US2441584A - Closed circuit grinding with twostage classification - Google Patents

Description

May 18, 1948. H. MQNEILL CLOSED CIRCUIT GRINDING WITH TWO-STAGE CLASSIFICATION Filed June 20, 1942 NEW F550 INVENTOR. HARRY L. M /VEILL 359 55 r k 4 m A wk xfikhuko .m mum wow 8 s m M T m m u Patented May 18, 1948 CLOSEDfClRQUIT'iGRINDING WITH-TWO- STAGE ICIIASSIFIGA-TION Harry;L.,McNeill, Denven Colmlip lication lunezo, 1942; SerialNo; 44mm 3 Claims.

This-invention relatesgto ore. treatments and more-partixiulairlytd improvements in ore dress? ing-i'lmethodsz In theearly daysofpur-metal"production, the valuable 'constitnents' were recoveredjfrom' high grade ores by; gravitymethods Tbefore grindin millshad reached a. commercial: development;

Batera's the ores became progressively lower in grade, grinding was employed to unlock'mineral-"of a'" quality formerly discardedfromthe treatment'as .a tailing. In' such operations a concentration stage was provided in the vtreatment following each reduction in' particle; size in the materialnnder treatment. 7 s

In-such-an operation manymachines .Were employednn relation to'the quantity ofiore treated andl'recoveriesseldom averaged; over 75%. Then with the advent'of froth flotation, fine grinding was. employed to unlock mineral particles of fthe type formerly lost in the tailin'gs' andrecoveries improved progressively until. today, most, l plant recover"90%'jor overof the mineral;

Likewise under modern practice mostvplants only; provide one stage in the circuit'where concentrates; areremoved; Ajlarge part ofithe min,- eral which initially is-freed in sizes too largevto float; consequently is retained in the grinding cire cuit-g andasaresu-ltinstead of having, one-1arge Darticlepermitting zit-100%" recovery; it is passed througnthe mill-repeatedlyuntilfine enough to overflowthe -classifier; and'in' the transformation becomes-thousandsof-smaller particles, someextremely fine, thereby rendering a l'0'0-%'- recovery an impossibility: I I

Experience. thEsWOl-ld over has shown that-the majonlossesiin thestailingsioccur in the: minus 200 meslrrfractiona Consequently aisubstantialeliminatiom oft-production ZOfgDElMiiOl-GS; imthe-zooimesh ontfinert ranges would appeanto vbe -onerneans of reducing tailings lossestl I Modern. ,practicet alsothasdeleterious-results in connection ,lwith- :the treatment ofitheiganglle-l or wastei contente Thusaganguetparticle. contain? ing no mineral and too, coarse to fioatis subjected tolrepetitions ofithetgrinding action until it, too, is'redu'ced'tota' state of fineness sufli cient to pass the classifieroverfiow':

Since the grinding stageisthe most-costly item in the operatiom of concentrating m-ills; it is apparent-=- that all eft'ort directed at areduction" of eral content of ores inlarger-and fewer particles than: haslbeen possiblewith methods heretofore Anotherobject of theinvention is to provide a simple, economicaljand eflicient; method of grindingthelmiddlingscontent'of an'ore in closed circuit.

A further object ofithe invention is to substantially eliminatesIimecondition's in the material under; treatment-in; an operation utilizing closed circuit; grinding sofrthemiddlings content of-an ore-so as to increaseu rinding. efficiency.

other-objects-ureside in the provision of: novel stepsv anditreatments all' of which will appear more fully iILthEzCO'llI'SGfOf the followingdescription-.-

The -presentrinvention has. been designed for use imawidevariety. of treatments and; in fact, is generally applicable wherever froth flotation has been: employed-to concentrate: one or" more constituent mineralsiof a given ore. Likewise the invention is beneficial :bothin treatments where a; single mineral islseparatedf from gangue or waste constituents and other treatments whereintwo or moreaminerals; are differentiallyseparated ands recovered-as concentrates. separate from the gangue constituents.

