USRE19240E - Process of treating black liquors - Google Patents

Description

July 17, 1934. E, GjGooDELL PROCESS OF TREATINGBLACK LIQUORS Original Filed Dec. 51, 1928 KE D IMQ/461 aodell EMA QD Shiva Ressued July 17, 1934 UNITED STATES P,AfIEIv'r OFFICE PROCESS F TREATING LIQUORS Edward G. Gooden,

StevensPoint, Wis.

Claims.

This invention relates to a process of treating black liquors and is more particularly directed to the regeneration of black liquors as obtained in the soda and sulphateprocesses for the manufac- '6 ture of pulp.

Accordingto both the sulphate and soda process for making pulp, the wood chips are cooked by steam under pressure in an alkaline cooking liquor composed, in the case of the sulphate proc- 4,lll ess, mainly o! sodium sulphide, sodium hydroxide,'and lesser quantities of sodium carbonate and sodium sulphate, and in the case of the soda process, principally of sodium hydroxide with relatively smaller quantities of sodium car- 16 honate but no sulphur compound. In certain cases, a combination or modification of both of .the processes is employed.

In am alkaline cooking process, however, the resulting spent liquors, -as separated from the pulp' after cooking are commonly referred to as black liquor and it is tothe processes'of recovery or regeneration of the alkali or inorganic content of said black liquors and the utilization of the heat 'values therein contained that this 26 invention is particularly directed.

Various methods are at present employed to recover the alkali *values contained in these yblack liquors. 'The successive Isteps lusually include a draining or ltering of the black liquors from 80 the cooked pulp and a concentrating ci' the `black liquor in vacuum or direct heat disc evaporators, or both, this step usually concentrating the black liquors from about 12 B. to labout 35 B. at which latter consistency the partially concern- .'35 trated black liquors are fed into a rotary :incinerator, the heavy black liquor is further dried and partly burned to a variable consistency, ranging from a heavy syrup to a solid ash or burning coal andin such condition is commonly referred 40 to as "black ash. This black ash is fed continuously into a :melting furnace, or smclting furnaces, where it is burned, and from which the heut' o! combustion of the organic constituents romainingln the black ash passes to the incin- In the regeneration of the black liquors from the sulphatoprocess, an addition ofsodiumsuiphate to replace the losses in the recovery cycle is made usually in the'black ash as it goes to ..50 the `smelting fmnaces, ywhere the sodiumsulpliate thus added and the sodium sulphate content of the black liquors ismore or less completely reduced to sodium sulphide, which in the sulphate process is an active cooking agent. The

ylili final productV from the smelting furnaces is melt (ICL 23-48) ed and run out into a dissolving tank, from which the solution is pumped to mixing and caustic'izing tanks, lters and the like, where the final steps in the preparation of the recovered liquors for reuse in cooking pulp takes place.

It is an important object of this invention to improve, simplify and cheapen the regenerating operation for the recovery of the alkali values contained inthe black liquors andto more effectively utilize the heat value contained therein.

It is a further important object of this invention to provide as a step in the regeneratingiprocess the dehydration of the partially concentrated black liquors to a dry formr without substantial loss or destructionofthe organic content of the black liquors and therefore without loss of the original heat values inthe black liquors.

It isa further important object of this invention to provide a process of treating black liquors wherein the black liquors are dried in a gaseous drying medium to a fairly uniform dried product, which because of its uniformity can be more efllciently handled in ithe subsequent operation for the recovery of the alkali values in the `black liquors.

Other and further vimportant objects of this invention will be apparent from the -disclosures in the specification and the accompanying drawing.

' This invention (in a preferred form) is illustrated in the drawing and Ahereinafter vmore fully described. y

My process for the treating of black liquors can best be described in connection with the accompanying drawing vshowing in a schematic Way the various operations'involved in the process and the general varrangement of the apparatus and flow of materials therethrough.

The starting material for my process is the socalled black liquors, or spent liquors, obtained ln either of the alkaline lprocesses for the manufacture of pulp, or in a modification thereof. As obtained directly from the draining of the pulp and trom the wash waters, the resulting black liquor is ordinarily too dilute for immediate use in my process and is accordingly concentrated in any well-known manner, as by means of vacuum ordirect heat disc evaporators, to about 35 B. It will be imderstood that this lpreliminary concentration of theblack liquors is not essential to my process but is preferable from an eccnomic standpoint.

