CA2184587A1 - A furnace - Google Patents

Abstract

A furnace comprises a furnace shell rotatable about a rotational axis. The furnace shell provides a furnace chamber for holding a solid particulate reagent as the furnace shell rotates. At least two electrodes are exposed to the chamber and are mounted electrically insulated fashion therein. The electrodes are spaced apart so that solid particulate reagent in the furnace chamber can be heated up by direct resistance heating thereof, utilizing the electrodes.

Description

WO 9Ci239-~8 PCT/GB9~C100~4~
21 ~45~7 A P~RNAC~

TXIS I~VENTION relate~ to a furnace. It relates aiso to a method of carrying out a reaction, ut~lizing the furnace.

According_ to a first aspect of the invention, there is - pro~ided a furrace compr sing a furnace shell rotatable 5~ about a rotational axis, the furnace shell providing a furnace chamber for holding a solid particulate reagent as the furnace shell rotateQ; and at least two electrodes exp~sed to t~e ~h~mher and being mounted in electrically ~sulated fashion therein, with the electrodes ~ein~ spaced apart 30 that 301id particulate reagent in the furnace ~h~h~r can be heated up ~y direct r-sista~ce heating thereo~, ut~lizing the electrodes.

The furn~ce shell will normally be cylindrical, and be located substantial~y horizontally so that the rotational axis extends substaneially horizontally.

The fu~nace i~ suieable for carrying out rea~tions whereby a solid particulate reagent is reacted with a further reagcnt at elevated temperature. The reactions may be endothermic or exothermi~. Examples of e~dothermic reacticns are nitriding o~ a titanium-contai~ing materiai as the solid par~iculate reagent, such as a titaniferous ore or slag, ~o convert titanium values therei~ to titanium ni~ride; nitriding of a ~anad ium containing material; and the carbiding of silicon-bearing mAterial. The solid reagen~ is thus heated tO said elevated temperature, and is supplied with heat ~or the endothermic reac~ion at said W095/~39~8 2 ~ 7 pcTlGss~/owJo eleva~ed temperat~re, 3y ~irect resls;ance heating .hereof in the f~rnace chamber The furnace can also be used for the regeneration of spent acti~ated carbon.

In other words, an electrlcal potential or ~oltage ~s S applied to the material of the solid reagene w~ereby an elect-ical c~rrer.t is passed by means of the electrodes, through the material, thereby generating heat within the material, to raise the temperature of the material to the ele~ated temperature, to supply heat at that temperature for the endother~c react~on (where app~icable~ and/or to enhance the reacti~ity of the reactants. Thus, the electrodes wil~ be o~ different polarity.

Thus, accor~ing to a secon~ aspect of the in~ention, there is provided a method of carrying out a reacticn wh~ch includes heating the solid reagent to the ele~ated temperature in the ~urnace chamber of a furnace as hereinbefore described.

The method may inc~ude introduc~ng a se~ond or further reage~ into the ch~her, to react with the solid reagent.
The ~urther reagent may be a further solid reagent, but is typically a gaseous reagent. The gaseous reagent can t~en be passed over the solid reagent in the furnace chamber and/or through ~t, eg by being introduced into ehe boetom of a ~ed of the material in the cha~ber.

~y 'partlculate' is meant any desired particle shape and size. Thus, the solid reagent particles can have an irregular shape and fall with~n a predetermined ranse of sizes, as would be the case when it comprises ore which has keen mined and m lled. Instead, it can be of resular shape 3~ and size, eg in the form of a powder, granules, pellets, briquettes, cr the ~i~e.

21 ~45~7 wo ~n3s4s Pcrl~ssloo~o The e~evated temperature ac which the reaction ta~es place may be 1000 - 1800C, preferably llO0 - 1600C and more pre~erably 1200 - 1350C, and the Yoltage app~ied to the material will be selected acc~rdingly, bearing i~ mind the resistivity of the material.

Certain solid reagent materials to which it is contemplated the method will in p~actice be applied, such as titan~fer~us ores (eg ilmenites) or titaniferous slags, which are to be reacted with gaseous nitrogen to ~itride t~tanium values therein, can be relatively non-conducti~e to electricity at am~ie~t temperatures. For such materials, i~itial heati~g may be by other methods, such as preheating the solid reagent by radiative, con~ectional and/or thermal-conducti~e meehods, tO raise the tempera~ure lS of the material from ambient temperature up to an intermediate value at which ohmic- or direct-resistance heating ~s effecti~e, after which the ohmic- or d~ect-resistance heating can be employed further to raiqe the temperat~re of the material up to its f~nal ~alue, and to supply the heat needed for the endothermic reaction.
Such intermcdiate ~alue may be 700 - 1300C, preferably 700 - lOOO~C, eg 700 - aoooc.

I~ctead or in addition, ehe method may include the step of mixing the solid reagent in particulate form with a pa~ticula~e solid electrical conductor, to pro~ide a mixture having increased electrical conducti~ity compare~
with that of the soli~ reagent. The mixt~re may be compacted or consolidated, eg by pelletisins, extruding or briquetting the mixture, ~urther to increase said conduct~ity. Fo~ the mixing, the solid reagent and electrical cond~ctor may be in finely di~ided ~orm, ha~ins a particle size of at most 1000 ~m, eg S0 - 200~m, and the consolidation, at leas~ in the case cf briquet~ing, may be by subjecting the mixture to a pressure o~ at least ~ -llMPa. Carbon may be employed as the electrical conductor, wo ~sn39 18 2 1 ,3 ~i ~ ~ 7 PCT/GBss/oo~o and has t~e advantase, ln the nitriding of tita~um-cor~tainin~ sol_d reagents, of pro~iding a reducing en~i-onment ~or the e~do;herm~c nieriding reaction. The pelle~s or bri~uettes may be in the ~ize range 5 - ~Omm, eg lO - 20mm.

When car~on is used as the electrica} conductor, i~ may form 13 - 9G~ by mass of the mixture, eg 12 - 60~ thereof.
The carbon may be in the form of coal, anthracite, coke, i~dustrial char, charcoal, graphite or the like, i~
par~ic~lar duff coal, which is readily obtainable and - inexpensive.

The ohmic- or direct-resistance heating is thus applied eo a moving bed of the solid reagent, eg a mo~i~g bed of said pel~ets or briquettes, in the furnace cham~er as it rotates so that any preferentia~ paths through the bed of material along which electrical currents pass in response to the applied ~oltage are continuously or interm~ttent~y di~rupted, and so that more or le~s u~iform heating of the particulate material i9 yLO.~O~ed.

The spacing between the electrodes may be lOO - lOOOmm, typically se}ected on the basis of ehe loading cr proposed loading o~ solid material in the furnace chamber, ie the furnace chamber capacity, the resistivity of the solid mater~al, and the required operacing ~olta~e. Such spacings permlt operating voltages conveniently of ~oo - 200 V to be used, al~hough higher spacings of up to 1,5 - 3m or more, requi~ing ~oltages of ~0 - 5QO v or more, ca~ in principle be feasible.

The power supply used may be AC or ~C.

In accordance with the method, the operating ~olta~e between the eleccrodes may be altered from tsme tO tsme, either manually or automatically by means of an autcmated wogs/23s48 PCTIGB95/00440 ~_ 5 control 5ystem, which may be elocl_onic, in response to changes in the temperature of the solid reactant in the interior of the fur~ace, ie in the furnace cham~er, which temperature may be sense~ eg by suitably located S thermocouples in the interior of the furnace. In this way, the operating ~o'tage can be increased to increAse the power supply to the fusnace and hence to increase the temperature of the solid reagent, or said voltage can be reduced to reduce the power supp}y and temperature. ~n one embodiment thsee voltages may be employed, eg 60V, llOV and 220V, the lowermost Yoltage being used when the solid reagent temperature exceeds a desired ~alue by more than a - predetermined amount, the uppermo~t Yo~tage being used when said reagent temperature falls shor~ of the desired ~alue by more than a predetesmined amo~nt, and the intermediate ~oltage being u9ed w~en the reagent temperature is closer to the desired ~alue tha~ said predetermined amounts.
Ingtead,eg 3~0V ~an be used for start-up, whereafter two ~oltages such as llOV and 200V may be used, the lower 2C ~olta~e being used when ~he reagent temperature is above the dcs~red temperature and the higher voltage bei~g used when the reagent temperature is below the desired ten~erature .

