CA2001832A1 - Process for calcining sludges of calcium materials and apparatus for carrying out the process - Google Patents

Abstract

ABSTRACT OF DISCLOSURE

A process and apparatus for thermal processing material especially sludges of calcium bearing material such as the recovery of lime sludge formed in causticizing of alkaline digestion liquors employed in the pulping of wood fibers. The process includes a suspension preheater and a suspension cooler and an apparatus for flash drying and calcining the material while in suspension in hot combustion gases. The calcining furnace itself includes a lower inlet for fuel to allow the material to be exposed to high temperature flame additional thermal energy supplied to the calcining vessel by separate combustion chambers close coupled at the upper levels of the calcining furnace. The gas flow in the calciner is helical.
Various points for introduction of fuel and material to be processed ace described.

2993S/bir-5

Description

~0~ 32 PROCESS FOR CALCINING SLUDGES OF
CALCIUM MATERIALS AND APPARATUS POR
CARRYING OUT THE PROCESS

BACKGROUND OF THE INVENTION

This in~ention relates to a proces6 and apparatus for thermal processing materials including specifically the calcining of lime sludge~which may be produced in the pulp and paper industry, wa~er treatment systems and in some cases in the sugar industry. The ap~aratus may also have application in the thermal processing of various ores such as gold ore or fertilizer material~ to drive off undesired compounds.

Lime sludge is a precipitated calcium carbonate in water treatment systems and CaC03 ~ludge p~oduced by the reaction of CaO or Ca(OH) in reaction with proce~s materials in the pulp and paper and sugar industrie~.

In the pulp and paper industry the CaO may be 61aked to . :
produce calcium hyd~oxide. The Ca(OH)2 i~ Leacted with Na2CO3 ~o Lecover sodium hydroxide which i8 then reused in the digestion operation o~ the pulping proces~. The reaction is a follows:

Na2C03 ~ C~O + H20 3 Since a continuous supply of CaO or Ca(OH~2 i~ required by various indu~tries, the resulting CaC03 sludge produced in 2993S/bir-5 -1-the pcocess i5 p~esently ~egeneLated to CaO in two known pieces of pcocess equipment. i.e. fluid bed ceactor~ and rota~y kilns. The cotacy kiln is the main piece of equipment for cegeneration of CaO. The ~eaction desi~ed is as follows:

CaC03 ~ CaO +CO2 I~ all cases the CaO generated must meet certain specifications for ceuse in the va~ious pcocesse~. Areas of major concern will be lime reactivity and availability, i.e., the amount of CaO that will react with watee to produce Ca(OH)2 and the rate of the reaction. Reacti~ity and availability is affected by heating including both the tempecature to which the material~ i5 exposed and the duration of exposuce and by the contaminants which may be contained in sludge that will reduce the potential CaO available in ~laking to Ca(OH) . As is genecally known, thssQ contaminants o~
impurities may be in the foLm of iron, ~iliaa, aluminu~ and sodium compound~.

.i The ~o~t com~on technique for calcining lime sludge has baen the use of a rotary kiln; 6ee for example U. S. Patent No.

2,178,586 issued Novembec 7, 1939. A modern kiln in6tallation will include a rota~y drum fil~er for increasing the percentage ,solids in the sludge ~o be regenerated or calcined. The dewateeed ma~erial i~ fed to a long kiln at the upper inlet oc feed end and fuel usually in the form of natural gas or oil is supplied to the lower discharge end of the ~iln ~or combustion therein for supplying the thermal energy necessa~y for drying the dewaeeced sludge and calc;~i~g the material to produce CaO. Material and gas flow ar~ counter-current ~o each other.

2993S/bic-5 -2-~)01~33~

The matecial discha~ge f~om the kiln may be cooled by use o~
various appa~atus such as attached tube coole~s and then conveyed to storage until the calcined lime is requi~ed for use in the process. The high capital investment and large space requi~ements as well as high fuel consumption rates ace d~awbacks of cotary kiln oparations.

A~tempts to utilize fluidiæed bed reactors for calcining materials have not always p~oved successful ~or operational conditions encountered on a commercial basis.

The invention desccibed herein i5 a 6uspension type calciner which will produce a high quality product at a saving of energy and capital investment ove~ both rotary kilns and fluid bed equipment for carrying nut similar p~ocesse~. Fuel consumption of both fluid bed apparatus and cotary kiln~ for calcining lime sludge a~e in the range of 6.0 to 12.0 x 106 BTU/ST of CaO with an industry avecage o~ ~ x 106 BTU/ST. It is anticipated that a commercial size apparatu~ for carrying out the proces~ accocdin~ to the present inverltion will be capable of producing CaO with a fuel consumption in the cange of ~.3 to 4.6 x ~o6 ~TU~ST. Power con~ump~ion of the overall sy~em is expected to be approximately 10% to l5% less than the systems presently in commercial u~e in the pulp and pape~
i ndus t ~y .

Apparatus for calcining calcium bea~ing materials while the material i8 in suspension in the hot ~as ~tream have been utilized in va~iou6 proces6es~ mos~ notably the cement manufacturing pro~e~s. Cement ~aw mateLial which utiliz~

2993S/bi~-5 _3_ 0~332 limestone as one of its primary ingredients has been thermal pcocessed in appa~atus including a suspension p~eheater ~ollowed by a stationacy ~uel fired furnace foc calcining the cement raw meal while it is in suspension within the stationary fu~nace and a rotary kiln for sintering or clinkering the thus calcined cement ~aw meal. The clinkered material is cooled by ambient air whe~eby the ambient air is heated by the hot clinker and used for combustion in both the rotary clinkecing kiln and the sepa~ately fired suspension calcining furnace.
The exhaus~ gases from the kiln and the calcining furnaca are used in the suspension pfeheater for preheating the caw material. An example of such a system i8 shown in U. S. Patent No. 4,381,916 issued May 3, 1983.

