electric field.

4. An electrostatic process for beneiiciating a nonmetallic mineral which comprises heating the mineral to a temperature of at least 150 F. while in a particle size of between about 14 and about 200 mesh; causing the mineral particles without chemical pretreatment, in the absence of an external electrical eld and while at a temperature of at least 150 F., to accept differential charges essentially by particle-to-particle contact; and thereafter subjecting the differentially charged particles as free falling bodiesand while at a temperature of at least 150 F; to an electrostatic eld to beneficiate the 5. An electrostatic process formbeneiiciating a non` metallic mineral which comprises comminuting the mineral to a particle size of between about 14 and about 200 mesh to substantially completely liberate the value Vclomponents from the gangue; heating the comminuted mineral to a temperature of at least 150 F.; causing the mineral particles without chemical pretreatment, in the absence of an external electricalfield and while at a temperature of at least150 F., to accept diterential charges; and thereafter subjecting the `differentially charged par,- ticles as free falling bodies and while at a temperature `of at least 150 F. to an electrostatic field to beneiciate the mineralwithout substantially altering the charge on the particles while in the electric field.

6. An electrostatic process` for beneiiciating` a nonmetallic, substantially non-conductive mineral `which comprises comminuting the mineral toa particle size of between about 14 and about 200 mesh to substantially completely liberate the value components from theV gangue; heating thecomminuted mineral to fa temperature of vat least 150 F., causing the mineral particles without chemical pretreatment, in the absence of an external electrical field and while at a Vtemperature of` at least `150 F., to accept differential charges essentially by particle-topar ticle contact; and thereafter subjecting the diierentially charged particles as free falling bodies and while at a temperature of at least 150 F. to an electrostatic field to beneficiate the mineral without substantially altering the charge on the particles whilein the electric eld.

7. An electrostatic beneciation process according to claim 6 in which the mineral treated is phosphatic ore.

8. An electrostatic beneciation process according to claim 6 in which the mineral treated is iluorite ore.

9. An electrostatic beneficiation process according to claim 6 in whichthe minerall treated is lithium ore.-

l0. An electrostatic beneficiation process according to claim 6 in which the mineral treated is b arite ore.

l1. An electrostatic beneciation process according to claim 6 in whichthe mineral treated is pegmatite ore.`

l2. An electrostatic process for beneficiating silicacontaining phosphate pebble ore which comprises comminuting said ore to apparticle size range of about -14 mesh to about +200 mesh, heating the, comminuted ore to a temperature of from about 150 F, to about 550" F., causing the particles of the heated comminuted ore while at a temperature of at least about 150 F. to accept ditferf ential charges, subjecting the diifer'entially charged parcharge on the ore particles while said particles are in the electrostatic tield.

13. An electrostatic process for beneficiating a rionmetallic, substantially non-conductive mineral which comprises heating `said mineral in a state of subdivision adequate` to effect substantially complete liberation of the desired mineral components` from the ga'ngue to atemperature of at least about 150` F.; causing the particles of said mineral, without chemical` pretreatment, in the absence of an external electrical field and while at a tem.- perature of at least about F. to accept differential charges bycontact with a donor element, andlsubjecting the differentially charged particles as free falling ,bodies and while at a temperature oftat least 150 F. to beneficiate said mineral without material alteration of the charge` on themineralY particles while said particles are in said electrostatic field.

References Cited ,in the le of this patent UNITED STATES PATENTS 959,646 Swart May 31, 1910 1,110,896 Comstock Sept. 15, 1,914 1,422,026 Brown July 4, 1922 2,135,716 Johnson Nov. 8, V1938 2,168,681 OBrien Aug. 8, 1939 2,197,865 Johnson Apr. 23, 19,40 2,198,972 Peddrick et al. Apr. 30, 1940 2,246,253 Johnson June 17 1941 2,357,658 Johnson Sept. 5, 1944 FOREIGN PATENTS 598,948 Germany June 21, 1934 OTHER REFERNcEs Milling lMethods, American InstituteV of Mining and Metallurgical Engineers, volume 134, 1939, pages 411 and 412.

Taggart: Handbook of Mineral Dressing, (1945), chapter 13, pages 45-46'.

Gaudin: Principles of Mineral Dressing, (1939), page 465. l l l l Fraas et al; Electrostatic separations of Solids', Industrial and Engineering Chemistry, volume 32, No. 5, 600- 605, May 1940. l

Fraas et al.: Contact Potential in Electrostatic Separa tion, Bureau of Mines, R..I. 3667, November 1942;`

Fraas et al.: An Electrostatic Separatorfor Fine` Powders, `Bureau of Mines, R. I. `3667, November 1942.

Fraas et al: The Electrostatic Separation of Several Industrial Minerals, AIMME, Technical Publication No. 2408, July 1948.

Fraas et al.: Notes on Drying for Electrostatic Separation of Particles, American Institute of Miningand Metallurgical Engineers, `Technical Publication No. 2257.` November 1947.A`

Von Szanthe: Electrostatic Beneciation in the Industry of Rocks and Earths', Tomindustrie-Etg und Keramische Rundschau, volume 77, 83-88 (1953).

Claims

6. AN ELECTROSTATIC PROCESS FOR BENEFICIATING A NONMETALLIC, SUBSTANTIALLY NON-CONDUCTIVE MINERAL WHICH COMPRISES COMMINUTING THE MINERAL TO A PARTICLE SIZE OF BETWEEN ABOUT 14 AND ABOUT 200 MESH TO SUBSTANTIALLY COMPLETELY LIBERATE THE VALUE COMPONENTS FROM THE GANGUE; HEATING THE COMMINUTED MINERAL TO A TEMPERATURE OF AT LEAST 150*F., CAUSING THE MINERAL PARTICLES WITHOUT CHEMICAL PRETREATMENT, IN THE ABSENCE OF AN EXTERNAL ELECTRICAL FIELD AND WHILE AT A TEMPERATURE OF AT LEAST 150*F., TO ACCEPT DIFFERENTIAL CHARGES ESSENTIALLY BY PARTICLE-TO-PARTICLE CONTACT; AND THEREAFTER SUBJECTING THE DIFFERENTIALLY CHARGED PARTICLES AS FREE FALLING BODIES AND WHILE AT A TEMPERATURE OF AT LEAST 150*F. TO AN ELECTROSTATIC FIELD TO BENEFICIATE THE MINERAL WITHOUT SUBSTANTIALLY ALTERING THE CHARGE ON THE PARTICLES WHILE IN THE ELECTRIC FIELD.