Alumina control scheme
I.Brief description of Bayer’s method
The Bayer’s process, which was proposedby K.J. Bayer in 1889-1982, is applicable to low-silicon bauxite and has theincomparable advantages in processing gibbsite bauxite. Above 90% of worldwidealuminum oxides and aluminum hydroxides are produced through Bayer’s methodcurrently.
The whole production process of aluminumoxide is composed of eight sections: grinding of raw materials, dissolution,red mud settlement, filter control, decomposition and classification,evaporation, filter of finished products and roasting, all of which will beintroduced as follows:
1. Raw material workshop
In the section of raw material grinding,the bauxite, lime and circulating mother liquor are added to rod mill and ballmill in proportion to produce raw pulp, which will be then classified inhydrocyclone to produce qualified raw pulp by overflow; the qualified raw pulpwill be sent to raw pulp tank and then sent to atmospheric desilication sectionin dissolution workshop through pulp pump. The underflow of classifier will bereturned to ball mill.
2. Dissolution workshop
The raw pulp from raw material workshopwill be sent to heating tank in atmospheric desilication section, indirectlyheated by steam room, heated to 100℃-105℃ from 80℃and then sent to desilication tank for continuousdesilication. After the Rp and temperature of raw pulp which passes desilicationare adjusted by mother liquor at the end of desilication tank, thequalified raw pulp will be sent to diaphragm pump in high-pressure pump room;the sand-returning pipe and pump are buried under desilication mechanism toreturn the rough sands to raw material processing section regularly in eachshift.
The raw pulp will be sent to casingpreheater in dissolution section through diaphragm pump, preheated to 174-180℃ through Level 6 casing heater, thenpreheated to 210-220℃through Level 6 preheating pressure boiler, indirectly heated to 260℃ in high-pressure new steam room and bemaintained for 60min. After dissolution, the pulp temperature will be reducedto 125℃ from 260℃ through Level 10 flash evaporation andthen sent to dilution tank. The red mud washing liquor from red mud washingprocess will be added in dilution tank and the diluted pulp will be sent toback dissolution tank and maintained for above 4h to remove impurities insolution such as silicon, iron and zinc.
The pulp is used in evaporator toproduce secondary steam, which can be used in casing preheater and heatingpressure boiler to preheat raw pulp; the condensed water of secondary steam isdischarged to condensed water tank from preheater; the condensed water isgathered in final stage condensed water tank after step-by-step flashevaporation and pressure reduction and then sent to hot water station forpreparing hot water.
3. Settlement separation and washingworkshop of red mud
The diluted pulp from back dissolutiontank, together with flocculant from flocculant preparation section, is sent toseparation and settling tank, which has a solid content of underflow of about38%-42%, be sent to washing and settling tank through pump and provided withfour times of back washing; the washing water is added from end tank; the solidcontent of underflow in the final washing is about 46%-54% and they are plannedto be sent to red mud yard for stacking through high-pressure diaphragm pump.
The overflow of separation and settlingtank is sent to coarse liquid tank in control and filter section, in which thecontrol and filter is performed by using vertical leaf filter and small amountof lime milks are added in coarse liquid tank as filter aid; the fine liquidobtained from leaf filter is sent to fine liquid plate heat exchanger section,the leaf residues are sent to filter residue tank and sent back to washing andsettling tank 1 through pump. The aluminum hydroxide washing liquor isadded in washing and settling tank 1 and the residue from caustification isadded in washing and settling tank 2.
4. Decomposition workshop
The fine liquid from control and filtersection is sent to fine liquid heat exchange procedure for two-level heatexchange in this section, cooled to 61-62℃from 100-105℃ and then sent to seedfilter for flushing of seed crystal. Level 1 is heat exchange between fineliquid and mother liquor and Level 2 is heat exchange between fine liquid andwater.
After flushing of seed crystal, the fineliquid is prepared as aluminum hydroxide with solid content of 800g/L and sentto No. 1 and 2 decomposition tank through seed crystal pump. The sand-shapealuminum hydroxide is made by using high concentration and high seed proportionprocess. Two sets of classification units of hydrocyclones which are properlyset at the end part of decomposition tank, together with the underflow ofclassifier of coarse particle aluminum hydroxide pulp, are used as the finishedproducts filter process in sending products of this workshop to roastingworkshop; the graded overflow returns to decomposition tank and the second lasttank in decomposition tank is seed discharge tank; seeds are discharged to seedfilter from proper position of tank and the seed crystal is sent to seedcrystal tank after filtration; the filtered mother liquor will be sent to conemother liquor.