Whe~ the particulate solid electrical conductor which is mixed with the solid reagent i9 carbonaceous~ eg du~f coal, the heating of the solid reagent to operating temperaeure can giYe rise ~o the production of a com~usti~le off-gas in the interior of the ~urnace, containing carbon monox~de, vaporized ~olatile coal constituents or the like. The method may include the step of burning this of~ gas tO
provide the heat used ~or preheating the solid reagent, as descsibed above, although elect~ical or any othe~ suitable heat~ng may ~aturally be used instead.

When the ~urnace is operated with a nitro~en atmosphere, as mentioned a~cve, a~d a carbonaceous particulate solid wos~/23948 2 1 36 1 ~ ~ 7 PcTiGBs~o~

electrical conductc~ _s use~, mixed with a solid reagent contai~ing titanium ~alues, suitable concrol of the reaction environment ln the furnace can permlt not only the ni~ ing cf the titan um values, but, instead or i-.
add7tion, the carbiding, c~rbonitriding or oxycarbon7tr~ding thereof, which permits the production, as desired, of titanium ~tride, titanium ~arbide and/or titanium carbonitride. It will further be appreciated that, although the description of the prosent in~ention emphasizes the nitrlding of titanium, it may easily, in ana~o~ous f~9hion, be applied to reactions in~ol~i~g other solid reagenrs, eg ~or the nitriding, car~iding or car~onitridi~g thereof, such as irl the production of sillcon c~rbide by reacting a solid rea~ent comprisin~
15 Qilicon with a 801id car~on-conta~ning reductant in an inert en~ironment in the ~urnace. Bearing in mind that the carbonaceous particulate material such as duff coal can have the functions, for a tita~7um-co~taining solid reagent, of both increaging electrical conductivity of the 2Q solid reagent and of pro~id~ng of~-gas for preheating, an exces~ thereof ig preferably used oYer the stoichiometric requirement for red~cing all the citanium (as the ox~de) ln the solid reagent, con~eniently double said stoichiometr7c requiremenr of carbonaceous material is uscd.

surprisingly, the Applicant has foun~ that, in the case where the solid reagent is vanadium-bearing maeerial, tita~ium-bearing materia~, or silicon-bearing mater~al, the increased conducti~ity of the solid ~eagent (whether or noe it is with any partic~late co~duct~r to raise ts 3 0 conducti~iey) achieved by preheating the solid reagent, is related to the rate of heating the solid reagent, a~d is related t5 the rate at which a~y carbonaceo~s material mixed wlth the solid reage~t is de~olatalized. It is acccrdi~gly desirable to preheat as quickly as po~sible, eg at least 20C/min, prefera~ly at least 80C/min.

wog~/23948 2 1 & ~ ~ i PCT/GB95iOo440 The method may be car_iea out batchwise, whe_eby a charge of solid reagent is charged into the furnace chambe~ and heated to cause the required react1on to take place, before being discharged and replaced by a succeeding charge; or it may be continuous, a stream of solid reagent passing continuously thrcugh the furnace, where it is subjected to required reaction.

The furnace may thus be constructed tO cause or permit passage therethrou5h of ~oth the solid reagent and the gaseous reagent, to permit the ~ontinuous operation, and may have an interior which is sealed off from t~e atmosphere. The furnace 9hell may comprlse an outer ski~
or wall, lined with ~ suitably inert shock-resistant electronically non-conductive and thermally i~sulating refractory lining, e~ a calcium silicate ant~or an a-alumina linlng; and the spaci~g of the e7ectrodes, which may b~ of copper, silicon carbide or prefera~ly of graphite, may be as deccribed above. The electrode material wi}l be selected according to the operating temperatures and conditions. Thus, at lower operating temperatures, copper electrodes can be used, while at higher temperature5, graphite electrodes can be used.
While the furnace may in principle have any suitable construction, such as a v~bratory table ~ocated i~ its fuxnace chamber, to convey the solid reagent through its i~terior, eg from an inlet to its furnace chamber to an outlet therefrom for continuous operation, the fur~ace is conveniently such that rota~ion of the furnace shell causes pas~age of solid reagent ehrough or along i~s c~mher The ~us~ace will naturally include suitable d~ive means for drtving the shell to rotate.

The furnace or kiln may be provided with an zlternating current (AC) or d~rec~ current IDC) power supp}y to the electrodes, via o~e or more suitable 61ip-rings mounted on the furnace. Similarly, the furnace may have a slip-ring wos~39~8 7 PCTIGBsSl~440 arra~gement connected e~ to one or more thermocouples arranged in the f~rnace chamber, fcr monitoring the temFerature in the chamber. The e~ectrodes may th~s be arranged ir. sne c- more pairs in the interior of ~he s furnace so that ~~ey are located at suitable location3 and spaci~gs whereby the passage of an electrical current between the electrode~ o~ e~ch pair in response to application thereCo of a sufficient electrical potential is promoted, and the passage of electrical current between electrodes of different paixs is diQcouraged.

_The potentlal difference between the electro~es of a pair, - measured through the sol~d reagent, is proportional to the distance ~etween the electrodes, so that the distance between the electrodes of a pair i8 in principle l~mited only by the voltage supply a~ail~ble; howe~er, the ~oltage i 9 also a function of the nature and resistivity of the solid reagent. Excessi~e ~oltages can cause difficulties related to unwa~ted electrical dischar~es ~etween the elect~ode8 across ehe-surface o~ the solid r~agent, along the surface of ~he insulating refractory lining of the furnace cr through the refractory ~ ining to the exterior ~f ~he furnace.

According to one embcdiment of the in~ention, each of the electrodes may be of annular fcrm and extend c rcumferentially along an inner surface o the furnace shell while protruding radially inward}y therefrcm, with the electrodes bein5 spaced axially or longit~inAl}y apart.

Howe~er, the in~ention also contemplates the pro~ision of a plurality of pairs of the electrodes in the furnace, the electrodes of each pair being spaced ~rom one another a~
the pairs of electrodes being arranged and located in the ~nterior of the furnace so that elect~ical discharge3 will take place only between the elec~rodes of said pairs, and wo9~/~948 ~ r, '-'7 2 1 ~ ~ 5 ~ ~ P~IGB9S/00440 g no- between electr~des o_ dif~erer.t pairs; and so ;hat a -elatively long rotary furnace can be used with relatlvely small spacings between the electrodes of each pair, thereby permltting reiatively low voltage9 (eg 100 - ~SoV) to be s used. Thus, for example, the pairs o~ electrodes may be spaced lonqltudinally from one a~other.

Accordi~g to another embodiment of the invention, one of the electrode5 ('the firYt electrode') may extend cenerally a~ong the rotational axis, with a plurality o~ the other elecerodes ('the second electrodes~) being pro~ided, the second electrode5 protrudins from and exten~in~ along an inner sur~ace Oc the furnace shell in a longit~in~l direct~on, and bei~ spaced circumferentially from cn-another. The central electrode will thus be of a particular po~ariey, wi~h the second electrode5 being of opposite polarity, to pro~ide for current ~lows ~etween the ceneral electrode and those- second peripheral electrodes which are at any time submerged by the particulate material in the furnace, the ~urnace bein5 operated with a bed of particulate material therein of sufficient depth to be i~
contact with the central electrode.

In yet a further emko~tment, a plurality o~ the e~ectrodes, arran~ed in pairs, eg th~ee pairs, and protruding from and extendin5 along an inner surface of the furnace 3hell in ~he longie~J~in~l direction, may be provided, with the p~irs bei~g circumferentially spaced from one another, and the electrodes of each pair being spaced circumfe-entiaily from each other by a spacing which is less than the spaci~g between adjacent pairs. In this case, a~ wi~h the electrodes discussed abo~e, the electrodes may stand proud of the surface of .he lining. They can then also a~t as lifters for lifting particulate material in the kiln as it rotates, thereby assisting in keepin~ the part~c~late mate_ial con~i~uously i~ motion and ~iyin~ it, to disrupt the paths of electrical currents ~lowing therethroush.

