Study On New Method For Determination Of Iron And Acid Washing For Removing Iron From Unbleached Straw Pulp

Wheat straw pulp in China is still a major non-wood raw materials, but the high contentof transition metal ions has brought many problems to the production process and productquality which restrict the depth and breadth of its application. Compared with other majortransition metal ions (such as Cu²⁺and Mn²⁺) in the wheat straw pulp, iron ion has the highestcontent and most difficult to remove, is the greatest danger of transition metal ions. Therefore,determining iron content accurately, researching the main controlling factors of removing ironions and optimal acid washing process conditions have important guiding significance foroptimizing pulp and papermaking technology, improving product quality and increasing theamount of wheat straw used.For these reasons, a sample pretreatment method-acid hydrolysis method and atriple-wavelength visible spectroscopic method for the determination of iron content in pulphave been developed. The pretreatment process involved: dispersing pulp in3mol/L sulfuricacid solution （the pulp consistency could be decided according to the content of transitionmetals in pulps）, and then put it in a sealed container for5hours at120℃. After the samplewas pretreated with acidic hydrolysis method, the color agent,1,10-Phenanthrolinemonohydrate, was added in the filtrate and then absorbances of416,510and700nmwavelengths was measured. Iron contents of the sample can be calculated used atriple-wavelength spectroscopic method. The results shows that the pretreatment method canovercame the problem caused by the silicon compounds in the ash method for straw pulpsample testing, and triple-wavelength visible spectroscopic method can efficiently deductinfluences of acidic soluble lignin and furfural compounds generated during the samplepretreatment and baseline drift caused by the tiny particles in the filtrates. The relativedifferences obtained by the present method and ICP-OES method’s is less than5%. Themethod is simple, precision and accurate.A visible spectrophotometry method for determining ferrous ion and ferric ion content inacid washing liquor was developed. The results shows that raw samples which pH higher than3.2should been acidize and pH of test sample should be controlled between2.1to3.0; othermetal ions (such as Fe³⁺, Cu²⁺and Mn²⁺) and acid-soluble lignin and other substances insample does not interfere with the test results. The method recovery rates of total iron andferrous reached （100±3）%. This method can determine ferrous ions and ferric ion content inacid washing liquor accurately, provide important data for researching the removal law ofdifferent valence iron ion and optimizing acid washing process. With iron hydroxide solubility product constant, the activation energy of hydrogen ionand mass diffusion theory, the effect of conventional acid washing process conditions on ironion removal was researched. The results shows that H⁺activation energy is37.06kJ/mol inacid washing process, and the acid washing effect is mainly affected by mass diffusion of H⁺and iron ion. Raising temperature can promote removal of iron ions, the appropriateconventional acid washing process temperature is90℃; The pH of system should becontrolled under3.2in the actual production, to ensure that the Fe（OH）₃were dissolved andremoved; the appropriate acid washing process time is40min; increasing pulp consistencycan improve iron removal rate when added the same amount of acid.Effect of acid washing on hexenuronic acid content in unbleached kraft wheat straw pulpwas analysis, and an empirical model for predicting HexA content was established. The modelpredicted results is well match with the experimental values （R²=0.92）, which be used asguidance for the removal of HexA from analogous wheat straw pulp in acid washingtreatment.The effects of acid wahing on the relations between physicochemical properties of pulpand acid washing liquor （HexA, Kappa, whiteness and content of methanol, oxalate andcarbonate） and iron ion content in pulp was investigated. The study also found that the acidwashing process can produce reducing substances, which can reduce ferric ion to ferrous ion.Thus, with the increase of the acid washing time, the proportion of ferrous ions to ferric ionsis increased.