CA1124016A - Catalyzing reduction in a kraft recovery boiler - Google Patents

Abstract

CATALYZING REDUCTION IN A KRAFT RECOVERY BOILER

Abstract A pulping process wherein iron rich green-liquor dregs are added to black-liquor in advance of a recovery furnace. The iron rich dregs serve as a readily available catalyst that increases the reduction of inactive sodium sulfate into active sodium sulfide to enhance the efficiency of the pulping process.

Description

CATALYZING REDUCTION IN A KRAFT RECOVERY BOILER
Background of the Invention In a Kraft chemical recovery process used in producing pulp from wood~ inactive sodium sulfate must be reduced to active sodium sulfide before it can be used ln the process. Efficiency of reduction is governed by a number of operating variables such as bed burning temperature, char bed depth, percent liquor solids fired, and proportion of primary air used.
Low reduction efficiency often necessitates the expensive operation of adding extra lime to the pulping liquor in a causticizing operation.
It has been shown conclusively in a paper ~'IHydrogen Reduction of Alkali Sulfate" by J. R. Birk, C. M. Larson, W. G. Vaux and R. D. Oldenkamp, Ind. Eng. Chem. Process Des. Develop., Vol. lO, No. 1~ 1971, pp 7-13) that iron compounds, especially iron sulfide (FeS) catalyzed the hydrogen reduction of alkali sulfates to sulfides to an important extent, 560% at 1112F. The catalytic action of iron was found to decrease with increasing temperature and to become substantially inactive above 800C. (1472F).

Summar~ of the Invention The present invention is accordingly dirested to a chemical recov-ery process by which an iron catalyst is added to spent black liquor in advance of a recovery furnace whereby inactive sodium sulfate contained therein is reduced to ac~ive sodium sulfide by carbon through the addition of an iron catalyst.
It has been discovered that the caustic pulping liquor circulating through the pulping system continuously picks up iron from the surrounding tanks, piping, digesters an~ furnace walls. Most of th;s iron precip;tates as green l;quor "dregs" containing insoluble ferrous sulfide and carbon, when residual smelt from the recovery furnace is added to water. These iron-rich green liquor dregs are recycled back to the system in advance of the recovery furnace to serve as an iron catalyst that reduces inactive sodium sulfate to active sodium sulfide.

Brief Description of the Drawing The single figure of the draw;ng diagrammaticially shows that por-t;on of a pulp ma~ ng process that is employed in the present invent;on.