W~ 9~123948 2 j ~ ~ 5 ~ 7 pcTlGBssloo44o In a still further embodiment, the electrodes may be of non-annular for~., and protrude from an inner surface of the ~urnace shell, with the one electrode ~ t~e first electrode'~ being spaced lo~gitudinally from the other electrode t~ehe second elect.ode~). A p}urality of che '~rst elec~rodes, circumferentially aligned and spaced apart circumferer.tially, and ~eing of the same polarity, as well as a plurality of the second e}ectrodes, circumferentially alisned and spaced apart ~0 circumferent'ally, and being of the same polarity, may ~e provided. Thus, the first electrodes will ~e in the form of a group, while the second e}ectrodes will also be in the form of a group, ~ith the groups being spaced axially or longitudinally and the electrodes of one group being of 1~ differe~.t polarity to ehose in the other groups. The electrodes of the f irst group may be aligned with those in the second sroup, in the longit~ n~ direction. If desired, a further group of the first electrodes, spaced axially or lon~it~in~l~y from the group of second eiectrodes ~o that the group of second electrodes is located ~etween the two groups of first electrodes, may be pro~ided.

The Appl~cant has found, that in certain cases, the resistiYity of the soli~ reagent decreases as the temperature of the solid reagent increases with heating thereof andlor as the sclid reagent reacts progressively wieh the gaseous reagent (thereby progressively cha~ging the composition of the solid reagent) so that, after such heating and/or reactis~, a relatively lower voltage i9 required to ~aintain a consiseent current flow in the sol~d reagent.

T~us the method may ~nclude the step of pass~ ng the solid reagent through a series of successive furnace chambers or reaction zones, each cham~er o~ zone including said at ~5 least two electrodes. The spacing ~etween the electrodes wo 95l2394~ ~ 1 8 ~ 7 PCr/GB9510044n in each succeedi~g zone-may ther. be greater than tha~ of the precedt~g zone. The method may in this case, in partlcular, inc~ude pas9ing the so~id reagent through the zones so that ~t forms a separate bed in each zone, with 5 the beds bei~g electrically isolate~ from each other.

Thus, the material dams up in each segment, so that a~
lea~t some parametexs can be controlled separately in each segment, eg temperature, re~idence time, and applied ~oltage.

In this way, by selecting zo~es of appropriate size or length for a partic~lar solid reage~t and a particular gaseous reagent, substantially the same voltage may be u~ed for each pair of electrodes in each of the zones, despite ~ariations of the spacing between the el~ctrode~ in the '~ different zones. In part~cular, in the ca~e of three zones, a sing~e three-phase source of pcwcr can be used with ane sa~d phase supplying power to each of said zones.

Thus, the furnace may inc}ude a ~eco~d sub~ta~tially horizontal cylindrical fu~nace s~ell rotatab~e abaut the rotational axis and spaced axially or longitudinally from the other or first furnace shell, the second furnace shell pro~iding a second fur~ace chamber or reaction zone which ig in cor~un~cat~on with the first fur~ace ~hamh~r or reaction zone and through which solid pa~ticulate react~nt from the f~r3t cha~ber can pass in the longitu~in~l direction, and wlth said at lea~t two electrodes also being proYided in the second furnace cham~er. ~f desired, at least one further similar horizont~l cylindrical furnace shell may be provided adjacent the second furnace shell, to provide t~e succcsai~e reaction zones.

The ~irst and second cham~ers may ha~e the same or dif f erent diameters. For example, the second rh~her may have a greater diameter than the ~irst ~h~mher.

WO 9~/23948 , I r; ~ L, ~3 C 1 ~ Lt ~ ) / PCTtGB951~0440 Additi~nally o- in~tead, the f irs_ and second '~rnace shells may be of :~e same cr differe~t len~th, and the spac~ng between the electrodes of the first furnace shell may be the same of ~ifferent to that of the electrode~ of S the second furnace shell.

In other words, the furnace can thus be segme~ted, comp.is~ng a series o~ ax~ally spaced portions ~r segments, each containing a pair of the eiectrodes. Each ~egmene may be of a different diameter from the adjacent p~rtion or segment. ~hus, the ~urnace may comprise a plurality of such segments increasin~ progresB~vely in diameter from one _ portion to the next, the portion of smallest diameter being at thc upstream end of the furnace relative to the d~rection of solid reagent flow.

Instead, the ~urnace may comprise a plural~ty of successive sesments of the same or generally similar diameter, each successive ~egment being longer, in a downstream direction, than the se~-.c--t precedlng it, and the distance between the electrode9 of each succe~sive segment being corres~on~in~ly greater. This construction ta~es advantage of the find~ng ~y t~e Appl~cant, referrcd to above, that the resistivity of the solid reage~t, and hence the voltage re$uired to cause passage 2f a current of a giv-n Yalue through a ~iven mass or volume of the solid reagent, decreases as the temperature af the solid reagent is i~creased and~or as the solid reagent progressL~ely undergoes reaction wit~ the gaseous re~gent.

For example, the fur~ace may comprise three successive segments, each ha~in~ first a~d second e}ectrodes, in which the distance between the electrodes in successi~e segments is 650-7~0mm, aso-gsomm and 1050-1250mm respectively, the inner diamete~ of the furnace chamber being 500mm.

w09~l2~948 PC'r/GB95100~40 - ` - 21 ~5~37 ~he inner su_faCe c~ the '~ g of the furnace is preferably smo~th and both non-porous and electrical~y lns~lating, so ~hat impre~nation thereof or coating thereof by solid reagent ~nd particularly by any particulate solid S conductor ~dded eo said reagent is discouraged. As mentioned abo~e, a-alumina ~uch ns castable a-alumina, has been found to be suitable for this purpose.

Preferably the furnace hag its interior closed of~ from the atmo~pher~ and/or is operable at a~o~e a~ospheric pre~sure to permit maintenance of a ccntrolled atmosphere thesein.

- The furnace axis may be tilted at an angle cf a~out 1-3, preferab~y about 2 to the horizontal, the downstream end being the lower end, to assist in passage of solid material through the o~her.

The furnace may be provided w~th longic~t~inAlly spaced annular isolating pastitions fos e~ectrically isolating sol~d reagent in one segment fro~ that in an adiacent s~ c.-t. The partitions will be of a refractory a~d preferably insulating material. The furnace may, further, be pro~ide~ with lifting mem~ers or bars which, as the furnace rotates, cause 50l? d material to be li~ted and transferred pr2gre9sively from one segment to an adjacent ~egment.

The ~urnace may include feed a~d extraction means ~or 2S feeding and extracting soli~ reagent and wa~te product therefrom, as well a~ gas feed means and gas ext_action means for feeding gas into and withdrawins ? t from the chamber respecti~ely.

T~e gas ~eed means may include a plurality of gas per~eable distributors in the furnace shell, the distri~utors bein~
circumferentially spaced from one another; gas delivery means for deliverin~ ga5 to the outsides of the os~n3s48 2 ~ 7 PCTIGB9~/0~0 14 -_ distributors, with the gas passin5 t~-oush the dist-iburors ~nto the furnace chamber; and gas flow control means operable, during rotation of the furnace, to deliver gas only to those distributors which are at or near eheir S lowermost position so that, in use, inflowing gas passes large}y thrcu5h a charge of solid particulate reagent in the furnace cham~er.

The in~ention will now be described, by way of example, with reference to the following Exampies and with reference to the accompanyi~g diagrammatic drawings, in which:
FIGURE 1 show~ a schematic sectional side elevat~on of a furnace i~ a~cordance with a f lr5t e~bo~; ment of the present in~ention; --FIGURE 2 shows a view, similar to Figure ~, of a furnace accordi~g to a second em~odiment of the invention;FI~URE 3 shows a plot of power input agai~st time for activated car~on used in a batch-type method according to the present ~n~ention carried out in the fur~ace of Figure l; .
2~ FIG~RE ~ show~ a plot of re~istance-~gain~t time for the activated carbon whose power input is plotted in Figure 3;
FIGURE S shows a ploe of resistance agains~
temperature for the acti~ated carbon whose power input is plotted in Fisure 3;
FIG~E 6 Qhows a plot, similar to Figure 3, for a mixture of ilmenlte and duf f coal;
FIGUR~ 7 shows a plot similar ~o Figure 4 fo~ the mixture of ~igure 6;
F~GC2E 8 shows a p}ot similar to Fig~re S for the mix~ure of Figure 6;
FIGURE 9 shows a pl~t similar to Figure 4 for a mixture of titanirerous slag and duff coal;
FIG~RE 10 shows a p~ot similar eo Figure S for the 3s mixture of Figure 9;

