**5. Chemi-mechanical pulping process and CMP pulp properties**

The chemi-mechanical pulping process, combined with multiple stages as chemical pretreatment, fiber mechanical separation, and enzymatic hydrolysis modification, has many significant advantages contrasting to chemical pulping process, i.e., higher pulp yield, lower pollution load, and easily treated effluent. In fact, chemi-mechanical pulping process has been developed rapidly and widely applied for wood pulping. In the early 1990s of the twentieth century, China's wood pulp production from fast-grown woodchips by promotion of chemi-mechanical pulping technologies, such as CTMP (chemi-thermo-mechanical pulping), APMP (alkaline peroxide mechanical pulping), P-RC APMP (preconditioning-refiner chemical APMP), and CTMP-c (chemi-thermo-mechanical pulping with enhanced chemical impregnation) processes, has increased rapidly [23].

Chemi-mechanical pulping from wheat straw studies lasted for more than 30 years in China. In the early 1990s of the twentieth century, Liu and Fang [24] investigated chemi-mechanical pulping properties from wheat straw using caustic soda impregnation, characteristics of effluent, and proper treatment process. It was proven that wheat straw can be used to produce chemi-mechanical pulps using small amount of caustic soda with advantages as high pulp yield, good physical strength, and proper biodegradable effluents [25–27]. A-grade corrugated medium was produced from this kind of wheat straw CMP pulps (**Table 9**), and also A-grade linerboard from WSCMP mixed with 15–30% kraft wood pulps was prepared [17, 24].

Zhang et al. [28] also investigated wheat straw chemi-mechanical pulping properties using two-stage caustic soda impregnation followed with two-stage atmospheric refining, while a total dosage of caustic soda 3–5% was applied at the impregnation stage, resulting pulp yield of 73.1% w/w–78.9% w/w, pulp tensile strength of 19.49–41.27 N.m/g, the specific refining energy consumption from 827 to 336 kWh/t pulp (**Table 10**), and COD load in effluent of 270.5–446.0 kg/t o.d. pulp (**Table 11**); the relationships between pulp yield versus caustic soda dosage and COD load versus pulp yield are illustrated in **Figures 5** and **6**.


**Table 9.** Quality specifications of corrugated medium from wheat straw CMP pulps [17].


**Table 10.** CTMP pulping properties of wheat straw [28].

run ability in paper machine, wheat straw pulp furnishes must be mixed with some portion

Variation % 23 ≤25

>1.5 mm<sup>2</sup> no Not allowed

Some paper mills produce Bible paper products from bleached wheat straw pulps, the formula of pulp furnishes is given in **Table 7**, and the quality specifications of Bible paper are

The chemi-mechanical pulping process, combined with multiple stages as chemical pretreatment, fiber mechanical separation, and enzymatic hydrolysis modification, has many significant advantages contrasting to chemical pulping process, i.e., higher pulp yield, lower pollution load, and easily treated effluent. In fact, chemi-mechanical pulping process has been developed rapidly and widely applied for wood pulping. In the early 1990s of the

**5. Chemi-mechanical pulping process and CMP pulp properties**

of bleached wood pulps or bamboo pulps occasionally [18, 20].

**Table 8.** Quality specification to bible paper products [18].

**Pulps Ratio (%) Beating degree (o**

**Table 7.** Portion of furnish for bible paper [18].

230 Global Wheat Production

Bleached wheat straw pulp 70–50 48–53 >2.2 Bleached softwood pulp 30–50 48–65 5–8

**Specific properties unit Bible paper product**

Grammage g/m<sup>2</sup> 40 40 ± 1.5 Density g/cm3 0.80 ≥0.8 Fold endurance CD Times 5 ≥5 Breaking length m 3000 ≥2500 Smoothness Average of T-B surfaces s 102 ≥100

Brightness % 80.0 ≥79.0 Opacity % 87.0 ≥85.0 Dust content 0.3–0.5 mm<sup>2</sup> Number/m<sup>2</sup> 40 ≤50

Moisture content % 6.0 6.0 ± 1.0 Ash % 19 —

**SR) Wet weight (g)**

**Produced product GB 1913–1989 (Grade A)**

illustrated in **Table 8** [18].


**6. Effluent treatment**

**Figure 7** [24].

Effluent discharge (m3

Ratio of BOD5

It is clear that pollution load from pulping process can be decreased by increasing pulp yield. Liu and Fang [24] summarized a relationship between COD load (y) and pulp yield (x) of

effluent from wheat straw chemi-mechanical pulping has a good biodegradability contrasting with that from OCC (old corrugated container) pulping. Effluent from wheat straw CTMP

The proposed flow diagram for installation of effluent treatment plant is illustrated in

Since less amount of chemicals is applied for wheat straw impregnation, the contaminant of such pulping effluent consists of low polymerized or monomolecular carbohydrate compounds, which are biodegradable. With consideration of effluent characteristics from each stage and quality requirements of process water, it is possible to reuse the used water in a short circuit after suitable individual specific treatment. If chemi-mechanical pulping line is going to be installed in recycled fiber pulping and papermaking mill, the integrated effluent from wheat straw CTMP or CMP line will contribute to improve biodegradability for recycled fiber pulping effluent, carefully considering the water balance, and it is possible to produce wheat straw chemi-mechanical pulps without consumption of any fresh water [17]. Therefore, the total volume of discharge effluent will not be increased. It means that the origi-

nal effluent treatment can be operated with improvement of efficiency [17, 25, 27].

pH 8.3 8.5 8.1 COD concentration (mg/L) 13,580 17,280 17,185 COD load (kg/BDT) 4.35 208.19 212.54 BOD concentration (mg/L) — — 7888 BOD load (kg/BDT) — — 97.57

SS (g/L) 3.7 11.81 11.60 TS (g/L) 7.7 19.04 19.32 Ash content in TS (%) 26.7 35.20 34.06 TN (mg/L) 110.0 146.0 145.0 TP (mg/L) 99.0 57.0 58.0

/BDT) 0.32 12.05 12.37

/COD — — 0.459

**Screw press after pre-steaming Pulp washing Integrated effluent**

, r<sup>2</sup> = 0.910. It was proven that

Wheat Straw Pulping for Paper and Paperboard Production

http://dx.doi.org/10.5772/intechopen.77274

233

wheat straw pulping, i.e., y = 4929.58426–104.0789x + 0.5883x<sup>2</sup>

pulping has been characterized and illustrated in **Table 12** [24].

**Parameters Wheat straw CTMP effluent**

**Table 12.** Effluent characteristics from wheat straw CTMP pulping [24].

**Table 11.** Characteristics of effluent from wheat straw CTMP pulping [28].

**Figure 5.** Wheat straw CTMP pulp yield decreases with increment of caustic soda dosage [24, 28].

**Figure 6.** Pollutant load of effluent decreases with increment of pulp yield [24, 28].