In= practicingthe-present invention, the classification stage is utilizedfto obtain a separation of constituent of theore as well as to grade thematerial according'to siZeS-.:. In so doing, it is possible toutilize a: seriesf "differential separations at eaclr 01% which: alvaluabie constituent is recovered separately-from waste matter or other matter .requiring'ladditional' treatmentbefore separation of its mineralcontent can be effected.v

- Before undertaking: a; complete description of the operating'procedure, it'will 'be advisable to define: certain-terms employed: in the specification, "to distinguish .from' other meanings sometimes applied. Wherethe expressions finished ,product 'ors"finished materialtare used, theylare intendew to designate; a-Mproduct groundfine enouglr-to overflow; present type classifiers and pass to subsequent treatment; The term gangue as-used in the. specification is intended to designate any-waste productwhich is barren of mineral.

To afford a better understanding of. the practice of the invention, reference-"is made tothe accompanyingdrawings, in which Fig. 1 is a fiow sheet in-whichatypical milling circuit has been illustr'atediembodying:the; features of the pres-v ent invention; and, Fig: 2iis -asection'taken'along thedine 2- -32; Fig: 1':

3 In the circuit illustrated, the grinding circuit in addition to the classification stage l and grinding stage 4, has concentrating stages 2 and 3 respectively, the details of which will be fully set fication operation which may consist of a single,

duplex classifier or a series of individual classifiers arranged as hereinafter described.

The classification stage of the present invention is utilized to produce three products, namely, (a) finished product, (b) a mixture of coarse fr e mineral with la er middlin 5 articles, and

e rg g p tice without reducing the capacity of the circuit (a) a mixture of fine middlings with coarse angue.

While a variety of apparatus may be utilized to produce the desired products at the classification stage, it will be essentially the same in function as apparatus of the type disclosed in my co-pending application Serial No. 289,466 now Patent No. 2,315,536, granted April 6, 1943, which is particularly suited for this purpose. Features disclosed but not claimed herein relating to the classification step per se, have been described and claimed in the aforesaid application.

The ball mill discharge constituting the classifier feed enters the tank I at the side through a feed box l2, the discharge of which passes through an opening 13, and through the provision of hindered settling means here illustrated as spigot compartments la and Im, the portion of said feed constituting finished material is caused to pass directly across the classifier to its overfiow lb the discharge of which passes through an opening l4. At the same time the hindered settling action is utilized to induce a differential settling of the other feed constituents, with the result that coarse free mineral mixed with still larger sized middlings particles are settled in one zone, while finer middlings particles together with coarser gangue constituents are settled in a secand zone. This hindered settling action may be produced either by introducing gas under pressure through a suitable valve-controlled conduit as shown at It in compartment l a, or water under pressure through a suitable valve-controlled conduit as shown at H in compartment Ir.

Each of the graded settled products is separately removed from the classification stage by suitable conveyor mechanism |5a or I511, with each product after removal from the unitary hydraulic classification treatment stage being subjected to a separation with only one of the products so separated returning to the mill for regrinding.

To this end, the mixture of coarse .free mineral and larger middlings particles preferably is subjected to a jigging treatment as indicated at 2, after which the doarse middling so separated is mixed with the entering feed to the mill 4, while the coarse mineral concentrate is collected as a product for smelting.

Likewise, the rake product of classification comprising fine middlings mixed with coarse gangue preferably is subjected to separation by froth flotation as indicated at 3, with the froth concentrate comprising substantially all of the mineral content being mixed with the entering feed to the mill for further unlocking, while the residue product comprising essentially coarse gangues is passed to waste.

Thus, the closed circuit grinding of the present invention difiers essentially from prior practice in that substantially all finished product, coarse free mineral and barren gangue are eliminated in one pass through the classifier while the product returned to the mill for regrinding consists almost entirely of middlings requiring further unlocking before separation of constituents can be attained,

By so doing, the useful work of the grinding mill is limited to an initial reduction of oversize and further unlocking of mineral-bearing material only. The elimination of finished product in the first pass through the classifier prevents sliming to any appreciable degree.

As a consequence of these economies in operat'ion,.the capacity of the grinding mill may be reduced from that required in conventional pracand in this way conomies in initial cost and operating cost are efi'ected.