wo 95~g48 ~ i 8 4 5 ~ 7 PCI'IGB95100~40 r IGUR~ ;1 shows a piot simi~ar ~o Figure 3 for the m~xture of Figure 9;
FIGUR~ 12 shows a ~iew, simllar to Figures 1 and 2, of a ~urnace according to a third embodiment of the in~ention;
FI~U~E 13 shows a view, similar to Figures 1 and 2 of furnace in accordance with a fourth e~o~ nt of the in~ention;
FIGURE 14 show9 a view, simil~r to Figures 1 and 2, of a furnace in accordance with a f~fth em~odiment of the i~ention;
FIGURE lS shows a view, similar to F~gures 1 and 2, of a furnace in accordance with a six~h embo~im~nt of the in~ention;
FTGURE 16 ~hows a view, similar to Figures 1 ant 2, of a furnace in accordance with a sevcnth embodiment of the inve~tion , with some detail omitted for clarity;
FIGURE 17 shows a seCtiohal ~iew through A-A in Figure 16, with some detail omitted for clarity;
FIG~R~ la shows a sectional view through B-~ in Figure 16, wlth some ~etail omitted for clarity;
FIGURE 1~ ~howq a sectional ~icw through C-C ~n Figure 16, with some detail omitted for clarity; and FIG~RE 20 shows, schematically, a representation of a power contrcl syseem for controlling power to two successi~e portions of a furnace according to the invention.

ln the drawings, the same or similar parts are indicated w-th the same reference numerals.

Referring to Figure 1 a ro~ary batch-operation furnace in accordance wit~ a fi~st embo~im~nt of the present inventton, and suitable for pilo~ scale batch operation, is designated ~y reference numeral 10. The ~urnace has a ho~low-cylindrical outer mild steel wall 12, close~ off ~y annular mild steel end plates 14 and 16 at opposite ends thereof. The central opening of the end plate 14 forms an woss/23948 ~ 1 8 ~ PCTI~B9S~OO~O

inlet eo the f~rnace ehambe- or interlor and opens into g2s iniet passage 1~ wlth an extenslon 20- The end plate i6 h~s an annular hinged door 22 provided with a spigot 23 throu~h which the furnace is loaded, discharged and sampled and which, thus, ~rms a samplin~ port into the interior of the furnace and a gas outlet therefor.

The wa~l 12 and end platc~ ~4, 16 are lined on their ~nner surfaces ~y a lining 24 of electrically insulating calcium silicate kricks, whlch lining 24 is, in turn, inter~zlly lined ~y a lining 26 of electrically insulating refractory -br~ks, a layer of ceramic fibre paper at 27 being pro~ide~
~ between the lini~gs 12 and 24. The l1ning 26 hag a cylindrical part on which is pro~ided a cylindrical internal lining 28, also of electrically insulating refractory brlcks, at opposite ends of which are mou~ted, on the lining 26, a pair of annular ~Yi~lly or ionsit~ n~lly spaced c~rcumferentially exten~ing graphite electrodes ~0 and 32, ~e a first electrode 30 and a second electrode 32. The i~ner p~riphe~ies of the electrodes 30, 32 stand radially inwardly proud of the inner ~urface of the lining 28. Two castable ~-alumina retorts, each 500mm in lensth and shown, schematically at 34, define a further non-porous inner llning which abuts the linin~ 28. The retorts 34 are recessed by about 10-lSmm from ~he in~er per~pheri-s of the elec~rodes 30, 32. The ioin between the retorts 34 is sealed wi~h refractory concrete (not shown~.
The electrodes 30, 32 are spaced ~par~ ~y lOOOmm and are connectet to a single-phase AC power supply.

The extension 2C t S pro~ided, on its outer surfa~e, with a plurality of siip rings 36, some of which are connected to ~arious thermocouples (not shown) in the furnace by electrical leads (not shown~, and two of which are connected respectiYely to the electrodes 30, 32 by electrical leads (also not shown).

woss/239~ 8 4 ~ 8 7 pcTtGss ' 17 Operation of the furnace o~ Figure 1 will now ~e described by way of illustrative non-lim~t~n~ example, with reference to the following Examples.

~XZ~h~PT.~ 1 A 20 kg batch of solid material was prepared by mixsng together a solid part~culate ilmenite reagent with ~
part~culate solid electrical conductor in the form of duff coal, there bei~g a58 g of duff coalJkg reagent, amounting to about double the amount of duff coal re~uired to provide sufficient carbon to reduce all the tttanium and iron (as - the oxides~ in the ilmenite. ~he i~menite had the - compositson given in the Table hereunder. By 'particulate ilmenite' is meant ilmenite as mined and milled, ant having irregular shaped partlcles of di~ferent 3izes.

The furnace lo was preheated ~y charg$ng it with 4 kg of granular act~vated carbon and by app}ying a potentlal of 380 V between the electrodes 30 and 32. The carbon was heated under nitrogen and reached 1~00C in 12 hours, heat~ng being monitored to keep it at this temperature by means o~ a reduced potential for a further 5 hours to heat up the furnace lin~ng to cause it to reach a steady state as re~asds temperature. The ~sln was rotated at 1 rpm.
Figure 3 is a plot of power input against time for the furnace preheating, showing that power supply decreased after 1 hour. ~igures 4 and 5 respectively show the change of electrical resistance of the acti~ated car~on against time and against temperature of said carbon. In this regard it should be ~oted that, naturally, if desiret, electrical preheating by mea~s of heating e~ementq embedded in the furnace ~ining or any other s~itable pretreating can be used instead.

After the kiln was preheated rotation wa~ stopped and the carbon was extracted under nitrogen, the 20 kg charge of ilmenite/duff coal then being l~aded into the furnace under W0 95n3948 2 1 ~ 4 5 ~, 7 PCI/GB9SiW4~0 nitrogen. Roeation o~ the furnace was restarted immediate}y after this loading and the temperature of the ~harge rose rapidly to a~out ~50C. A potential of 220V
was app~ied as ~oon as rotation restarted, to heat the charge to 1300C, with the potential being reduced whenever power input exceeded 22 kW. The operating temperature of 1300C was reached after 1 hour and was maintained for a further 3 hour~ by select~vely altering the ~oltage of the potential applied to the electrode~ 30, 32 to appropriate Yalues to keep as clo9e to 1300~C as po9~ible. The interior of the furnace was ed with nitrogen 50 th~t the charge was maintaincd under ~itrogen atmosphere, and so that the titanium sn the charge became fully nitr~ded.

M~ m power input was nct allowed to exceed ~2 kW and temperature was not aliowed to exceed 1300C to guard against thermal shock to the furnace lining a~d three input potentials were used a~ter 1300C had been reached, namely 60 V, 110 ~ and 220 V. Power input for heatins the charge is plotted i~ Figure 6 a5ain~t time; re~ista~ce of the rharge i- plotted in Figure 7 against eime; ~d re~istance of the charge is plotted in Figure 5 agai~t temperature of the charge.
.
Riln r~tatlon was kept at 1 rpm and the nitrogen feed rate was 2,19 ~gJhr, nitrogen eed being continuous, the nitrogen feed amounting to 3 times the stoich~ometric requirement to nitride the titanium in the charse. After 3 ho~rs of nitrogen feed the power supply was cut off and the fur~ace was allowed to cool naturally with constant rotati~n at 1 rpm w$th the charge under nitrogen. The charge was removed when the temperature ~o~c~ to under 400~C. In the nitrided charge ~35~ of the titanium was found to ha~e been converted to non-stoichiometric titanium nitride.

wo 95/239-18 2 ~ ~ 4 5 ~ 7 PCr/GBgS~O~
lg Similar results were o~tained whe~ Example 1 was repeated using the charge mixtur~ in the form of ~i) pellets of a 10mm ~iame~er, co~taining 2~ ~y mass bentonite binder, although lt was found that a substantial proportion of the pellets had disin~egrated by the end of the test;
br~quettes having a size of 45mm x 20mm x 20mm; and (iiil larger and smaller particles than the pellet~ and briquettes.