It is also known to thermally pLoce~s calcium bearin~
materials while that material i8 in suspen~ion in a stationa~y calcining furnace with the use of a suspension ~ype p~eheatar and a suspension type cooler. Such an apparatus i8 al~o ~hown in U. S. Patent No~ 4,3~1,916 including speciically Fig. 5 o~
that ~at~nt and in U. S. Patent No. 4,483,831 issued November 20, 1984 and other publications. In flash calcining apparatus, the gas and material f 1QW may be generally counter current or co-current. Neither of these patent~ is directed to the calcining of sludge mate~ials while in suspension.

Sludge bearing materials such as those utili2ed in the pulp and paper industry have a much finsr particle size when d~ied than are encountered in either the calcining of lime stone materials which will have a pact~cle SiZQ in the range of 100%
smaller than 20 mesh (~50 micron) and limestone contained in 2993S/bir-5 _4_ 21~ 33Z

cement caw meal which will have a particle size in the range of 80~ smaller than 200 mesh t75 miccon). In lime sludge applications, the particle size of the dcied mateLial will be 100% smaller than 200 mesh, 99% smallec than 325 mesh (45 micron) and the bulk of the mate~ial may be in the 15 to 30 miccon size range.

It has been known to utilize su~pension type calciners for calcining sludge materials. An early such arrangement i~ shown in U. S. Patent No. 2,505,617 issued April 25, 1950. In this arrangement there is shown a cage mill for drying the sludge material which is then finish dried and preheated in the flue gase6 discharged from the calcining furnace. Material is then supplied to the calcining furnace from which the product is discharged by conveyors. Other su~pen~ion systems for calcining sludge materials are shown in U. S. Patent Nos.
4,098,871 issued July 4, 1978~and 4,201,541 i~ued May 6, 1900, but it is not believed that these devices take full advantage of the available heat in the sys~em and further, it is believed that they will re~ul~ in overbu~ning the material.

In the thecmal processing of certain matecials, it i6 important to compleee the desired thermal proce~sing while at the sa~e time avoid overbu~ning the material. For example, in the calcining of calcium bearing materials such aæ li~e sludge, the fine size of the particles ~esults in a greater surface area for a given quantity o~ material. I~ this sur~ace a~ea i~

exposed to flame, it can be easily overburnt, particularly if the material i~ exposed to the high tempe~atuLe ~lame after calcining is already underway. If the ~a~erial is oveEburnt, 2993S/bir-5 -5-when it is attempt~d to slake o~ ~eact the CaO with wate~ to produce the desired calcium hydcoxide the overburnt material is less reactive as ~he water cannot penetrate the surface of the mate~ial. Consequently, the product i6 les~ desirable fo~ its ultimate use due to its low reactivity. On the other hand, if the material is underburnt~ the product is less than desicable ~ue to the unavailability of Cao, i.e, some of the CaC03 is not conver~ed to CaO. The loss on ignition (LOI) is highe~
than desirable. Therefore, it is impoLtant to expose the material to sufficient heat to achieve substantially complete calcina~ion of the material without exposing the material to too high a temperature which would resul~ in overburning the material. Oveeburning can occur if the material i8 exposed to an initial high temperature as by passing through a flame and then subsequently adding additional fuel to the calciner so that the material again passes through a flame. In designing a system for thermal proces~ing sludge mate~ials, it i~ importan~
to conside~ factors such a~ whether the system can operate on a sus~ained basi~ handling materials which a~e chemically contaminated, whether the ~ystem can be shut down and ~estacted in a short peciod of time, whether the system can be put on "hold" while repairs to auxiliary equip~ent ace being made.
Another factor to consider is cefractory life.

.

SUMM~RY

It is therefore the principle object of this inventio~ is to pLovide an economical process for ther~al pcocasging materials such as ~ludges of calciu~ bearing material6 which will cesul~ in ~he production of a product having a low los~ on ignition while avoiding being overburnt.

Z993S/bir-5 -6-8~

It is a further object of this invention to pcovide an apparatus f OL caccying out the process of the present invention.

In gene~al, the fo~egoing and othe~ objects of this invention will be carried out by pcoviding in a pcocess for calcinînq sludges of calcium bearing matecials including the steps of ~lash d~ying the sludge to produce a dried material and ~eeheating the d~ied material while the material is suspended in a hot gas stream, calcining the dcied and pceheated matecial while in suspension in hot combustion gases within a vectically ociented calcining vessel having a lower end and an upper end, and cooling the calcined material, the impcovement compcisinq introducing fuel and pceheated material to be calcined into the lowec end of the vessel, inducing and maintaining an upwacd helical flow of ga~e~ in the ves~el, dischacging calcined matecial and spent combu~tion g~se~ from the uppec end of the ve6sel, sepacating the calcined material fcom the combustion gases befoce the step o~ cooling the calcined mate~ial, cooling the calcined mateLial while suspeQded in ambient air wheeeby the ambient aic i8 heated by the calcined :matecial, and supplying the thus hea~ed ambient air to the lower end of the vessel ~o secve as pceheated aic for combustion in the ves~el.