Description of the Preferred Embodiment The present invention relates to a Kraft process for pulping wood wherein wood chips are digested in a mixture of sodium hydroxide and sodium sulfide known as "white liquor". Spent or used digesting liquor is separ-ated from the resulting pulp, and the pulp is further ref;ned as necessar~for eventual use. The recovered liquor known as "black liquor" contains some organic mat~er extracted from ~he wood pulp, together with other chemi-cals not consumed in the digestion process according to standard pulping procedure.
The black liquor is concentrated and then injected into a chemical recovery furnace where the organic content is burned and some sulfates reduced to sulfides.
A molten ash or "smelt" is discharged therefrom into a quantity of water to form "green liquor" that is clarl~;ed and circulated further for use in the pulping process.
The "green liquor" contains sodium carbonate, sodium sulfide and other salts along with a quantity of finely divided material commonly known as green liquor "dregs" that precipita~e when the "green liquor" ls added to water. The "dregs" are comprised chiefly of ferrous sul~ide together with some carbon that is carr;ed over with the smelt.
The iron of the green liquor "dregs" normally occurs because of the continuous corrosive attack on the tubes, digesters, tanks, furnace walls, and because of the impuritiPs present in the make-up chemicals. Most of this iron from the green liquor preciptates quickly as ferrous sulfide 5FeS) when the green-liquor is added to water. The "dregs" also contain some carbon from char carried out of the furnace with the molten smelt. The "dregs" are washed to remove all sodium salts therefrom.
Such "dregs" are commonly considered as a waste product and are discarded continuously in normal plant practice.
; 30 In my invent;on, however, I utilize the waste iron of the green liquor "dregs" as an effective catalyst in the recovery furnace to enhance the reduction of inactive sodium sulfate contained in the pulping liquor to active sodium sulfide. Furthermore, residual carbon of the "dregs'~ is recycled along with the waste iron of the "dregs" to provide an additional source of fuel to be used in the recovery process.
Accordingly, pulping liquor containing inactive sodium sulfate together with residual organic matter is injected into the inlet 10 of a recovery furnace 12 where the carbon is burned and oxygen of the inactive sodium sulfate combines with carbsn ~o for~ active sodium sulfide and char.
Inasmuch as sodium sulfate is inactive and sodium sulfide is the active alkali that is required in the process of pulping wood, a high efficiency 3L~4~3 in reducing sodium sulfate to sodium sulfide is to be desired. However, an efficiency of approximately 85C~ is typical of most pulping processes, so a significant part of the inactive sodium salts remain in the pulping liquor.
A molten ash resulting from burning the pulping liquor in the recovery furnace is discharged as "smelt" through outlet 14 into a quantity of water from a source 16 to form "green-liquor" and an amount of finely divided precipitate called "dregs". The green liquor contains a quantity of sodium sulfide and sodium carbonate while the prec;pitate contains chie~ y residual carbon and ferrous sulfide.
The green liquor is separated from the "dregs"in clarifier 18 and directed out through a passageway 20. Lime may be added to the green liquor from a source 26 to convert the sodium carbonate thereof ~o caustic soda.
The resulting clear fluid i 5 the "white liquor" that is the original solvent in the pulping process.
The dregs containing FeS and carbon are passed through a pumping means 22 and recirculated into the pulping liquor in advance of the furnace 12 where the iron content of the dregs serves to catalyze the reduction of inactive sodium sulfate to active sodium sulfide. Moreover, the carbon of the "dregs" may again be subjected to combustion in furnace 12 so that it becomes an additional souroe o~ fuel utilized in the reduction process.
To achieve maximum benefits claimed from recycling gre~ liquor "dregs" back to the recovery furnace, the temperature of the smelt should be maintained below 800C and the concentration of iron in the smelt should range from .05% to 5% by weight as Fe203.
Compounds of other alkali me~als such as potassium or lithium may be added to the pulping liquor anywhere in the system in the manner shown by numeral 24 to provide a temperature depressant that lowers the melting tem-perature of the smelt to a range wherein the iron compounds thereof may function suitably a~ a reduction catalyst.

C7~0140

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A chemical recovery process in which a spent pulping liquor bearing inactive alkali sulfates is burned in a chemical recovery furnace to form an inorganic ash bearing alkali sulfides, said process comprising the steps of introducing the spent pulping liquor to the furnace, burning the spent pulping liquor in the furnace to a molten smelt, removing the smelt from said furnace, adding said smelt to a quantity of water to produce green liquor that contains finely divided dregs bearing iron, removing said dregs from the green liquor, and introducing said dregs to the spent liquor in advance of the furnace to add an iron catalyst thereto that enhances the reduction of alkali sulfates to alkali sulfides.

2. A chemical recovery process as defined in claim 1 wherein the green liquor dregs including iron therein are added continuously to the spent liquor in advance of the furnace to provide a continuing reaction.

3. A chemical recovery process as defined in claim 2 wherein the iron contained in green liquor dregs comprises a quantity of FeS.

4. A chemical recovery process as defined in claim 3 wherein the concentration of iron ranges from .05% to 5% by weight as Fe203 in smelt.

5. A chemical recovery process as defined in claim 4 wherein the alkali sulfate in the pulping liquor comprises a sulfate of sodium.

6. A chemical recovery process as defined in claim 5 wherein the temperature of the molten smelt is less than about 800°C.

7. A chemical recovery process as defined in claim 6 wherein other alkali metal compounds are added to the pulping liquor to increase the reduction of the sulfates to sulfides.

8. A chemical recovery process as defined in claim 7 wherein the green liquor "dregs" contain a quantity of carbon.