~y~yPr.~ 2 Example 1 was repeated using an 20 kg charge w~ich was a mixture of titan~fercus slag a~tai~ed from ~ish~eld Steel and Vanadium COL~ tiOn ~Proprietary~ Limited mixed with tuff coal i~ a proportion Of 350 g ~oal/kg slag. This was double the stoichiometric amount of coal rec~uired completely to reduce the titanium ~as the oxide) i~ the slag. The charge wa~ pelleelzed using 2~ by mass bentonite binder, into 10mm diameter pe~lets.
r The compos~tion of this slag, ~nd that of the ilme~ite u-~ed ~or Example 1, are set forth ~n the following Table:

TA~E

Chem~cal Comro3itlons of Titaniferous Sl~g and Ilmenite Constituent Titaniferous ~lme~ite Slag (mass ~) ~mass % ) T~ 30.5 48.8 2~ SlO. 20.7~ 1.3 MqO 14.10 1.0 CaO 16.8 0.04 Al.l 13.65 0.7 Cr.Ol 0.1g co.o~
FeO ~.15 47.0 v2~ 1.05 0.12 MnO ~.6g 0.82 ~1 ~4~37 woss/23948 PCT/GBs~/o~o The TiO~ in ~he glag was prese~t pr~n~ipally as fassaite [Ca~T~,Mg,Al)tSi,A~)2O6J~ and perc~skite [CaT~03~, and to a lesser exee~t as pseudobrookite [Fe2O3TiO2 and ulvospi~el ~Fe2TiO4] .

The charge was heated to 1300C as soon as possible, power inpu~ being restricted to 20 kW to resist si~tering the charge, and u~ing a potential of 380 V. The operatins te~perature of 1300C was reached after ~,~ hours an~ was maintained thereafter ~or 3 hours by electrically applying ~oten~ials o~ 230 V or 110 V, as re~ui~ed. The nitrogen ~ flow rate du_ing the 3 hour reaction period was 0,4 kg/hr, wh~ch amounted to 4 times the stoichiometric re~uirement to nitr~de the titanium in the charge.

A co~version of ~92~ of the tita~ium in the charge to no~-stoichiometric titan~um nitride was achieved. Figure 9 ~hows a plot o~ resistance of the charge against time;
F~gure 10 shows a plot of resistance of the~ charge against the temperature thereof; ~d Figure 11 showæ power supply to the fur~ace plotted aga~nst time.

~MPLE 3 A meta~ oxide-carbon mixture was prepzred by mixing l5,6 kg ~23 with 4.2 kg pitch coke (consisting o~ 82,5~ fixed carbon and lS,~% volatiles) ~nd 0,4 ~g stabilised polymer emulsion plus starch ~inder.

The mixture was formed into pellets, ha~ing a diamerer of approximately 10mm, on a disk pelletiser and cured. ~he expected chemical reac~ion was V23 ~ 3C + ~2 = 2vN ~ 3Co.

The furnaoe 10 was pre-heated and loaded w~th the cured pellets as i~ Example 1. The ~harge was hea~ed to 1350C
in S hour~ and maintained at 1350c for 2 hours. Power W09~/23948 PCTtGB9~loO~n ~,, input was restrict~d ;o ~2 ~W to pre~ent 'ocalised sintering of the charge- The potential difference settingS

applied across the electrodes 30, 32, in order to ensure sufficient power input, were limited to 60, ~1~, 220 and ; 380 ~. The kiln was rotated at 1 rpm during t~e re~ction, and ~ nitrosen flow rate of 2,19 kg~h was maintained during the procedure.

After reactio~ had been compl~ted, the furn~ce was al~owed to cocl under nitrogen- The char~e was unloaded at am~ient temperature to pre~ent ~e-oxidation of the vanadium - c~rb~nitride produ~t. The product, whic~ was hard and ~ dense and h~d a volume about one half of the orisinal vo~ume, co~tained co~tains 77, 2~ ~anadium, 2, 7~ carbon, 17,6~ ni~rogen and 2,6~ oxygen.

1 5 13XAMpt .~ 4 A metal oxide-car~on mixture was prepared by mixing 9,6 kg SiO2 with 10,4 kg of carbon (precipitated from a coal ~olutio~ by e~aporati~g the sol~e~t).

Bri~ueetes ha~lng a size of 40mm x 23mm x 12mm were prepared from the ~ix~ure by compressing at lS00 psi a~d curing at 230C. The expected reaction is given by the equation SiO2 + 3C = SiC + 2CO
The fu~nace 10 was pre-heated and loaded with the ~ure~
briquettes as fcr ~xample i. The charge was hea~ed to lS~O~C o~er a period of 8 hours and maintained at 1500C
for 9 hours. Power input was restricted to 25 kW to prevent localised sinter~ng of the charge. The potaneial dif_erence se~tings applied over the electrodes 30, 32, in order tO ensure su~ficient power i~pUt, were limited to 60, 110, 220 a~d 380 volts. The kiln was rotated at 1 rpm du~ing the reaction, whi~e an argon flow-raee of 10 ~/min was maintained.

2F (~4L~7 WOg~l23948 PCT~GB95100440 22 -~
After the reaction, the furnace was allowed to cool to 600C unde_ argon. The charge was unloaded and cured at 600~C for lC hours to remove any excess carbon. After curl~g the p~oduct was found to consist of lOO~ SiC.

~luted carbon from a gold extraction process, and having a moi~ture conter,t of 42~, was loaded into the preheated furnace (800C) u~der nitrogen. Rotation was started immediately a~ter loading, and ~ol~age was applied across the bed o~ wet carbon- Typical voltages throughout the regeneration p.rocess ranged from 3B0~-60V as the resisti~ity of the carbon changed. Steam was emitted in the first fi~ e m~nutes of the process ~efore regeneration as such commenced. The residence time of the carbon in the furnace was 20 minutes ~t a temperature of 720OC. This facilitated the dr.ving off of the organics from the porous car~on thereby reactivatln5 it and preparing the carbon for deli~ery to the adsorption 9ection of the gcld extraction plant.

The organics thae are dri~en off the carbon during regeneration come from rea~ents added upstream of the elution process. These oxga~ics wastefully occupy sites on the car~on that ext~acred gold should occupy duri~g adsorption, rendering the adsorption process i~ef~icient.

When the residence time o~ 20 minutes has been co~pleted the car~on w~s discharged ir.to a quench tank of water where lt was cooled, and then pumped ~ack to the adsorption sectlon. Quenching the carbon inhi~itc oxidizins thereof a~d pro~ides rapid cooling.

~eferring to Figure 2, a furnace in accordance with a second em~odiment of the in~entior. is also generally designated 10. The fur~ce 10 of Figure 2 is, in co~.rast 2i~4j&7 wo9sl23s48 PCT/CB95/0~40 ~, 23 to that of Figure ', ineended for continuous operation and is also opera~le by means of an AC power supply.

Accordingly, the en~ of the feed passage lB remote from the wall 12 is fed by a soiids feed chute 38 which i9 supplied by a worm feeder 40 fo~ extracting feed pellets from a pellet ~upply hopper 42. The hopper 42 is in ~urn fed from a pe}let drying hopper 44 by a rotary star feeder 46.

The central opening of the end p7ate 16 forms an outlet for the furnace lO and is pro~ided with ~ hood 43 sealed to an ou~let passage 50 protruding from said central opening by a circumferenti~lly extending bear~ng seal 52. The hood 48 has an off-gas outlet du~ ~4 extending to a com~ustion chamber (descri~ed hereunder~. The hood 48 also has a sigh~ glass 56, a solid9 discharge de~ice 58 and an l~ adjustable chute 60. The chute 60, when the furnace is in steady-state operation, allows the flow issuing from the furnace via solids d~scharge device 5B temporarily to be ~ncreased, when desired.

The duct 54 extends to a combustion ch~or 62 enclosing the upstream end of the passage 18, which cha~ber is sealed to said passage lB by a~nular bearing se~ls 64. The chamber 62 has a pi}ot burner 56 and an outlet-p~o~ided with an extraction fan 5~ and a flow control slide valYe 7~. A gas duct 72 leads from the fan 68 to the drying hopper 44.

In Figure 2, the slip rings 36 o~ Figure 1 are omitted and repiaced by elect-ode connection boxes 74. The h~nged door 22 and spigot 23 of F~ gure ' are also omit~ed from Figure 2.

3C A particu~ar fea~ure of the fur~a~e lO of Figure 2 is that i~ compr~ses two axially aligned portions or segments, namely an upstrea~ portion 76 Oc re7atively reduced 2~ ~sg7 f ~ :
woss/23s48 PCTtGBg~100440 -diameter and a downst-eam portion 78 of relatively increased diameter. Each portion 76, 78 has a pair of ~raphite elec~~odes 3~, 3~ Ypaced apart by S~Omm, and each portion 76, 78 is o~ broad~y simila~ construction eo the S furnace lO of ~igure l.

