Process for demineralization of crustacea shells

#The present invention relates to processes for demineralizing crustacea shells and the like which are rich sources of chitin, a polymeric carbohydrate material containing 2-deoxy-2-acetyl-amino glucose as its building unit. Many uses have been found for products derived from chitin, such as chitosan or deacetylated chitin, as described in, for example, our U.S. Letters Pat. No. 3,533,940. Recovery of chitin from shells of crustacea species is desirable in many locations throughout the world where shrimp, prawns, lobsters, crab, crayfish, etc. are harvested.

#One of the difficulties in processing crustacea shells to recover chitin, is the removal of the large amounts of mineral matter, primarily calcium carbonate, which also are present in the shell. Generally, dilute hydrochloric acid is used for this purpose since it reacts rapidly with the calcium carbonate (and calcium phosphate), and the products, calcium chloride and calcium acid phosphate, are soluble and are readily removed. Hydrochloric acid, however, is relatively expensive and where markets do not exist for the reaction products, it becomes a major item in the cost of chitin manufacture. Hydrochloric acid, furthermore, is a hazardous and costly commodity to transport and store, adding considerably to its cost in remote localities. There is some evidence, moreover, that hydrchloric acid may be too strong an acid, and causes some degradation of the chitin during the demineralization treatment, as described by C. V. Lusena and R. C. Rose, Journal of Fish. Research Bd. of Canada, 10:521 (1953).

#For the above reasons, in some applications, it is desirable to find a practical alternative to hydrochloric acid for demineralization of crustacea shells. Sulfuric acid, while cheaper and less corrosive than hydrochloric acid, is unsatisfactory since the reaction product, calcium sulfate, is insoluble and precipitates in the interstices and on the surface of shell particles. Acetic acid, while obtainable from chitin as a by-product in the manufacture of chitosan, is too weak to dissolve carbonates from the shell at a practical rate, and its regeneration for reuse from calcium acetate solutions would be a costly procedure. Sulfurous acid, moreover, though weaker than hydrochloric acid, is still much stronger than acetic acid; but calcium sulfite is even less soluble than calcium sulfate, so that if used in the ratio of one mol sulfurous acid to one mol of calcium carbonate, an insoluble precipitate is formed and demineralization is not accomplished.

#The invention will now be described with reference to the accompanying drawing, illustrating a process flow chart of a preferred embodiment.

#Moist deproteinized shell from a continuous centrifugal washer is fed to a rotary drum gas contactor 1 and passes counter-current to the stream of off-gases from an auger type continuous demineralizer 2. Contact is provided by wall flights lifting the shell and dropping it through the gas stream. A small portion of the calcium carbonate in the shell is converted to calcium sulfite and, since the ratio of shell to off-gases is large and the gas velocity is low, substantially complete sulfur dioxide absorption is assured. In the demineralizer 2, the shell is continuously treated with a sulfurous acid solution. Calcium carbonate and calcium sulfite formed in the rotary contactor are dissolved as calcium bisulfite. The chitin from the demineralizer 2 is washed and dewatered in a continuous centrifuge 3 and is subsequently converted, as is well known, to chitosan, or is otherwise recovered, as desired. The calcium bisulfite -- sulfurous acid solution from the demineralizer 2 and the centrifugate from 3 are passed to an indirect vacuum stripper 4. Sulfur dioxide and water vapor from 4 are condensed in the condenser 5 and the resulting regenerated sulfurous acid solution is combined with fresh sulfurous acid solution from a conventional sulfur burning and adsorption system 6. The slurry of precipitated calcium sulfite and weak calcium bisulfite liquor is pumped from the stripper 4 to a continuous centrifugal decanter washer 7. The calcium sulfite may be treated by one of the means previously described to recover sulfur dioxide, or may be marketed as a by-product, or may be discarded. The centrifugate liquor containing some residual calcium bisulfite may be cooled at 8 and used to absorb sulfur dioxide from the sulfur burner at 6 for regenerative re-use. Excess, roughly equal to the water used to wash chitin and calcium sulfite in 3 and 7, may be discarded without substantial loss.

#Advantages of the process of the invention for demineralization of crustacea shells include the following:

#1. Sulfurous acid causes less degradation of the chitin in shell than does hydrochloric acid under comparable conditions, producing a superior product for many applications.