Since 1979 the Rice Research Board has supported research to determine if advances in industrial chemistry can produce pulp from California rice straw that is competitive in price and quality to pulp produced from wood chips. Specific objectives are to develop pulping conditions suitable for preparing bleachablegrade pulps from rice straw and to determine the properties of the pulp for papermaking and for dissolving-grade or chemical-grade celluloses.

Rice straw pulps prepared by the conventional soda and kraft processes have high ash contents and bleach requirements. Even if the properties of kraft pulp - the major process used in North America - were satisfactory, it would be difficult to install a new kraft facility in California because of the costly equipment required to control odors produced by sulfur-containing compounds formed in the process.

Paper pulp prepared from rice straw. (450x)

To overcome the problems associated with conventional pulping processes, a new sulfur-free process is being sought for manufacturing a low ash content pulp suitable for a dissolving-grade pulp. As a first step in this process, it has been shown that fines leafy material which contribute little to pulp yields and have a high silica (inorganic ash) content - can be reduced in the feedstock when the rice straw is first chopped and dry screened.

Using rice straw prepared in this manner, a modified soda pulping process - the NOS process - was investigated. In this process the mechanically processed straw is pretreated using nitric oxide followed by oxygen. The NOS process requires remarkably short times, the shortest of any of the processes studied. Unbleached NOS pulps have tensile properties equal to kraft pulps and lower tear and burst properties. These properties are superior to those obtained from soda pulps. The ash contents of and requirements for bleaching chemicals by the unbleached NOS pulps are significantly lower than for either kraft or soda pulps.

A simple three-stage, conventional process readily bleaches the NOS pulps. The resulting bleached pulps have a high brightness (whiteness) and an ash content well below 1 percent. This ash content was significantly lower than that of any of the other pulps prepared.

These results indicate that NOS pulping of rice straw is technically feasible for producing a bleachable-grade pulp. Used in conjunction with a simple conventional bleaching sequence, a bleached rice straw pulp can be prepared having properties required for a dissolving grade pulp.

A model cellulose was prepared from rice straw to compare with a commercial dissolving-grade cellulose pulp manufactured from wood for making acetate rayon fibers. A technique was developed for obtaining dissolving-grade cellulose from impure rice straw pulps. The feasibility of obtaining cellulose acetate by direct acetylation of the rice straw is also being studied. A yield of 42.4 percent cellulose acetate based on the weight of rice straw has been achieved.

The suitability of rice straw as an ion-exchange material for the removal of heavy metal ions from industrial and mining waste waters is being studied. The results obtained so far indicate that sulfonated rice straw could be a useful polyelectrolyte.