According to my process, the partially concentrated black liquor is sprayed or atomiz'ed into any suitable form. of dehydrating chamber 1 wherein the dehydration of the black hqiior vis effected through direct contact with a gaseous dehydrating medium. The temperatures of the gaseous dehvdrating medium maintained within the dehydrating chamber are such asto preserve the organic content of the black liquor in a substantially unchanged condition, thereby at the same time conserving the heat value vof the black liquor; Furthermore, under the temperatures employed there is no decomposition or destruction of the inorganic content of the black liquor,

such as, sodium-sulphur compounds. As illustrative of the range of temperatures withi the spray drier, the temperature ofthe entering gascous medium may be approximately" 300 F., with an upper limit say of 500. F., and a temperature of say as low as 150 F. for the exit gases.

In addition to the control of the temperatures oi the drying gases, it should be understood that the customary precautions normally used in spray drying operations will be observed. For example, the evaporative heat capacity of the gases supplied to kdehydrate the sprayed black liquor is balanced with respect to the amount o! heat required to substantially dehydrate black liquor so as to effect the dehydration without destroying the organic matter therein. This means a regulation oi the temperature and rate of flow ot the gases, and the time of contact of the hot gases with the sprayed liquor in the drying chambers. f 'The major portion o! the dried black liquor is wlected in powdered form from the bottom o! the dehydrating chamber but a certain small quantity will be carried over in the exit gases and be recovered in a dust collector 2 or other type of Separator. A The waste gases from the separator are preferably drawn through an induced drait ian 3 and discharged through a stack 4 in any suitable manner.

From the spray drier 1 and dust collector 2, the dried, powdered black liquor, if derived from the soda process for the making of pulp, is burned, preferably in a rotary furnace. to recover the caustic alkali values in the form mainly of soda ash. The heat of combustion from the burning. maybe used for the generation of steam and the heated gases of combustion may be used to dehydrate further quantities of black liquor. If, however, the black liquor is derived from the sulphate process, the dried black liquor obtained from the spray drier and from the dust collector is together with the required quantity oi' sodium sulphate, into a smelting furnace 5 frmins bed thereimwherecombustion of the liquor is-eiiected under reducing conditions to produce mainly sodium sulphide and soda ash. In either case, the heat of combustion obtained from the burning of the dried black liquor is utilised in the concentrating and dehydrating oi further quantities of black liquor.

In thecase of dried black liquor obtained from thesulphate process, ,t0 which my invention is particuhrly directed, tively little actual on o( the organic content n! the black liquortakes the melting turnace, but them arganic constituents are: passed from the smelting furnace 5. into a suitable devicedor or their heat values, as for instance a steam boiler 6, where said volatile constituentsarewholly or partly burned and the heat oi' combustion utilized in the production oi' steam tor-uaein the concentration oi further quantities of black liquors.

The burning of the black liquor solids, whether from the soda or sulphate process, is

carried out under conditions such as will produce the desired inorganic salt values, soda ash in the case of the soda process and sodium sulde and soda ash in the case of the sulphate process. Where, as in the preferred form of my invention, the heat values are to be used rst in the generation of steam, the burning operation is carried out under such conditions as will produce the maximum amount of heat of combustion. In the burning of sulphate process black liquor solids, this requires a complete burning of the volatilized organic matter resulting from the partial burning in the smelting furnace of the organic matter in said solids under reducing conditions.

The waste gases and products from the combustion of the volatile constituents in the steam generating plant 6 are led into the spray drier 1, where they serve as the gaseous dehydrating medium for converting further quantities of black liquors into dried, powdered form. In this way, not only is a great economy of heat values effected but also a considerable economy in the recovery of the alkali values in the black liquor, since any inorganic dust carried in the exit gases from the smelting furnace is precipitated in the spray drier or recovered in the dust collector.

The solid or semi-solid product from the smelting furnace is run into a dissolving tank 7 and workedA up from there on in the usual manner.

'Ihrough the use of standard regulating devices, the various operations are largely automatic, continuous, and self-contained. As compared with present methods, the greatly increased amount of heat generated by the combustion of black liquor in a completely dried condition. forms an important and distinguishing feature of my improved process. The heat derived from the partial combustion of the dried black liquors serves in the smelting furnace for the obtaining of the necessary temperatures for reduction of the sulphate to sulphide and the heat of combustion of the volatile organic constituents serves not only to furnish the steam required to evaporate the black liquor in vacuum evaporators, or the like, to the consistency required for spray drying, but also to eiect the dehydration of the partially Vconcentrated black liquors in the spraydrying process. 'I'he spray-drying, itself, is carried out at a comparatively lowk temperature. using the flue gases from the steam boilers at approximately the nue gas temperature common to usual steam boiler practice, thus completing by a. new and improved method and to a degree not heretofore attained, the utilization of the heat available from the combustion o! the organic constituents of the black liquor.