In eaeh portion 76, 78, the lnner surface of the lining 26, upstream of the first electrote 30 and downstream of the second eleetrode 32, is provided with a plurality of axially extending c~r~um~e~entially spaced extractor ~ars or ri~s ~0, standing radially inwardly proud o~ the inner -surface of the linir,g 26, for keeping solids ~n the fusnace ~ l~ in motlcn as it rotates and for assisting in mo~ing the solids axially throug~ the furnace.

The connection boxes 74 are arranged in f our rings arou~d i5 the furnace shell lZ, each ring comprising four equally circumfe~entially 6p~ced boxes 14 mounted on the shell 12.
Each box is ro~ected ~y an electrically insulated elect~ical lcad ~2 leading to the associated electrode 30 or 32 as the case may be. The boxes serve to connect the electrodes 30, 32 by means of slip rings ~ro~ shown) to an AC power supply (not shown~.

An eiec~rical preheater 84 is shown enclosing the ~ownstream portion of the passage 18.

A feature of the ~urnace lO of Figure 2 is that the ~ncrease ir. diamerer from the portion 76 to the portion 76, which is in the form of a step in diameter at 86, promotes electrical ~solation of the electrode 32 o~ the portion 76 from the electro~e 30 of the portion 78, by causing a brea~
or di~conti~uity, in use, between solids in the por~ion 76 and solids i~ the portion ~8, so that there are separate beds of so~ids ir. the portions 76, ~8, which beds do not merge into each other. ~n other words, ~here is a ~mmj effec~ in each portion or ~egment, so that each port~on ca~

21 845~7 woss/23s~8 Pc~/Gs9~/0~40 be operated and controlled, eg as regards applied ~oltagee and residence times, substantially independently of each other.

The operation of the furnace iO of Figure 2 will be essentially similar to that of the ~urnace of Figure l, but on a continuous rather than a batch basis. Thus, optionally, after preheating the furnace lO using granular acti~ated car~on fed throu5h the furnace under nitroger.
while applying a suitable ~ol~age untll the interior of the furnace is at a steady state temperature of 1300C, feed of -a pelleted rea~tion mixture, similar to those of Examples ~ l or 2, can be started.

Pellets are fed from hopper 44 by feeder 46 to hopper 42 and thence they are fed by feeder 40 ~ia chute 38 into passage l~. In passage 13 they are heated by the elec~rical heater ~4. As the pellets pass through the rotati~g p~rt~ons 76 ~d 78 they are heated by electrical currents flowing throu~h the pellets between the elec~rodes 30, 32 of each portion 76, 78. Suitable potentials (eg ~D Examples 1 and 2) are applied to the electrodes 30, 32 to maintain pellet temperature at l300C and nitrogen at a su~eable stoichiome_ric rate (see Examples 1 and 2~ is fed into the furnace along duct 20. The pellet ~eed rate is selected su~h that the Fellets have a residence time of 3 hours in the furnace at l3000r Off-gas from the pellets in the furnace is ducted along duct s4 by f an 68 to combustion chamber 62 where it ~s ignited by pi~ot burner 66. Heat frcm the burning off-gas asslsts ~ n prehea~ing ;he pellets before they enter ehe furnace lO, and combustion ga~ from the chamber 62 is fed along duct 72 by fan 68 to hopper 44 where it dries the pellets.

2 i ~8$5~87 ~r wo gSI23948 PCT/GB9~/OO~n Product is extracted from hood 48 via dischar~e device 58 and can be sampled by means of the adiustable samp7ing chute 60. The reaction in the furnace can be monitored vicuaily by means of the sight glass 56, and the ; temperature ~t ~arious places in the furnace ca~ ~e monitored by means of suitably located thermocouples ~not shown). The connection boxes 74 are used to feed current via the leads e2 to the electrodes 30, 32 as required, and are u~ed to receive inputs from the thermocoupies and to tr~nsmit them to external monitoring devi~es (not shown).

A parti~ular feature of the ~nvention, as demonstrated with reference to the Figures, is that constant motion of the solids ~harge in the furnace continually disrupts the paths of electrical discharses between the electrodes 30 and 32, new discharge paths c~ntinually being established. Local o~erheating of the charge ~s a~oided ~as could take place in a fixed bed) ~nd miYi~ of the charge promotes an e~e~
te~perature thereof.

~eferring now to Figure 12, reference numeral 100 generally indic~tes a third em~odiment of a rotary continuous operation furnace in accorda~ce with the present in~ention.
The furnace lOo generally resembles the furnace lO of Figure 2.

The furnace 100 differs from the furnaoe lO of Figure 2 in that it lacks the solids feed chute 3~ and, i~stead, the worm feeder 40 feeds feed material direct~y into the furnace. Further, ins~ead of the off-gas outlet duct 54 and the combustion chamber 6~, the hood 48 of the furnace 100 is provided with a burn-off burner 102. In this embodiment of the invent_on the solids discharge device 5~
feeds direc~ly into a sealed storage hopper 104 wh-ch ~s provided with a wor~ extractor 106 for discharging solid material. If desired, the storage hopper 104 can be provided with a sui~ahle gas inlet (not shown) , eg i~ it . 2 1 ~537 WO9S/23948 PCT/G895/oo~o -is required to conrrol the atmosphere in t~e hopper. The entire assembly from the wor~ feeder 40 to the worm extractor 106 is more or less gas tight.

The furnace lO0 differs, further, from the fu~nace lO of ~igure 2 ~ n that it comprises three axially aligned portions or segments namely a first se~ment 110, a second segment 112 and a third segment 114, segment 110 being upstream of segment 112, and segment 112 being up~tream of segment 114. The se~ments 110, 112 and 114 all ha~e an inter~al diameter of 500mm but d~ffer in length. In additio~, the distance ~etween the electrodes 30, 32 of ehe first segment is 700mm, that between the electrodes 30, 32 of the second se~m-nt is 910mm, and that between the electrodes 30, 32 of the third segment is 1120mm.

The first, second and third segments llO, 112, 114 are separated from one another by annular partit~ons, or orifice rings, of electrically insulatin~ refractory ~ricks 116. The furnace 100 is further pro~ided with lifti~g bars ~not shown) ad~acent the or~fice rings 116 for transferring solid material l;B from one segment to the next as the fur~ace 100 rotates, through the central opening of the assoclated par~ition 11~. The entire furnace is inclined a~ an angle of 2 to the horizontal to fac~litate the passage of the solid materlal 118 through the furnace lOo, whose downstream end is its lower end. The orifice rings 116 ser~e electrically to isolate the solid material llB in one adjacer~t segment from solid material lla in the other ad~acent segment. The electrodes 30, 32 of the separate ~egments llO, 112, 114 are connected via the connec~ior.
boxes 74 by electrical connectors (not shown~ to a single rhree-pha9e source of electrical power, one phase beins connected to eac~ of the segme~ts.

The construction of the furnace 100 ta~es advanta~e Oc the fact that the resisti~iry o. the solid material 11~

21 84~
.~

wo~/23948 PCl'JGB95tO044~

2~

prepared according tO the method of Example 1 descri~ed above, reduces as th^ material is heated and as reaceion of the c~tanium and iron in the material proceeds so that although the distan~e between the electro~es 30, 32 of the third segment 114 is greater than that between the electrodes 30, 32 of the second segment 112, the same ~oltage ca~ be usea to achieve the same current flow in both segments. The same holds for the segments 112 and 110 .

~0 Referring to Figure 13, reference numera} 200 generally -indicates a iourth embodiment of a rotary ~ continuous-operation furnace in accordance with the present in~ention. The furnace 200 generally resembles the furnace 10 of Figure 2.

la The fur~ace 200 di~fers from the furnace 10 of Figure 2 in tha~ che combustion chamber 6~ of the furnace 10 is a~sen~

in the furnac~ 200, the off-~as outlet duct ~4 se~ing simply ~o ~ent the of~-gasses. In ~his embo~ nt of the invention, the electrical pre-heater 84 alone serves to preheat the feed pel~ets and in}et gas. The furnace 200, ~urther, includes thermocouples 202, 204 projecting, respecti~ely, ~nto the interior of the portions 76,-7~. In this e~bodiment of the inYention, there are three hoppers 44 (of which only one is shown in the drawing~ each of 5 - 8 ~on capacity for holding pellets or particulate materials. The hopper 42 is a 2 ton supp~y hopper. ~he o~erall length of the portio~s 76, 7~ ls 2/2m.