The objects of the inven~ion will al~o be caLcied out by providing an appacatus foc thermal pcocessing materials while suspended in hot combustion gases compri~ing an elongated ve~sel having a lowec end and an uppec end, the lower end o~
said vessel having an inlet fo~ ~uel and an inlet for ai~ ~o~
combustion ~a6es whereby combu~tion o~ the fuel takes place in 2993S/bic-5 -7-~()0~L~il32 the vessel; means for supplying mate~ial to be processed to the lower end of the vessel; the uppec end o~ said vesseL having an outlet ~oc spent combustion gases and p~ocessed material whe~eby the flow of material and combustion gases is co-curcent from the lowe~ end to the upper end of the vessel; at least one combustion chamber having an inlet ~or fuel and an inlet for air fo~ combustion and an outlet for hot combustion air flow connected to ~aid vessel at at least one location intermediate the lower end and the upper end of the ves~el; means for establishing and maintaining a helical flow of gas in said vessel: and a gas solids separato~ flow connected to the outlet of said vessel for separating proce6sed material from the spent combustion gas and havlng an outlet for spent combustion gas and an outlet fos processed material.

The equipment of the present invention includes a rotary drum filter for dewatering the sludge material to a moisture content of between 20~ and 40% thereby increasing the solids content of the material to be processed. A flash dryer dries the thus dewatered material while sus~ende~ in hot gas supplied ~rom the calcining and preheating section6 of the appaLatus.
The invention ~u~ther includes one o~ more ssrially connected cyclone separators which utilize ~pent combustion gas f~om the calcine Eo~ preheating the dried material. A two stage preheatee should be designed so that the material discha~ged from the ereheate~ for supply to the calcinar will have a temperature in the ~ange of 750 F. to 1100 F. The calcine~ vessel will preferably be an elo~gated vertically oriented vessel with fuel injected at the bottom or lower end of the calciner. The temperatuee range within the calciner 2993S/bir-5 -8-8~;~

should be maintained between approximately 1600 F. and app~oximately L8000 F. with the pcesent invention, burners and hot combustion gas inlets will be provided with gene~ally tangential inlets to maintain a cyclonic or helical flow through the elongated calcining vessel. In the pre~erred form, the hot gas and uel inle~s to the calcining vessel will be at an angle o~ 20 to 40 from tangential in order to maintain the desired helical upward flow of gas through the calcining vessel. The matecial will be calcined while in suspension in the hot gases of combustion. In a preferred focm, a mechanical impellee or spinn~c is p~ovided in the bottom of the calcining vessel foL establishing the initial helical flow of gases.
Alternately this helical flow can be established by a tangential inlet for preheated air for combustion.

The calcined product is discharged fcom the upper end of the calciner entrained in spent combustion gases and supplied to a collection ayclone which will separate and collect the solid calcined product fcom the hot spent aombu~tion gases.
The sepacated hot ga~es are then supplied to the preheater.

Hot pcoduct from the gas-solids separator or collection cyclone a~ the outlet of the calcinec is supplied to a suspension ~y~e cooler which may consist of one or moce secially connected cyclones. This coolec will reduce the product temperatuce to the range of 200 to 450 F. with a preferred temperatuce in the range of 250 to 350 F.

Coolec off gases which aLe heated by the hot pLoduct will be used as combu~tion aic in the calcining vessel.

2993S/bir-5 -9-20~18~2 If desi~ed, so~e o the calcined material from the co~lection cyclone may be ~eci~culated to the calcining vessel in a manner beoadly similar to the ~eci~culation disclosed in U. S. Patent No. 4,381,916. This may incl~de an adjustable splitter installed in the solids discha~ge leg of the collection cyclone to allow a recirculation rate of up to eiqht tO one of product to the ~lash calcinec.

The cooled product will go to storage silos or in some cases directly to lime slakers. Off gases from the top of the flash dryer will go to a high efficiency dust collection system such as a scrubber, bag house or electrostatic precipitator to collect dust to be returned to the vessels.

The apparatus used for carrying out the prsces~ of the present invention will require less land space when com~ared to prior processes such as the rotaLy kiln pcoces~. This can be particularly important in a pulp and paper plant where land space may be a~ a premium.

Th~ proce~s of the present invention iR believed to have ~lower energy regui~ements when compa~ed to existiny processej including both fuel consumption and electric power u~age. It is also believed ~hat the present invention will permit higher ca~acities in single lines compaeed to other systems and will require less capital investment on an installed basis fo2 equivalent capacity.

Further advantages of the present inven~ion include the ability to handle chemically contaminated liquor bettee than 2993S/bir-5 -lO-~0~8~

rotary kilns and 1uid bed processes. Carbon con~amination f~om black liquo~ in the paper industry should not affect operation. It is believed that refractocy life will be impLoved with the ~rocess of the pcesent invention.

Another advantage of the present invention is that it has the capability of sho~ter shutdown and cestaLt time including initial start up time. This is also advantageous so that the system may be put on hold while repairs are made to associated equipment without loss of product quality.

BRIEF DESCRIPTION OF THE VRAWINGS

The invention will be desc~ibed in connection with the annexed drawings wherein:

Fig. 1 is a diag~amatic view oE the apparatus of the present invention used foc carrying out the proce~s of the present invention;

Fig. 2 is a diagcamatic ~iew of an alternate embodiment of a flash calcining furnace utilized in the presen~ invention;

Fig. 3 is a diagramatic view of a calcininq vessel according to a furthec modified version of the present invention: and Fig. 4 is a vie~ of a still ~urthec modif ication of the calcining vessel of the pcesent invention.