In starting up my process, the smelting iurnace 5 may be heated by means of wood, oil or other fuel, in the absence oi a supply of dried black liquor, in order to raise the temperature o! the flue gases yentering the spray drierto the desired degree.

As; hereinbeiore stated,l the chemical commanly used to replace theloss -oi caustic alkali content in the recovery cycle of the sulphate process is (sodium sulphate), NaaSOi, whichis-usuallyfaddedto the black ash as it falls fromthe rotary ineinerator into the smelting `furnaces. "'Bhejvariable consistency o! the blackfvash obtainedfrom the inclnerator accordingto the common practice; however, isnot conducive-to maximum percentage of reduction of the sulphate to sulphide, since the black ash may and frequently -does vary widely from a thick syrup containing considerable water to a glowsu unchanged chemically and collecting said product for separate utilization else- 9. In the process of regenerating black liquor obtained in the manufacture of pulp by the lsulphate process, the steps of dehydratiug black liquor whilelin suspended nely divided state in a hot gaseous medium under conditions preventing-simultaneous oxidation of the resulting dried black'iiquor solids.- burning said dried black liquor solidsA and utilizing the gaseous products of combmtionfromsaid burning Operation as the gaseous medium of said dehydrating step.

10.'In the process of regenerating black liquor obtained in the manufacture of pulp by-the sulphate process. the steps which comprise intrcducim dried and powderedfblack liquor solids as such intola smelting zone to form abcd therein, electing reductionin said bed of the sulphate content thereof by the organic content normal to said solids withoutvcomplete combustion-of said organic content andcompleting the combustion of volatilized organic constituents and derivatives inthe presence of a boiler for generating stearm Y.

11. In the process of regenerating black liquor obtained inlthe manufacture of pulp by the sulphate process, the steps which comprise spraying black liquor in a stream of hot gases to obtain dried black liquor solids having substantially allof `the original organic content 'of the black liquor, heating said solids under reducing conditions to volatilize the organic conten and fuse of! the alkaline values, burning the organic contents vclatilized therefrom, and utilizing the heat of combustion of said organic content for dehydratlng and steam generating purposes.'

12. 'Ihe process of regenerating blackliquor obtained in the manufacture of pulp by the soda or sulphate process,` which comprises spraying apartially concentrated biackrliquor into a gaseous dehydrating medium -under conditions which will reduce saidfllquor to a 'substantially dry powder without substantially changing its organic content, then'burning the organic content under conditions such as will produce ythe desired-inorganic salt values and generate the maximum amount of heat of' combustion, utilizing the heat of combustion inthe generation of steam and thegaseous products of combustion as the gaseous medium in subsequent dehydratng operations and recovering the inorganic content of said dry powder.

13. The process of'regencrating black liquor from -the sulphate process of making pulp', which comprises partially concentrating the black liquor, dehydrating the partially concentrated liquid to substantially dry form under such conditions as will preserve its organic content, heating the dry material under reducing conditions in a separate operation with combustion of the organic content, of said dried material, utilizing the heat of combustion of said organic content for further dehydrating operations and recovering the inorganic content of the dried black liquor in a reduced state.

14. In the process of lregenerating black liquor obtained in the manufacture of pulp by the sulphate process, the steps which comprise drying sprayed black liquor under conditions that will yield black liquor solids in substantially dry andI in substantially unchanged chemical condition, burning said dry solids under reducing conditions in a separate operation, burning the resulting gases from said reducing operation, and utilizing -the heat of said gaseous products of combustion immediately for the generation of steam and subsequently thereto for the drying of sprayed blackliquor.

i5. In the process of regenerating black liquor obtained in the manufacture of pulp by the sulphate process, the steps of introducing black liquor solids as such and without any accompanying free aqueous liquor into a zone heated' by the partial bln'ning of the organic content of said solids and utilizing the heat from said burning and from the burning of the volatile products thereof first in the generation of steam and subsequently in the dehydration of black liquor.

* EDWARD G. GOODE'LL.

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