The product overflowing the classifier at lb is then taken as the feed'of the flotation circuit, here represented as a plurality of rougher cleaner cells. The concentrate of this operation is then passed to a thickener 6, and the collected solids of the thickening operation are filtered as shown at l and are then ready to go to a smelter 8 which may or may not be associated with the milling operation.

The concentrate of the jig 2 also is taken to smelter 8 and where shipment is necessary to get these concentrated minerals from the mill to the smelter, they may be mixed for convenience in transportation.

The elimination of slimes from the feed to flotation has beneficial eifects of a pronounced character. In the first place, a granular flotation concentrate requires smaller thickener and filter areas. Likewise, a granular filter cake contains less moisture than a slimy one, and where such products must be shipped to a smelter, a substantial savings in shipping expense results.

Furthermore, a granular concentrate has a much lower dust loss in the smelting operation and consequently a further economic gain results from the treatment of the present invention.

From the foregoin description of the practice of the present invention, it will be understood that the purpose of the initial separations is to free the product recirculating through the grinding mill of substantially all its barren gangue content, and all its free mineral, irrespective of size.

To this end, the finished product containing both free mineral and gangue, in sizes fine enough to pass the classifier overflow, is discharged from the grinding circuit without re circulation and is then in ideal condition for the subsequent flotation separation as the slime formation factor has been eliminated.

Likewise, the graded settling of the oversize products permits isolation of the middling by screen or gravity separation methods. Consequently, the material returning for regrinding is almost exclusively mineral-bearing and. for the most part Of a size permitting further reduction without producing slimes.

For example, in the operation a given middlings product of the mill may be substantially oversized and comprise gangue and 10% mineral. When returned and further reduced, it is split into one oversize piece of gangue, and two smaller size pieces, one being classed as coarse middling and the other fine eeeeigcsa risiddlrnga Maltese-array co-nrprls'efirouglily m /s mian Wamiii eral; 1 I I At this stage -the 'speclfie grayitiesiofi these iniddiihgsi coastituents readilyi identifies them ir'omitlia gangue and from tlie-mineral.

This condition permits making the-' tail ing at a substantiali' oarserhrindithamis possiblein I :cticel With tlie"process"I -ha-ye developed it is possible; to isol'ate themiddli-ng atethe screen size at which tlrel r'a-ti'c'i off. mineral to gangue in a particle gives it a specific gravity whicir: idritifiesiit -from' the free minerai -i on one rhanugandatheiganguezon tlierctherx While jigging has been desfgztratc. as; pres ferried: treatment: for: separatiorrl .of coarsea free mineral and larger sized middlings, it willibe understood that screening or other forms of gravity separation may be substituted where desired.

Similarly, froth flotation has been designated as the preferred treatment for collecting the fine middlings separately from the associated gangue, but screening or other forms of gravity separation also may be substituted in this stage as well.

As previously suggested herein, the preferred structural arrangement at the classification stage is a duplex-type classifier having a transverse trough or channel for the direct movement of the finished material through the classification stage and out of the treatment. To insure prompt elimination of substantially all finished material, the classifier feed preferably is subjected to hindered settling influences but other forms of treatment apparatus capable of attaining the same results may be substituted in place of the classifier hereinbefore described.

As the aforesaid treatment is not dependent upon chemical change or reaction except to the extent that froth flotation or leaching is recommended, it will be understood that all ores containing mineral which can be recovered on a commercial scale by ore dressing methods may be treated efiectively by the methods of the present invention.

It is to be understood further that the invention is in no sense limited to the arrangement of treatment steps set forth in the accompanying flow sheet which is intended merely as a typical example, but rather embodies as essentials the elimination of substantially all ore constituents other than middlings in the product returned to the grinding circuit.

In the middlings flotation treatment the product treated is of relatively coarse mesh size and a large amount of reagent is required to collect all of the particles of varying mineral content in the froth concentrate. However, with slimes eliminated from the feed as described, the operation in this treatment becomes quite efiicient, whereasif the slimes were permitted to remain in the feed the reagent would carry up all fines, both middlings and gangue, resulting in further sliming and wasteful grinding in the mill.

For this reason, flotation is recommended for the treatment of the mixture of fine middlings and coarse gangue. But as pointed out hereinbefore, a variety of treatments may be employed at this and other stages within the spirit and scope of the invention as defined in the hereunto appended claims.