The mother liquor from seedprecipitation is delivered through pump; parts of it are sent to aluminumhydroxide for classification and another part is sent to fine liquid heatexchange process to have heat exchange with fine liquid; after heat exchange,the mother liquor is heated to 85-90℃from 50-55℃ and then sent toevaporation raw liquor tank. The wide-runner plate heat exchanger is properlyset on the top of decomposition tank as intermediate cooling equipment topromote decomposition and productivity.
5. Evaporation workshop
The mother liquor from decompositionworkshop is sent to evaporation raw liquor tank in workshop; the section isprovided with partial evaporation process of mother liquor, part of which willbe partially sent to VI effect evaporator and discharged to mixing tank throughquintuple effect evaporator; the another part will be sent to IV effectevaporator and then be partially sent to forced-circulation evaporator andmixing tank of circulating mother liquor through III,IIand Ieffect three-stage flash evaporator.
The evaporation station is composed ofone VI effect falling-firm evaporator and one forced circulation crystallizingevaporator and three-stage flash evaporator; the countercurrent flow is used inevaporation.
The discharging temperature of Ieffect is 140℃; the solution is sentto three-stage flash evaporation system and its temperature is reduced throughgradual flash evaporation; the discharging temperature through three flashevaporations is about 92℃;the evaporated mother liquor discharged after three flash evaporations is sentto mixing tank of mother liquor for preparation of circulating mother liquor.
A certain amount of mother liquors fromevaporation and two flashes are subject to mandatory effects to increase thesteam concentration above Na2Ok320g/l. The discharged materials from mandatoryeffects are sent to salt settling tank; the underflow is sent to salt filterand then sent to causticization tank after it is diluted by filtered Na2CO3filter cake. The pulp from caustification is sent to separation filter and theresidue is sent to washing and settling tank 2 for settlement, separation andwashing of red mud. The strong alkaline liquor is returned to have four flashevaporations and then sent to mixing tank of circulating mother liquor forpreparation of mother liquor.
The supplemented alkali is produced byusing liquid caustic with NaOH concentration higher than 42% and then sent tomixing tank through pump.
6. Roasting workshop
The aluminum hydroxide pulp obtainedfrom decomposition is sent to horizontal disc filter by pump through aluminumhydroxide pulp storage tank to carry out separation and washing of aluminumhydroxide and the water content of filter cake after washing is 6%-8%; then itis sent to feed box of roasting furnace or aluminum hydroxide bin through beltconveyor; the filtered mother liquor is sent to cone mother liquor tank of seedfilter and the aluminum hydroxide washing liquor is sent to red mud washingprocess.
The aluminum hydroxide from finishedproduct filter process or aluminum hydroxide bin is discharged to 50m3 feed boxin roasting process; the material level has interlock with belt metering feederand the feed quantity of roasting furnace is controlled. The aluminum hydroxidewith water content of 6%-8% is sent to Venturi dryer through belt conveyor andscrew feeder; the dry aluminum hydroxide will be sent to Level 1 cyclonepreheater by airflow and the flue gas and dry aluminum hydroxide will be separatedagain; the aluminum hydroxide from Level 1 cyclone preheater will enter Level 2cyclone preheater and have heat exchange with flue gas mixture which comes fromheat separator and has a temperature of about 1000℃; the temperature of aluminumhydroxide will reach 320-360℃and have its attached water basically removed; the aluminum hydroxide whichpasses preroasting will be discharged in the cone of roasting furnace togetherwith flue gas from cyclone preheater; the combustion air used by roasting furnaceis preheated to 600-800℃and it enters from bottom of roasting furnace; the fuel mixes with air andburns, the aluminum hydroxide which passes preroasting has full mixing with hotair at furnace bottom and the roasting of aluminum hydroxide is completed inabout 1.4s in the furnace.
The roasted aluminum hydroxide and hotflue gas are separated in hot separator. After passing aforementioned Level 2cyclone preheater and heat exchange between Venturi dryer and aluminumhydroxide, the hot flue gas is cooled to 145℃,sent to electric precipitator and then sent to chimney through exhaust fan tobe discharged into the air.
The aluminum hydroxide from hotseparator is cooled to 80℃after cooling in two sections, in which the aluminum hydroxide is cooled to 260℃ from 1050℃ through Level 4 cooling cyclone inthe first section and the air required by fuel combustion is preheated to 800℃; the aluminum hydroxide is cooledto 80℃ from 260℃ by indirect cooling of water throughfluidized bed cooling machine in the second section. The aluminum hydroxidefrom fluidized bed cooling machine is sent to aluminum hydroxide bin throughpneumatic chute and belt conveyor and then sent for stacking after packaging.