Referring now tc F~re 14, reference numeral 300 generally indicates a fifth em~odiment of a rotary continuous operation furna~e in accor~ance with the in~entior,. T~e furnace 300 general}y resembles the furnace lo of Figure 2.

21 845~
W095l23948 pcTlGBs5loo44 The furnace 300 dir~rs frcm the furnace lQ of Fisure 2 only in that the port~cns or segments 76, 78 are of the same diameter so ~ha. the step 86 in diameter ~s absent.

The capacity of the hoppers 42,44 and the overall lengths of the portions 76, 78 are substantially the same as those of t~e furnace 200 of Figure 13.

Referring to Figure 1~, reference numeral 400 generally indicates a sixth embodiment o~ a furnace in accordance with the invention. Again, the furnace 400 rcsembles the -furnace 10 of Figure 2.

The furnace 400 d~ffers from the furnace 10 of Figure 2 in th~t it includes a further portion 402, in addition to the portions 76, 78. The portion 402 has a larger diameter than the portion 78 with a further step in diame~er at 86 ~etween the portions 7B, 4~2. This step also ser~es to promote electrical isolation of the electrode 32 of the portion 78 from the electrode 30 of the pcrtion 402, as described abo~e, by a ~reak in continuity betwcen solids i~
the portion 1~ and solids in the portion 402.
.
Referring to Figu~es 16 - lg, reference numeral sOC
genera~ly indicates a seventh embo~i~ent of a continuous o?erat~on rotary furnace in accordance with the present in~ention.

The furnace 500 also has a hollow cylindrical outer mild 2S steel wall 12, whic~. is ii~ed on its inner surface with z linir~g of insulating tiles ~52 which lining is, in turn, interna~ly lined by a layer of refractory concrete 504.
The furnace 500 is DC operable.

Eighty four generally elongate porous or permeable plugs or 3C distributors ~06 of porous refractor~ material, eg silicon carbi~e, each ha~ing a generally square cross-section, and arranged in seven groups of twelve distributors 506 each, project inwardly form the wall 12. The distributors of each group are arranged in an annular fashion and are circumferentially spaced from each other as can be seen, in particular, in Figure 18, and are aligned axially. The groups are axially spaced from each other as can be seen, in particular, in Figure 16. Each distributor 506 is embedded in linings 502, 504 with an inwardly directed face thereof flush with the inner surface of the refractory concrete lining 504.
Each distributor 506 is connected to a nitrogen inlet manifold 508, which is mounted to the furnace wall 12 and thus with the rotation of the furnace. Each manifold 508 has nitrogen inlet conduits 510 so that the axially aligned distributors 506 of each group are served by a single or common manifold 508. The manifolds 508 are in turn connected to a ring manifold 509 mounted to the wall 12.
The ring manifold 509 slidingly abuts a stationary annular manifold component 511 so that the ring manifold 509 moves relative to the manifold component 511 as the wall 12 rotates. A conduit 513, connected to a nitrogen source (not shown) leads through the manifold component 511 at a low level. Thus, as one of the manifolds 508 comes into register with the conduit 513, nitrogen thus flows along that manifold, thereby to inject nitrogen sequentially only into those distributors 506 which are at their lowermost position during rotation of the furnace, so that the nitrogen passes into solid reagent material located at the bottom of the furnace chamber.

The furnace 500 includes riding rings 512. Unlike the furnaces 10, 100, 200, 300 and 400, the furnace 500 has twelve non-annular or elongate electrodes, arranged in three axially spaced groups 515, 517, 519 each comprising four electrodes. The electrodes of each group project radially inwardly and are spaced circumferentially from one w~ y~ 4 5 ~ 7 ~ "~

~otr.er at angies cf gO~. Each eiecc~ode has a generally squ~re c_oss-section.

Figure 19 shows the four elec~rodes 514, 516, 518, 520 of the group 5i7 of electrodes. The sroup 515 cf electrodes, of which only two S~1, 522 can ~e seer. in Figure 16, is positioned ~ear to the inlet end of the furnace S00 and the group 519 cf which also only two electroàes 526, 528 can be seen in Figure 16, is positioned near to the outlet end of the furnace. The third group 517 of electrodes 514, 516, 518, 520 (Figure l9~ ~ 5 posirioned near to the middle of the furnace S00. The electrode~ in each group are aligned circumferentially while the e}ectrodes of the three groups are aligned in ~he longitudinal dlrection.

Each electrode has an i~er e~d which stands ~nwardly lS radi~lly proud of the refractory c~nc~ete lining, as can be seen in Figures 16 and 18, ~nd each is mounted in a mo~nting bracket 529 Each electrode comprises tWO parts engaged with one another spigot and socket fashion. 3y way of illustration, the ~0 electrode 518 con~ists of an outer part 518.1 and an inner part 51~.2, the outer part 518.1 haYing a spigot portion 525 which is enga5ed, ~y a friction ~it, with a socker por~ion SZ7 in the inner part 51~.2. Thus, ~s the inner part 518.2 is abraded away during use, the outer part 519.1 is pushed progressive7y i~wardly until ~t eYentually replaces the inne_ part 518.2 and a further outer part 51~.1 is inserted behind i.. In this way the electrodes of the furnace 500 are continuous}y replaced.

As msntioned hereir~efore, the furnace 500 is powered ~y a DC electrical supply. Thus, the group 517 o electrodes is maintai~ed at a negati~e polarity, while the ~roups 515, 51~ cf eiectrodes are maintained at positi~e polarity so that ourre~t flow is towards the ceneral group 517. The ~ ~, wossl239~8 32 PCTIGss5/~o pocential difference ~etween the gro~p 517 ~nd the groups 515, 519 wil' ~epend on the material with which the 'urnace 500 has been chargec and the process taking place and, _n the case of nitriding ilmenlte in order to recover titanium will typically ~e 350-~oO vol~s.

The f~rnace 5~0, furthert includes a feed chuee 522 which is supplied by a worm feeder, or feed scroll, 40 for extracting pel}ets frcm a pellet supply hopper 44. A lower porticn 524 o~ the hopper 44 which is circular cylindr~cal ~0 ~n shape is surrounde~ by a cylin~rical shell 528 of - rèfractory concrete in a mi~d steel casins 529. The shell 528 has walls 531 and upper and lower inwardly directed annular portions S33, 535 between which extend eight cylindrical silicon car~ide electrodes 530 in a circumferentially spaced symmetrical arra~seme~t. The shell S28 and electrodes 530 act as a shaft-type pre-heater for ~he ~ater~al in the lower portlon 524 of the hopper 44, and, in use, 9er~es to pre-heat the materi~l passing through t~e lower port~on 524 to a temperature of about 800C. The off-gasec from the furnace can, opt~onally, ~e dire~ted to atmosphere in a counter current direction to the material ~n the feed scroll 40, vla the walls of the feed scroll 40 to ensure sustained pre-heating af-the material fed into the furnace.
.
The furnace 500, further, has an outlet 530 leading to a coo}er 532 provided with a worm screw 533, a water sprayer 535 and a sump 537. The worm scr~w 533 feeds ccoled product into a discharge chute 534 pro~ided with a screw conveyor 535 which L~",oves the material when i~ has reac~ed a temperaeure below 200C.

Referri~g to Figure 20, a control sys~em 600 for the ~urnaces operates primar~ly on ~eedback 602 from the chermocouple readings in the furnace, which prov~ted measured values 604. Set values 606 for power and w095/239~8 ~ 5 ~ ~ PCT/Gs tempe~ature are supplied to a control lnstrument 608, whic:~
rece~es also the measured value 604. The contrcl ~nstr~ment 608 is connected to two thyristor dri~es 610 i~
ser~es, each drivins a separate f~rnace segmen~.
Naturally, a 5reater ~umber of the drlves will be pr~vided if there are a greater num~e- of individually dr~ven fur~ace segments. The thyristors are driven by independent ~ransformers 6~2, delta connected on the secondary side of each transformer. The thyristors 610 are connected to the furnaces by means of transformers 614.