2993S/bi~-5 -Ll-8;~:

DESCRIPTION OF THE PREFERRED EMBODIMENT

ReferLing to the drawing, the invention includes a vertically oeiented calcining system particularly designed for ca~cying out a process for calcining sludges of calcium bearing matecials such as the lime sludge of a pulp and paper manufacturing operation.

Sludge to be calcined i6 supplied from a source (not shown) such as the ~ecausticizing a~ea of the pulp mill to a rotary drum filter generally indica~ed at 1 and well known to those skilled in the art. The drum filter applies a vacuum to a layer of wet material to withdraw moisture from the sludge and produce a product having a moisture content of between 20% and 40% with a moisture pe~ cent prefer~ed to be on the orde~ of 25% or les6. The dewatered material i8 ~upplied to a conveyor 3 from which it is supplied to a ~urge bin 4. The surge bin compensates for variations in the eed ~uch as short peciods of shut down of the ilter 1 for maintenance purposQs such as washing the filter. A conduit 5 supplies material ~rom the surge bin 4 to a conduit or duct 6 thLouqh a valve 7 capable of handling a high moistuce material. The valve 7 may for example be of the type shown in U. S. Pa~ent No. 4,497,122 issued February 5, 1985. The duct 6 supplies gas of ~elatively high temperatu~e on the order of L000 F. for entraining and conveying sludge ~o a flash drying cyclone 10. A comminuting device LL may be provided in the conduit 6 in the focm of an apparatus which allows a f~ee flow o~ gas through the comminuting device 11. Any lump~ of mate~ial which are supplied through conduit 5 to conduit 6 which a~e too large to 2~93S/bic-5 -12-2~ 332 be entrained in a hot gas f low will be b~oken up by the comminutec ~ so that they may be entrained in the gas stream in conduit 6 and conveyed to the drying cyclone lo.

In the conduit 6 and flash dryer 10, the moist material is dsied to a powder with moisture content of approximately 0.5-1.0%. The cyclone 10 includes an inlet lB for gas and entcained sludge and an outlet for gas connected to the conduit 13 and an outlet 19 for dried material flow connected by conduit 21 to a conduit Z3. Thus the dryer 10 serves to carry out the process step of dLying the sludga while the sludge is in suspension in hot spent preheater gases. Exhaust gas from the flash dcyer 10 is supplied through conduit 13 and fan 14 to a high efficiency dus~ collector 15 ~uch as a scrubber or fabric filter bag house with the cleaned air being supplied to a stack 16 for exhaust to atmosphere.

The dried material disch~rged from dcier 10 through conduit 21 is entrained in hot ga~ flowing through conduit 23 and admitted to a cyclone 25 having an inlet 26 for gas and entrained pulvurulent material and an outlet 27 for sepa~ated gas and an outlet 28 for preheated material. The material discharge from cyclone 25 is supplied ~o a conduit 30 where it is again entrained in hot gas and supplied to a cyclone 32 having an inlet 33 for gas and entrained m~te~ial, an outlet 34 for gas and an outlet 35 for preheated material~

The pair of serially connected cyclones 25 and 32 and the associated duct work 30 and 23 constitute a preheater 36 fo~
pLeheating the deied material while suspended in a stream of 2~)0~L8~

hot gas. It should be understood that the prehea~e~ may be a single stage OL cyclone oe moce than two stages may be used.
The flow of material in the peeheater is generally counte~
cueeent to the flow of hot gas. Thus the peeheatee 36 includes an inlet (conduit 21) ~oe dried mateeial to be processed, an outlet 27 for spent preheating gas and an outlet 35 for preheated material.

The apparatus of the present invention includes a vertically ociented elongated calcining furnace generally indicated at 42 and defined by a vessel 43 having a lower end 44 and an upper end 45. The lower end 44 has a tangential inlet ~7 for fuel, and an inlet 49 foe air for combustion in the vessel 42 whereby combustion of the fuel takes place in the vessel 42. The lowe end of the vessel also includes means oc inlet ~8 for supplying preheated material to be processed to the lower end of the vessel. This inlet is flow connected to the outlet 35 of the preheatee by conduit 40.

The upper end 45 of the vessel 43 has an outlet 51 (peefeeably tangential) for spent combustion gases and ~rocessed material so ~hat the ~low of combu~tion gases and entrained material is co-cureent from the lowee end 44 of the vessel 43 to the uppee end 45. While the peeheated ~ine material is suspended in the hot combustion gases~ it is calcined.

A gas-solids sepa~ator or collecting cyclone 55 has an inlet 56 for gas and entrained proce~sed mate~ial an outlet 57 for saparated gas and an ou~let 58 foe processed solid 29g3S/bir-5 -14-2~ 32 material. The cyclone 55 is flow connected to the outlet 51 of the calcining vessel 43 by conduit 52. The outlet 57 foc gas is flow connected to the pceheate~ 36 by duct 30 and this duct 30 defines the inlet for spent combustion gas of the pLeheatec flow connected to the outlet 57 for spent combustion ,gas of the gas-solids separatoc 55.

