The paper presents a mathematical formalization of regularities during the alkaline treatment of rice straw as one of the stages of oxidationorganosolvent pulping. The purpose of the research is to maximize the yield of technical pulp, to extract minerals and to remove lignin. It has been found out that the following input values are optimal: the duration of alkaline treatment of rice straw is 240 minutes; the concentration of alkali is 0.69 N. Under the given conditions of alkaline treatment with oxidation-organosolvent pulping, commercial pulp is produced with a yield of 58.8%, the mineral content being 0.05%, and residual lignin being 2.5% of bone dry raw materials.
Currently, many studies are focused on the production of budget-friendly sorbents produced from natural materials,
the functional groups of which are able to retain reagents on the surface of the carrier without interfering with the visual
identification of the metal ion content [
One of the aims of processing RS is to obtain commercial cellulose pulp. As is known, in its composition RS
contains from 10 to 30% of mineral components, the remaining part being bone dry raw materials (BDRM). To remove the
mineral component from the RS during the oxidation-organosolvent pulping, an alkaline treatment stage is introduced.
The action of the alkaline solution on the lingo-carbohydrate complex leads to the removal of the mineral component
and part of the lignin, as well as to the degradation of the polysaccharides, reducing the yield of commercial cellulose
pulp [
Thus, the purpose of this paper is to study the regularities of alkaline RS processing in order to maximize the yield of commercial pulp, to extract the mineral part and remove lignin. To achieve the goal, the following tasks were set: - to carry out preliminary studies of alkaline RS treatment with various ratio of sodium hydroxide to BDRM and various processing time; to conduct analysis of fibrous material;
- on the basis of realization of a two-level two-factor full factorial experiment, to obtain experimental statistical models of the yield of fibrous material and the content of lignin and mineral components in it;
- to solve the optimization problem of searching for optimal values of technological factors that ensure a high yield of fibrous material from the RS with a minimum content of mineral components in it;
- to conduct oxidation-organosolvent delignification of the fibrous material obtained under optimal conditions to produce technical pulp and perform its analysis.
As an object of research, the RS from Krasnodar region, harvest of 2015, was used. The composition of the raw
material: the content of Kürshner-Hoffer cellulose is 47.5 ± 1% [
Obtaining technical pulp from RS was carried out in two stages: the first stage was alkaline treatment, the second
was oxidation-organosolvent pulping [
Stage 1. RS treatment with aqueous NaOH solution under the following conditions: 1:10 hydromodule; NaOH concentration 0.2 ... 1 N; the consumption of alkali 9.6 ... 48.0% of the BDRM, treatment temperature of 90oC; the duration of the temperature rise is 15 minutes; duration of alkaline treatment is 60 ... 240 minutes. The obtained fibrous material was washed to neutral state, dried in air and analyzed.
Stage 2 (oxidation-organosolvent pulping). Treatment of fibrous material with equilibrium peracetic acid under the following conditions: liquid module - 1:10; treatment temperature of 90 оС; the temperature rise time is 20 minutes; processing time - 90 min; the consumption of the pulping composition in recalculation for the equilibrium peracetic acid is 0.8g per 1g of the mass of the BDRM. The content of the pulping composition: equilibrium peracetic acid, decomposition stabilizer of peroxide compounds in an amount of 0.01% of the mass of the BDRM, water. At the end of pulping, the used pulping composition was separated from the commercial pulp through the drain valve of the reactor system. Distilled water was fed to the reactor and the obtained commercial pulp was washed until the state of a neutral reaction of the washings was achieved. The analysis of cellulose obtained as a result of pulping showed the following characteristics: humidity - USSR State standard specification 16932; lignin content - USSR State standard specification 11960; content of mineral substances (ash content) - USSR State standard specification 18461.
The results of preliminary studies of alkaline treatment of RS in single-factor experiments are shown in Fig. 1, 3 and 4. .
It can be seen from Fig. 1 that, the higher the concentration of alkaline solution, the more effective the mineral substances are removed from the plant raw material. During the first hour of processing of mineral substances, 67.3 to 71.3% of BDRM is removed depending on the concentration of alkali. For concentrations of 0.2 ... 0.5 N, further processing is performed with the same intensity and allows to remove from 6.0 to 9.5% of BDRM of the mineral component. The process is most effective at the maximum concentration of alkali when 14.5% more of BDRM are removed. Thus, when using 1 N concentration of alkali, 86% of mineral substances are removed from the fibrous material, and in other cases - 73.5 ... 79.6%.
One of the features of RS is the presence of a lipid layer on the outer and inner surfaces of the stem. The lipid layer has hydrophobic properties and performs a protective function. The hydrophobic layer prevents the penetration of chemical agents into the ligno-carbohydrate matrix and makes it difficult to extract other components. Alkali saponifies and partially dissolves the resins and fats in the lipid layer, making the ligno-carbohydrate complex accessible (Figure 2).
×500 ×100 the inside of the stem × 1000 × 500 the outer side of the stem
Figure 2: Internal and external side of the stem of rice straw after alkaline treatment 1N. alkali solution This creates favorable conditions for the effective separation of minerals and partly lignin. However, aggressive and prolonged alkaline treatment negatively affects the carbohydrate component and, accordingly, the yield of the fibrous product (Figure 3). Fig. 4 shows that the bulk of lignin amounting to 64.7 ... 70.6% of the initial lignin content in BDRM is removed in the first hour of processing. Subsequent 180 minutes of treatment allow to remove from 71.8 to 84.3% of the initial li gnin content in BDRM depending on the concentration of alkali.
From the data presented, it can be concluded that when the alkali concentration is 1 N, and the treatment time is 240 minutes, a significant degradation of the carbohydrate component is observed, and further increase in the concentration of alkali and the duration of treatment is not advisable. Treatment at an alkali concentration of 0.2 ... 0.3N even with a duration of 240 minutes does not reach the required values for the yield, the content of minerals and the content of residual lignin.
In order to find the optimal values for the concentration of alkali and the duration of RS treatment, a two-level twofactor complete factoral experiment 22 was planned and implemented, with a three-fold repetition of each iteration.
Based on preliminary studies of alkaline RS treatment, the following areas of variation of the natural values of the factors (Zi) were chosen: the concentration of alkali (Z1) from 0.5 up to 1 N (with the hydromodule 1:10), the duration of RS treatment from 60 up to 240 min.
Areas of change in input factors are presented in Table 1.
Y1 - yield of fibrous material,%, Y2 - content of residual lignin,%; Y3 - content of mineral substances in the fibrous material,%, of the initial BDRM,%.
To obtain the experimental statistical model, a regression analysis of the experimental results was carried out for the
first-order plan 22 [
According to this plan, an experimental statistical model with normalized values of input factors can be presented in the form of a polynomial:
Analysis of the dependences of the output parameters on the values of the input factors shows (Figures 5-7) that with an increase in the values of the input factors all output parameters decrease. In this case, it is visually impossible to determine the optimal values of the input factors that ensure a high yield of technical cellulose from RS with a minimum content of mineral components in it. To find the optimal conditions, it is necessary to apply mathematical methods for solving optimization problems.
The research has found out the following regularities of alkaline RS processing: maximum preservation of the high yield of commercial pulp - 58.8%, extraction of mineral part (residual content of mineral substances) - 0.05% and removal of lignin (residual lignin - 2.5%). The results show that the following values of input factors are optimal: duration of RS alkaline treatment is 240 min; the concentration of alkali is 0.69N. es.