-The furnaces 10, lOO, 200, ~Oo, 400 and 500 will naturally include suitable dri~e means for dri~ing the furnace shells to roeate. The dri~e mea~s may include an AC elect ic motor and reduction-gear box with ~ariable speed dri~e, togerher with, for smaller furnases, ~ chain and sprocker mechanism for drlving the casing to rotate, or, for larger f~rnaces, spur gears or driv~n support rollers for driving the sheli or casi~g to rotate.

,~ ~

Claims (20)

1. A furnace characterized in that it comprises a first cylindrical furnace shell located substantially horizontally and rotatable about a rotational axis which thus extends substantially horizontally, the furnace shell providing a first furnace chamber for holding a bed of solid particulate reagent as the first furnace shell rotates; and at least two electrodes exposed to the first furnace chamber and being mounted in electrically insulated fashion therein, with the electrodes being spaced apart so that the solid particulate reagent in the first furnace chamber can be heated up by direct resistance heating thereof, utilizing the electrodes;
a second substantially horizontal cylindrical furnace shell rotatable about the rotational axis and spaced axially from the first furnace shell, the second furnace shell providing a second furnace chamber which is in communication with the first furnace chamber and through which the solid particulate reagent from the first chamber can pass in a longitudinal direction, and at least two electrodes exposed to the second furnace chamber and being mounted in electrically insulated fashion therein, with the electrodes being spaced apart so that the solid particulate reagent in the second furnace chamber can be heated by direct resistance heating thereof, utilizing the electrodes; and drive means for dividing the furnace shells to rotate.

2. A furnace characterized in that it comprises a cylindrical furnace shell located substantially horizontally and rotatable about a rotational axis which thus extends substantially horizontally, the furnace shell having a first segment providing a first furnace chamber for holding a bed of solid particulate reagent as the furnace shell rotates, as well as a second segment spaced axially from the first segment and providing a second furnace chamber which is in communication with the first furnace chamber and through which the solid particulate reagent from the first chamber can pass in a longitudinal direction, with the solid particulate reagent being held in the second furnace chamber in a bed separate from that in the first furnace chamber and electrically isolated therefrom;
at least two electrodes in each furnace chamber and exposed to the furnace chambers, the electrodes in each furnace chamber being mounted in electrically insulated fashion therein, with the electrodes in each furnace chamber being spaced apart so that the solid particulate reagent in that furnace chamber can be heated by direct resistance heating thereof, utilizing the electrodes; and drive means for driving the furnace shell to rotate.

3. A furnace as claimed in Claim 1 or Claim 2, characterized in that each of the electrodes is of annular form and extends circumferentially along an inner surface of the furnace shell while protruding radially inwardly therefrom, with the electrodes in each of the chamber being spaced axially apart.

4. A furnace as claimed in Claim 1 or Claim 2, characterized in that one of the electrodes ('the first electrode') in each of the chambers extends centrally along the rotational axis, with a plurality of the other electrodes ('the second electrodes') being provided, the second electrodes protruding from and extending along an inner surface of the furnace shell in a longitudinal direction, and being spaced circumferentially from one another.

5. A furnace as claimed in Claim 1 or Claim 2, characterized in that a plurality of the electrodes, arranged in pairs, and protruding from and extending along an inner surface of the furnace shell in a longitudinal direction, are provided in each of the chambers, with the pairs being circumferentially spaced from one another.

6. A furnace as claimed in Claim 1 or Claim 2, characterized in that the electrodes in each of the chambers are of non-annular form, and protrude from an inner surface of the furnace shell, with the one electrode ('the first electrode') being spaced in a longitudinal direction from the other electrode ('the second electrode').

7. A furnace as claimed in Claim 6, characterized in that a plurality of the first electrodes, circumferentially aligned and spaced apart circumferentially, and being of the same polarity, as well as a plurality of the second electrodes, circumferentially aligned and spaced apart circumferentially, and being of the same polarity, are provided in each of the chambers with the polarity of the first electrodes being different to that of the second electrodes.

8. A furnace characterized in that it comprises a cylindrical furnace shell located substantially horizontally and rotatable about a rotational axis which thus extends substantially horizontally, the furnace shell providing a furnace chamber for holding a bed of solid particulate reagent as the furnace shell rotates;
drive means for driving the furnace shell to rotate;
a plurality of non-annular first electrodes exposed to the furnace chamber and being mounted in electrically insulated fashion therein such that they protrude from an inner surface of the furnace shell, the first electrodes being circumferentially aligned and being spaced apart circumferentially, and all being of the same polarity;
and a plurality of non-annular second electrodes exposed to the furnace chamber and being mounted in electrically insulated fashion therein such that they protrude from the inner surface of the furnace shell, the second electrodes also being circumferentially aligned and being spaced apart circumferentially, and all being of the same polarity which is different to the polarity of the first electrodes, with the second electrodes being spaced in a longitudinal direction from the first electrodes.

9. A furnace as claimed in Claim 8, characterized in that the furnace chamber is provided by a first segment of the furnace shell, with the furnace shell including a second segment spaced axially from the first segment and providing a second furnace chamber which is in communication with the other or first furnace chamber and through which the solid particulate reagent from the first chamber can pass in the longitudinal direction, with the solid particulate reagent being held in the second furnace chamber in a bed separate from that in the first furnace chamber and electrically isolated therefrom, and with the furnace including a plurality of non-annular first electrodes exposed to the second furnace chamber and being mounted in electrically insulated fashion therein such that they protrude from an inner surface of the furnace shell, the first electrodes being circumferentially aligned and being spaced apart circumferentially, and all being of the same polarity;
and a plurality of non-annular second electrodes exposed to the second furnace chamber and being mounted in electrically insulated fashion therein such that they protrude from the inner surface of the furnace shell, the second electrodes also being circumferentially aligned and being spaced apart circumferentially, and all being of the same polarity which is different to the polarity of the first electrodes, with the second electrodes being spaced in the longitudinal direction from the first electrodes.

10. A furnace as claimed in any one of Claims 1 to 7 or Claim 9, characterized in that the first and second furnace chambers have the same diameters.

11. A furnace as claimed in Claim 10, characterized in that the first and second furnace chambers are of the same length, and in which the spacing between the first and second electrodes of the first furnace chamber is different to that of the first and second electrodes of the second furnace chamber.

12. A furnace as claimed in Claim 10 or Claim 11, characterized in that the or each shell comprises an outer skin, and an inner lining of a non-porous material against the outer skin.

13. A furnace as claimed in Claim 12, characterized in that the non-porous material is .alpha.-alumina.

14. A furnace as claimed in any one of Claims 1 to 7 inclusive, or any one of Claims 9 to 13 inclusive, characterized in that it includes an annular isolating partition between the first and second furnace chambers for effecting the electrical isolation of the solid reagent in the one chamber from that in the other chamber.

15. A furnace as claimed in any one of the preceding claims, characterized in that the or each furnace chamber is closed off from the atmosphere, and which includes feed means for feeding solid reagent thereinto, extraction means for extracting waste product therefrom, gas feed means for feeding gas thereinto and gas extraction means for withdrawing gas therefrom.

16. A furnace as claimed in Claim 15, characterized in that the gas feed means includes a plurality of gas permeable distributors in the furnace shell, the distributors being circumferentially spaced from one another; gas delivery means for delivering gas to the outsides of the distributors, with the gas passing through the distributors into the furnace chamber; and gas flow control means operable, during rotation of the furnace, to deliver gas only to those distributors which are at or near their lowermost position so that, in use, inflowing gas passes largely through a charge of solid particulate reagent in the furnace chamber.

17. A method of carrying out a reaction, characterized in that it includes heating a solid particulate reagent to elevated temperature in the furnace chamber(s) of a furnace as claimed in any one of Claims 1 to 16 inclusive.

18. A method as claimed in Claim 17, characterized in that a further reagent is introduced into at least one of the furnace chambers of the furnace, to react with the solid regent.

19. A method as claimed in Claim 17 or Claim 18, characterized in that the solid reagent is heated to a temperature of 1000°C - 1800°C, optionally after pre-heating it to a temperature of 700°C - 1300°C before introducing it into the furnace chamber.

20. A method as claimed in any one of the Claims 17 to 19 inclusive, characterized in that it includes, prior to introducing the solid reagent into the furnace chamber, pre-mixing it with a particulate solid electrical conductor to form a particulate mixture which is then fed into the furnace chamber, optionally after consolidating the particulate mixture by pelletizing, extruding or briquetting it.