A material cooler is generally indicated at 65 and is a device foc cooling the matecial by suspending it in ambient aic and is shown as a paic of serially connected cyclones 66 and 67 each having an inlet foc gas and entrained matecial, an outlet for sepacated solids and an outlet foc separated gas. Ambient aie is supplied fcom atmosphere by means of a fan 68 through a conduit 69 to the lower c~clones 66. A duct 71 intecconnect~
the cyclones the outlet for .se~arated ga~ o cyclone 66 with the inlet for gas and entrained ~olids of cyclone 67. The solids outlet of cyclone 67 is connected by duct 74 to conduit 69. Hot proce~sed matecial discharged f~om ga~ ~olid sepacator is 6upplied through conduit 62 to conduit 7L where it i6 entrained in cooling ga~ being conveyed thLough conduit 71 to cyclone 67 wheceby the product is cooled by ambient air and the ambient air is heated. The cyclone 67 separates the product from the cooling gas and supplies it thlough an outlet 74 to conduit 69 where it is again ~ntrained in the cooling gas further cooled and conveyed to cyclone ~6. The cyclone 66 dischacges pcoduct or peocQ6sed matecial through outlet 76 and pceheated cooliny gas through conduit 71. The cyclone 67 discharges thus preheated air foc combustion through an outlet 77 to the inlet 49 for aie for co~bustion of the calciner furnace 42. Thus, the coole~ 65 haM ~n inlet tdefined by 2993S/bir-5 -15-~0~8~

conduit 62 supplying mateLial to conduit 71) ~or material flow connected to the outlet foc erocessed matecial of the gas solids separatoc, an inlet 69 for ambient air, and an outlet 77 heated ambient ai flow connected to the inlet for air ~oc combustion at the lower end of the vessel.

A mechanical spinner of any suitable type is provided at 80 for inducing and maintaining a helical motion to the aic foc combustion. Those skilled in the art will know how to design such an apparatus for imparting a helical motion to the preheated air for combustion, but such a device may take the form of stationary helical vanes (not shown) on the inside of the duct, an impeller which is rotated by an exteLnal motor, oc othec suitable means.

The process o~ the present invention also includes recirculating a portion of at least partially calcined material back to the calcining fu~nace ~2. This process is carried out by providing a splitter valve 59 at the outlet 58 o~ cyclone 55 and a conduit 60 connecting cyclone 55 to the lower end 4~ of the ve~sel 43. The splitter 59 controls the flow of matecial to either conduit 60 oc conduit 62 with the usual practice being to supply part of the material to conduit 60 for recirculation to the calciner for ~u~ther calcining and the balance of the matelial is discharged ~o coolar 65 through conduit 62; see generally U. S. Pa~ent No. 4,38~,916 foc the recircula~ion of material in a flash calciner and the benefits obtained in a cement and limestone application. Prio~ to th~
present invention i~ is not believed that cecirculation was even at~empted wi~h the fine materials encountered in sludges 2993S/bic-5 -16-33~

as it was belieVed that such ~ecieculation could cesult in oveebu~ning.

With the present invention, the pce~erred focm of supplying the~mal energy to the calcining furnace is in the form o~
direct injection of fuel to the lowee part of the calcining furnace with ma~e~ial supplied through conduit 40 to the calciner above the fuel inlet 47. The preheated matecial will d~op down near the flame generated by the fuel injection in lower cone 47a and is initially contacted by the high tempecature associated with direct combustion within the calcinee. While the inlet for peeheated mate~ial is above the inlet for fuel, because of gravity flow, the preheated material may tend to drop through the flame generated by the injection of fuel within ~he calciner. This contact with the flame i~
believed to cause a peompt calcination of at least the sueae of the material. The inner core of the material is processed by maintaining the calciner at the desieed temperature so that the eesidence time of the mateeial in ~he high temperature vessel completes the peoce6s.

With some materials it may be necessary to add additional heat to the materials to calcine the innee core of the mateeial, and not rely solely on the combustion of fuel at the loweL end of the vessel. In some cases, additional thecmal energy ~an be supplied in the form of direct injection of small amounts o~ ~uel to the vessel ~3 at points in~eemediate the lower fuel inlet and the upper outlet 51. With othec mate~ials, it is desieable not to provide additional high tempeeature flame contact foe ~he ma~eeial in order to avoid 2993S/biL-5 -17-o~eLburning the su~face o~ the mate~ial. With these materials fuel is not di~ectly injected into the vessel at the upper levels. Therefore, with the p~esent invention, exte~nal combustion chambecs 90 are pcovided to supply additional thecmal energy between the lower end 44 and upper end ~S o the calcining vessel. ~oth fuel as indicated by the solid line aLcows 91 and ai~ for combustion as indicated by the dotted lines 92 a~e supplied to each of the combustion chambecs 90.
The close coupled combustion chambeLs a~e mounted so that hot gases of combustion a~e injected into the caLcining vessel ~6 at vertically ~paced apart points above the fuel inlet 47 and the matecial inlet 48 of the vessel 43. It has been found that it is impoctant to maintain the helical flow of mate~ial through the ves~e~ 43 which wa~ initially established by the spinner 80 and this is maintained by having the hot combustion air supplied to the calcine~ by sub~tantially tangential inlets as illust~ate-l in Figs. l to 3. Prefecably, these inlet~ may be at slight angles such as 20 to 30 to the tangential.
The burner ~7 should also be po6itioned tangentially o~ nearly so at 20 to 30 ~com tangential. This helical flow of hot gases pcevents the material f~om sticklng to the sides of the calcining fucnace. Furthe~, it has been ~ound that with the helical flow and injection of hot combustion ga~as rathe~ than the use of flame, tha~ a product loss on ignition or LOI of between 1% and 2% can be maintained.

