Pruned branches of Citrus limon 2 years old had moderately high average basic density 572.1 kg m− 3 and green density 577.3 kg m− 3 ( Table 2) according to Bin (1970) classification[27],Citrus linon fall within the range for commercial pulp wood ( 350–650 Kg m-3) according to [28]. The yield of pulp per unit volume is directly related to basic density as Citrus limon chips expected to occupy normal digester space for given pulp making capacity. The wood density is highly related to cell wall thickness and corresponding to collapsibility and conformability of the fiber. Fibers of moderately density are less flexible, however the wood of moderately high density is expected to give moderate to high yield with good quality paper. The average bark- to wood ratio by mass and volume (Table 2) were low (6.7 and 5.2% respectively) for pulp woods. It is well known the presence of bark and dirt influences the quality of pulp, the presence of bark in low ratios affects positively on economic value of the pulp production by decreasing the cost of transportation and debarking processes.
The moderate ash content of Citrus limon (Table 3) may be attributed to the rapid growth, as the result for need of macronutrients in metabolic activities in first years. Silica content was more or less negligible. Solubility of hot water, alcohol and alcohol: cyclo hexane(1:2) were low indicting low extractives in Citrus limon wood with high solubility of 1% NaoH indicated normal cooking without pretreatments and normal chemicals amounts to be used with high screened yields to be expected. However the normal Kurschner- Hoffer cellulose and high pentosans indicated good yield could be expected. On the other hand the moderate lignin content and high 1% NaOH (18.9%) as shown in Table 3, which favorable for moderate active alkali charge. The high amount pentosans indicted easy beating and fiber-fiber bonding, however the low total extractives indicted that there will be no pitch problems and easy cooking.
Pulping of pruned branches of citrus limon was carried under four chemical processes namely Soda, soda-AQ, alkaline sulphite anthraquinone (AS-AQ) and alkaline sulphite anthraquinone with methanol ASAM. Optimization of delignification and results of yields were given in Table 4.
The active alkali charges for the four processes were in range (12–17%) with more or less similar cooking conditions. The slight differences in time to reach the needed for maximum temperatures applied to AS-AQ and ASAM methods which were increased with 10 min. It seemed the addition of anthraquinone to Soda, AS-AQ and ASAM trials and lately methanol to ASAM cooking accelerated the delignification and preserved the carbohydrates ([29–34]. The contribution of AQ was supported further by addition of methanol in ASAM pulping resulted in lowest Kappa number 19.5 compared to other three methods (Table 4). The pulping results for branches of Citrus limon (Table 4) show that, this raw material is easy to delignify. With all processes applied, bleachable kappa numbers (19.5–26.3) could be reached at good to excellent screened yields (41.9–60.1%). ASAM pulping resulted in highest screened yield (60.1%) with negligible amount of rejects (0.2%) at bleachable pulp had low kappa number 19.5.
Comparison of strength properties of different Citrus limon unbleached pulps (Table 4), indicated in general the high tensile strength of the ASAM pulps. The high tensile strength, which mainly based on the good bonding ability of the fibers, results from the high carbohydrate content of ASAM pulps due to the high stability of xylan and cellulose in the outer cell wall layers. However, The Soda Citrus limon pulp had lower tensile strength compared to ASAM and AS-AQ pulps which can be explained by the lower pentosan preservation during cooking. The overall evaluation of pulp properties showed the superiority of ASAM pulps in all strength properties and fluctuations in grammage and thickness, however the Soda process showed inferior results in strength properties. When gaur gum was added to Soda-AQ( CL3) dramatically increase the physical properties especially tensile and burst strengths compared with Soda-AQ (CL2)