Kraft Liquor Systems

As in the kraft digester, the vessels in the kraft liquor system can be brick lined to prolong their useful life. Carbon brick linings are normally not used be-... 

See the NACE Papers Oliver W. Siebert, Correlation of Laboratory Electrochemical Investigations with Field Applications of Anodic Protection, Materials Performance, vol. 20, no. 2, pp. 38-43, February 1981 Anodic Protection, Materials Performance, vol. 28, no. 11, p. 28, November 1989, adapted by NACE from Corrosion Basics— An Introduction. (Houston, Tex. NACE, 1984, pp. 105-107) J. Ian Munro and Winston W. Shim, Anodic Protection— Its Operation and Appheations, vol. 41, no. 5, pp. 22-24, May 2001 and a two-part series, J. Ian Munro, Anodic Protection of White and Green Kraft Liquor Tankage, Part I, Electrochemistry of Kraft Liquors, and Part 11, Anodic Protection Design and System Operation, Materials Performance, vol. 42, no. 2, pp. 22-26, February 2002, and vol. 42, no. 3, pp. 24-28, March 2002. 

Kraft processes are applicable to nearly all species of wood, and effective means of recovering spent cooking chemicals for recycle in the process have been developed. Some sodium and sulfur losses do occur and are replenished in the cooking-liquor system by adding sodium sulfate at the recovery boiler, where it is converted to sodium carbonate and sulfide. Ill order lo maintain a proper sulfur-lo-sodium ratio in the recovered chemicals, other chemicals, such as sodium carbonate, sodium sulfite, and sulfur, are sometimes used for chemical makeup. 

Liquors of the kraft process itself contain a large amount of free alkali, which must be neutralized by sulfuric acid before the acetic and formic acids are freed. The corresponding amount of sodium sulfate produced from the free alkali and from the acetate and formate decomposition may be more than can be reused in the recycle liquor system. On the other hand, since the makeup of liquors for a kraft process requires the addition of fresh sodium sulfate as well as the sulfate radical from... 

Anodic protection of a Kraft liquor tank was first successfully realized at the end of 1984, and the success of this system resulted in many commercial installations. Unfortunately, unexpectedly high corrosion rates were reported at localized areas in several of the tanks even though the remainder of the surfaces corroded at rates less than 0.13 mmyi.i Most of the problems experienced have been attributed to incomplete understanding of the electrochemistry of carbon steel in these liquors and the coexistence of active and passive areas, which had not been addressed properly in earlier control strategies. 

Kraft pulping is a common process in the paper industry. Figure 8.4 shows a simplifled flowsheet of the process. In this process, wood chips are reacted (cooked) with white liquor in a digester. White liquor (which contains primarily NaOH, NaiS, Na2C03 and water) is employed to dissolve lignin from the wood chips. The cooked pulp and liquor are passed to a blow tank where the pulp is separated from the spent liquor weak black liquor which is fed to a recovery system for... 

Kuban P. and Karlberg B., On-line monitoring of kraft pulping liquors with a valveless flow injection-capillary electrophoresis system, Anal. Chim. Acta, 404, 19, 2000. 

Ammonia-based liquors, when incinerated, result in a loss of nitrogen as N2 in the flue gas. This difficulty is highly responsible for the domination of Kraft pulping. Magnesium-based liquor incineration is most easily accomplished, and can be achieved either in a Tomlinson furnace or a Copeland fluidized bed system. 

These bricks find relatively frequent use in Kraft mills because of the amount of alkaline media encountered. They are used in the liquor regeneration systems, the smelt tank, the lime slaker, the causticizer, the liquor storage tanks, and the caustic storage tanks. They, of course, can be used in any industry having corrosive alkaline conditions. As with any corrosion resistant material of construction, proper care must be exercised to insure the proper application of Portland cement brick. 

A feature of the kraft process vital to continued success is its integral, well-tested chemical recovery system. The digestion liquor for each batch of chips to be pulped is mainly obtained from the chemicals recovered from the spent liquor of previous digestions, and has approximately the composition given in Table 15.8. For kraft pulping it is usual to specify all of the components of the digestion liquor on a NaiO equivalent basis. This puts all the active constituents on the same sodium ion content basis. Thus, the actual concentration of sodium hydroxide present for a 73 g/F, NaiO equivalent is given by Eq. 15.12. 

The soda-AQ proeess ean use the same ehemieal recovery system as for kraft pulping but is teehnieally well suited to a simpler, more compact system known as DARS (direet alkali reeovery system), whieh has been explored on the pilot plant seale. Here ferrie oxide is added to the blaek liquor in a fluidized bed furnace. The residue of sodium ferrite partieles is removed and on leaching under controlled eonditions regenerates sodium hydroxide and ferrie oxide ... 

Some important automated instruments based on ion-selective electrodes measure sodium in boiler feedwaters, fluoride in public water-supplies, and water hardness (Ca " and Mg +) in water-conditioning systems. The sulfide electrode is extensively used in the paper industry for monitoring and control of sulfidity in paper pulping liquors (for instance, the Kraft process), for waste-treatment control of excess sulfide using a process that oxidizes sulfide with air, and for monitoring... 

The electrolyte systems of kraft black liquor are well known (4). In principle this data can be applied to the product mixture of the thermal treatment process, allowing the following conclusions to be drawn ... 

Calcium carbonate scale is even more commonplace in Kraft pulp mill systems such as heat exchangers, where, compared to paper machines, it often forms a thick deposit leading to loss of performance of the heat exchanger. Calcium comes into the system with either the wood or from poor recovery liquor clarification and carbonate is formed from degradation of sugars in a side reaction during pulping conditions. Dissolution of filler (calcium carbonate) will increase the concentration of both calcium and carbonate in the system. 

---