The use of extecnal combustion chambecs allows g~eatec control ovec the quanti~y of thermal ene~gy that can be supplied to the upper levels of the ve~sel compared to the direct injection of fuel into the ve6~el and make6 it easie~ to 29g3S/biL-5 -18-%O(:JlB~2 maintain a uniform temperatuLe throughout the vessel 42 thereby achieving moce uni~ocm calcining o~ the matecial as a whole.
Thus, with separate co~bustion chambers, maximum thecmal energy can be supplied by ~uel inlet 47 and supplemental thecmal energy supplied at the upper level~ but in some applications it may be desirable to add the majority of the thermal energy at the upper levels. This configuration allows the necessary flexibility to achieve optimum operations.

With the pcesen~ invention, it has been found desirable to maintain the tempecature within the calciner approximately between 1600 F. and 1800 F. This also includes maintaining exit gas tempeLatUre at outlet 51 in the ~ange o~
approximately 1650 to 1750 F. P~oduct discharged from cyclone 55 will have a tempçrature on the ocde~ of 1500 to 1700 F and ideally approximately 1600 F. The temperature at gas outlet 34 will be on the order of 1100 to 1540 F.
with a preferred range of 1300 to 1470 F. and the gas exit temperature at 27 will be approximately 1000 F. These temperatures are accomplished by maintaining combustion tempecature~ in the auxiliary combustion chambers 90 on the order of 2000 F. to 2500 F. Thecmal couple6 (no~ shown) may be provided in each of the combustion chambers to provide for proper contcol of the temperature ~ithin the calcining furnace 42. It i6 to be understood that the temperatures given are for calcining lime sludges and that for theLmal processing of other ores, some other ~emperature may be appropriate and those skilled in the art will be able to achieve the desired tempsrature through routine sxperimen~ation.

2993S/bir-5 -19-~0~ L8~:~

Also, with the calcining o~ ~ine lime such as contained in calcium bea~ing lime sludge materials, it has been found that ~he oxygen content within the calcining furnace should be maintained approximately in the range o~ 0.5 to 2% whiLe the oxygen content in the duct 52 should be maintained between approximately 0.5 and approximately 1~.

The product temperature may be in the range of 200 tO
4S0 F and ideally between 250 to 350 F.

Modifications to the present invention are illustrated in Figures 2, 3 and 4. In Figure 2, rather than supplying the preheated material directly to the calcining fucnace 42, the preheated mate~ial is supplied through condui~ 140 to an inlet ~49 for hot preheated air foc combuRtion from the cooler 65~
The pceheated air foL combustion and en~rained preheated mateLial may be suppliéd through a tangential inlet into the calciner 143 to establish the helical flow of gas and material upwardly throuyh the vertical vessel 143. The ve~sel may have fuel injected at 147 and additional combustion chambers 190 for maintaining the helical flow of gases upwardly through the vessel. A tangential outlet 151 may be provided.

Referring to Figure 3, in this embodiment an upwardly tapered calcining vessel 243 is provided to improve material residence time. The arrangement of the ~eed inlet and fuel inlets may be similar to that shown in Figure 2 with preheated air for combustion being supplied to tangential inlet Z49 and fuel being supplied through inlet 247 and auxiliary oc additi.onal thermal ene~gy being supplied through tangential inlet~ from combustion chambers 290.

2993S/bir-5 -20-83~

ReEe~ring to Figu~e 4, in this modi~ication the calcining vessel may be similar in shape to that shown in Figuce L with a conical bottom 347a and an inlet 347 foc fuel and a mechanical means 380 Lor establishing a helical flow of ~reheated air foc combustion ~rom combustion air inlet conduit 377. In this case the preheated material to be processed is supplied thcough conduit 340 from the p~ehea~er 35 at a location above the fuel inlet 347 wheLeas the recycle material f rom condui~ 360 is supplied to a point above the inlet for preheated air for combustion and below the inlet 347 ~or fuel. In this embodiment additional fuel is added at the bottom of the vessel as at 365, upstream in the direction o gas f low of the inle~
348 for ereheated material and downstLeam of the inlet 377 for preheated air for combustion. This ~uel inlet will supply between 25% and 50% of the total fuel ~equicement~ of the calciner. Recirculated material inlet ~60 may be located near this fuel inlet to dampen the 1ame temperature at thi~ point . It is believed that this a~rangem0nt will improve the thermal pcoce3sing of the matecial. Outlet 351 is providad for calcined material and spent combustion ga6.

Exam~e Typical lime sludge pulp and papeL filtec product ~ed ~o rota~y kiln ~or regeneration proce~s obtained at 32.75 moistuce.

2993S/bir-5 -21-2C~ 33~

Chemical Analysis sio2 0.56 2 3 . 4 Fe23 0.14 CaO 53.35 MgO 0.76 K2O 0.02 Na20 0.82 so3 0.25 P205 0.03 Tio2 0.02 LOI @ 900 C 43.41 Cl . 0.016 2 41.55 Total Los~ Free CaO 94.27 Size Analy6i~ - 100% passing 200 me~h ; 99~ pas6inq 325 mesh 50~ passing 30 mu 10~ pa6sing ~ mu BUlk Density - Loose: 41.28 lbS/cu. t.
Tapped down: 67.S2 lbs/cu.ft.