More specifically, certain operations will involve leaching operations for the recovery of mineral in place of the froth flotation concentration 'hereinbefore described. The present process may utilize leaching ateeitheriofithestagesdesignated 3 or Ssinthe fiow sheet.

However best results in leaching operations will? be attained when the second commitment o.2the-.classi fier is used f61 .Sa|IldS=-S1im-. separationsaridpacli product of tliislseparation thereafter is subjected to a difirent type of'leaching.

In such an operation, the=slimes or finished materialswill be passed from the-igrinding stageeto leaching with agitationatqaastage actingas an alternate forthe. flotation stage 5 of the fiow sheet. The sandsso' separatewwill'pass to a still eechm goperatiomservinaasanalternatez-rorzthe sta e: 31: illustrated. Otherwise the treatment steps will be substantially asz hereinberore: des

cribed.

What I claim and desire tov secure by Letters Patent is:

1. In an ore dressing circuit, the steps of grinding an ore in a mill to produce a product containing some coarse free mineral, passing the grinding mill discharge containing granular material with only a minor quantity of slimes sizes directly into a two-stage classification separation, inclusive of a hindered settling treatment in which finished material, inclusive of slimes, is

maintained at or near the surface and eliminated directly from the circuit, controlling the feed of hydraulic water to the hindered settling zone to produce two settled products, one consisting predominantly of coarse middlings together with some free mineral and the second consisting predominantly of fine middlings together with coarse gangue, elevating said segregated products through the liquid of the classification stage so that the discharge products are essentially de- Watered, subjecting the coarse middlings and free mineral to a concentration treatment for the recovery of the free mineral, subjecting the fine middlings and coarse gangue to a difierent concentration treatment for the collection of the fine middlings and elimination of the coarse gangue from said circuit, and subjecting only the said middlings products so separated to regrinding in said circuit.

2. An ore dressing circuit of the character described in claim 1 wherein the concentration treatment to which the coarse middlings and free minerals are subjected comprises a jigging treatment.

3. An ore dressing circuit of the character described in claim 1 wherein the concentration treatment to which the fine middlings and coarse gangue are subjected comprises a gravity concentrating treatment.

HARRY L. McNEILL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,710,522 Trumbo Apr. 23, 1929 1,930,709 DeLore Oct. 17, 1933 1,939,119 Holt Dec. 12, 1933 1,955,039 Weinig Apr. 17, 1934 2,065,722 Munro Dec. 29, 1936 2,106,290 Akins Jan. 25, 1938 2,107,289 Denuy Feb. 8, 1938 2,147,234 Bird Feb. 14, 1939 2,156,245 Mead Apr. 25, 1939 2,163,285 McNeill June 20, 1939 2,180,574 Wood Nov. 12, 1939 (Other references on following page) 7 UNITED STATES PATENTS Number Name Date 2,194,139 Davis Mar. 19, 1940 2,312,865 Bird Mar. 2, 1943 2,330,158 Tartaron Sept. 21, 1943 2,336,854 Ferris Dec. 14, 1943 FOREIGN PATENTS Number Country Date 294,210 Great Britain Nov. 12, 1929 OTHER REFERENCES Denver Equipment Index, published by Denver Equipment 00., Denver, Colorado, 1936, pages 82, 83, 84, 100, 104, 106.

Milling Meth'ods, 1934, Transactions of American Institute of Mining and Metallurgical Engineers, volume 112, published by American Institute of Mining and Metallurgical Engineers, New York, 1935, pages 563, 564, 579, 580, 581, 707, 708, 709, 711, 712, 715, 867, 868, 869, 870, 871, 889, 890, 891, 394.

Milling Methods at Concentrator of Miami Copper Company, Bureau of Mines, I. C. 6573, Apr. 1932, Figures 1, 2, 3, pages 2, 3.

Truscott, Text Book of Ore Dressing, 1923,-page 297.

Reports of Investigation, Bureau of Mines, No. 3328, pages 5 to 35 and. 9 unnumbered pages; No. 2618, pages 1 to 6,

Gaudin, Principles of Mineral Dressing, page 413.

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