Specific Gravity ~ 2.63 2993S/bir-5 -22-zq3~332 The material as p~ocessed in flash calciner system utilizing a single stage pceheatec, two stages of cooling and a calcinec with multiple vertically spaced ~uel inlets and a collection cyclone ~eoduced a pcoduc~ with:

L.2% Residual CaCO3 91% Availability (out of potential 94.25%) ASTM C-llo Hydration test: 40 C rise - 1.25 min.
Particle Size: 99~ - 325 mesh The temperature within the calciner was ~aintained in the cange of ~700 to 1800 F. The preheated feed material was supplied to the calciner at a temperature of approximately lZ00F. The pcoduct wa~ dischaeged at a temperature of approximately 325 F. Exit gas ~emperatures were maintained at approximately 250 F. Air flow into the system was maintained as necessaey to obtain combu~tion o~ the fuel and has a exit gas 2 content at 4~. The ~uel requirements for the pcoce6s were met by 70S o~ the theemal energy supplîed to the lowest burner 47, 30% to the upper combustion chambers, divided equally between the chambers 90.

From the ~oregoing de~cription the process o~ the present invention should be apparent. Sludges of calcium bearing materials aLe flash dried by hot spent combustion/preheatinq gases in cyclone dcyer 10 to produce a dried material. The dried material is pceheated in pceheater 36 while suspended in hot combustion gases f rom calciner 42. The material is calcined in the vertically oriented calciner 42 then cooled in cooler 65 while suspended in cooling gas. The improved pLocess incLudes inteoducing fuel, pceheated matecial to be calcined and air foc combustion to ~he lower end 44 of vessel 43. ~n upwaed helical flow of gases is induced and maintained by 2993S~bie-5 -23-L8~Z:

spinner 80 o~ tangential gas inlets (Figs. 2 and 3) and tangent.al gas outlets. This helical gas flow is also maintained by tangential fuel inlets 47 and tangential inlets ~oc hot gases of combustion fcom chambers 90. Calcined matecial and spent combustion gas is discharged ~rom the vessel ~3 th~ough tangential outlet 51. The calcined material is then separated from the hot spent combustion ga~ before the material is cooled. In one embodiment, additional thermal energy, in the focm of hot combustion gas~ is added to the vessel at at least one point 90 spaced down6tceam in the dicection of gas ~low from the point 4a at which p~eheated matecial is introduced into the vessel.

From the foregoing, it should be apparent that the objects o~ the invention have been ca~ried out. A syet~m hae been provided which is capable of economically producing a proce~sed product such as calcined lime from ~ludges ~rom a pulp and pape~ operation. An appa~atus and proc~6s ha~ been pcovided which permits the ultimate product to be controllad by means o~
controlling the additional thermal energy supplied ~o the calcining ve~el. The initial calcining is achieved by direct injection o~ fuel into the calcining furnace in the aeea below the inlet for matecial. The final pcocessi~g of the material is accomplished by the addition of thermal enecgy in the form of hot gases of combu~tion oc the alternate form of additional small quantities o~ ~uel added to the calcining furnace downstream in the direction of gas ~low. I~ ~uLthec processing is required, recirculation o~ at least pactially t~eated material may be accomplished through a r0cycle operation.

Because of the ability to control the calcininy process and 2993S/bic-5 -24-200~B3Z

because of the intimate mixing of the ~ine material with the hot combustion gases, a the~mally economical system may be provided.

It is intended that the focegoing be me~ely a description of a preferred embodiment and that the invention be limited solely by that which is within the scope of the appended claims.

Z993S/bi~-5 -25-

Claims (28)

1. The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
   l. In a process for calcining sludges of calcium bearing materials including the steps of flash drying the sludge to produce a dried material and preheating the dried material while the material is suspended in a hot gas stream, calcining the dried and preheated material while in suspension in hot combustion gases within a vertically oriented calcining vessel having a lower end and an upper end, and cooling the calcined material, the improvement comprising introducing fuel and preheated material to be calcined to the lower end of the vessel, inducing and maintaining an upward helical flow of gases in the vessel, discharging calcined material and spent combustion gases from the upper end of the vessel, and separating the calcined material from the combustion gases before the step of cooling the calcined material, cooling the calcined material while suspended in ambient air whereby the ambient air is heated by the calcined material, and supplying the thus heated ambient air to the lower end of the vessel to serve as preheated air for combustion in the vessel.
2. 2. In a process for calcining sludges according to claim l further comprising the step of supplying additional thermal energy to said vessel at at least one point spaced downstream in the direction of flow of the gases and material from the point at which preheated material is introduced into the vessel.
3. 3. In a process for calcining sludge according to claim 2 wherein said additional thermal energy is supplied by adding hot combustion gases from a combustion chamber positioned external and flow connected to said calcining vessel.
   
   2993S/bir-5 -26-
4. 4. In a process for calcining sludges according to claim 3 further comprising the step of controlling the temperature of the hot combustion gases supplied to the vessel at said vertically spaced apart points.
5. 5. In a process for calcining sludges according to claim 4 wherein said combustion gases are supplied to said vessel in a direction to assist the helical flow of gases in the vessel.
6. 6. In a process for calcining sludges according to claim 1 further comprising the step of maintaining the temperature throughout the vessel at a temperature of between approximately 1600° F. and approximately 1800° F.
7. 7. In a process for calcining sludges according to claim 6 further comprising the step of supplying additional hot combustion gases to the vessel downstream in the direction of gas flow, said hot combustion gases have a temperature approximately in the range of between 2000° F. and 2500° F.
8. 8. In a process for calcining sludge according to claim 2 further comprising the step of recirculating a portion of the separated calcined material to the vessel for further exposure to the hot combustion gases in the vessel.
9. 9. In a process for calcining sludges according to claim 8 wherein the oxygen content inside the vessel is maintained approximately in the range of between 0.5% and Z%.
   
   2993S/bir-5 -27-
10. 10. In a process for calcining sludges according to claim 9 wherein the oxygen content of the hot spent combustion gases discharged from the vessel is maintained between approximately 0.5% and approximately 1%.
11. 11. In a process for calcining sludges according to claim 10 wherein the preheated material to be calcined is supplied to the vessel between the point at which the fuel is introduced to the vessel and the point at which the additional thermal energy is supplied to the vessel.
12. 12. In a process for calcining sludges according to claim 10 further comprising the step of maintaining the temperature throughout the vessel at a temperature of between approximately 1600 F. and approximately 1800° F.
13. 13. In a process for calcining sludges according to claim 12 further comprising the step of supplying additional hot combustion gases to the vessel downstream in the direction of gas flow. said hot combustion gases have a temperature in the range of between approximately 2000° F. and approximately 2500 F.
14. 14. In a process for calcining sludges according to claim 8 wherein the portion of the separated calcined material that is recirculate to the vessel is introduced into the vessel upstream in the direction of gas flow of the point at which fuel is introduced into the vessel and downstream in the direction of gas flow of the point at which the air for combustion is introduced into the vessel.
    
    2993S/bir-5 -28-
15. 15. In a process for calcining sludges according to claim 8 further comprising the steps of introducing into the vessel both the portion of the calcined material that is recirculated and the preheated material downstream in the direction of gas flow of the point at which air for combustion and the fuel are introduced into the vessel.
16. 16. In a process for calcining sludges according to claim L wherein the preheated material to be processed is introduced into the preheated air for combustion outside the calcining vessel, is entrained in and supplied to the calcining vessel with the preheated air for combustion.
17. 17. In a process for calcining sludges according to claim 1 wherein said preheated material is supplied directly to the calcining vessel independent from the air for combustion.
18. 18. In a process of calcining sludge according to claim 17 wherein some of the fuel is introduced to the vessel upstream in the direction of gas flow of the point at which preheated material is introduced and downstream of the point at which preheated air for combustion is introduced into the vessel.
19. 19. Apparatus for thermal processing materials while suspended in hot combustion gases comprising an elongated vessel having a lower end and an upper end, the lower end of said vessel having an inlet for fuel and an inlet for air for combustion gases whereby combustion of the fuel takes place in the vessel: means for supplying material to be processed to the 2993S/bir-5 -29-lower end of the vessel: the upper end of said vessel having an outlet for spent combustion gases and processed material whereby the flow of material and combustion gases is co-current from the lower end to the upper end of the vessel; at least one combustion chamber having an inlet for fuel and an inlet for air for combustion and an outlet for hot combustion air flow connected to said vessel at at least one location intermediate the lower end and the upper end of the vessel; means for establishing and maintaining a helical flow of gas in said vessel; and a gas solids separator flow connected to the outlet of said vessel for separating processed material from the spent combustion gas and having an outlet foe spent combustion gas and an outlet for processed material.
20. 20. Apparatus for thermal processing materials according to claim 19 further comprising a preheater for heating material to be processed including an inlet for material to be processed, an inlet for spent combustion gas flow connected to the outlet for spent combustion gas of the gas-solids separator whereby the spent combustion gas serves as preheating gas for preheating the material to be processed, an outlet for preheated material flow connected to the inlet for material to be processed of said vessel and an outlet for spent preheating gas.
21. 21. Apparatus for thermal processing materials according to claim 20 further comprising a cooler for cooling processed material having an inlet for material flow connected to the outlet for processed material of said gas-solids separator, an inlet for ambient air whereby the ambient air cools the 2993S/bic-5 -30-processed material and is heated by the processed material, an outlet for cooled processed material and an outlet for heated ambient air flow connected to the inlet for air for combustion of the vessel.
22. 22. Apparatus for thermal processing materials according to claim 21 wherein the material to be processed is a sludge, the apparatus further comprising a flash dryer including means for supplying hot gas to the dryer, means for supplying sludge to dryer and an outlet for dried material connected to the inlet for material of the preheater.
23. 23. Apparatus for thermal processing materials according to claim 22 wherein said means for supplying hot gas to the dryer includes a duct flow connecting the outlet for spent preheating gas of the preheater to the flash dryer, said means for supplying sludge to the dryer includes said duct having an inlet for sludge whereby the sludge is entrained in the spent preheating gas and conveyed to the dryer and further comprising comminution means mounted in said duct upstream in the direction of gas flow from said inlet for sludge.
24. 24. Apparatus for thermal processing materials according to claim 21 wherein the outlet for heated ambient air of the cooler is connected to the inlet for air for combustion of the vessel by a second duct and material to be processed is supplied from the outlet for preheated material of the preheater to said second duct to define the means for supplying material to be processed to the lower end of the vessel.
    
    2993S/bir-5 -31-
25. 25. Apparatus for thermal processing materials according to claim 20 wherein first conduit means flow connects the outlet for preheated material of the preheater directly to the vessel near the inlet for fuel to define the means for supplying material to be processed to the lower end of the vessel.
26. 26. Apparatus for thermal processing materials according to claim 25 further comprising second conduit means extending between the outlet for processed material of the gas solids separator and said vessel for recirculating a portion of the processed material from said gas solids separator to said vessel.
27. 27. Apparatus for thermal processing materials according to claim 26 wherein said second conduit means is connected to said vessel below said inlet for fuel and said first conduit means is connected to said vessel above said inlet for fuel.
28. 28. Apparatus for thermal processing materials according to claim 27 further comprising means for supplying at least some of the fuel to said vessel upstream in the direction of gas flow from said inlet for material to be processed and near the point at which said second conduit is connected to said vessel.
    
    2993S/bir-5 -32-