• International Journal of Technology (IJTech)
  • Vol 15, No 2 (2024)

Silica Removal of Oil Palm Empty Fruit Bunch Fiber

Silica Removal of Oil Palm Empty Fruit Bunch Fiber

Title: Silica Removal of Oil Palm Empty Fruit Bunch Fiber
Hapip Ramadhan, Jaroslav Stavik, Yin Ying Hng, Moh Fahrurrozi

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Cite this article as:
Ramadhan, H., Stavik, J., Hng, Y.Y., Fahrurrozi, M., 2024. Silica Removal of Oil Palm Empty Fruit Bunch Fiber. International Journal of Technology. Volume 15(2), pp. 321-331

Hapip Ramadhan 1. Department of Chemical Engineering, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia, 2. Riau Andalan Pulp and Paper, APRIL, Pangkalan Kerinci 28300, Riau, Indonesia
Jaroslav Stavik Riau Andalan Pulp and Paper, APRIL, Pangkalan Kerinci 28300, Riau, Indonesia
Yin Ying Hng Riau Andalan Pulp and Paper, APRIL, Pangkalan Kerinci 28300, Riau, Indonesia
Moh Fahrurrozi Department of Chemical Engineering, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia
Email to Corresponding Author

Silica Removal of Oil Palm Empty Fruit Bunch Fiber

This research aimed to optimize sodium hydroxide concentration and temperature in silica removal of oil palm empty fruit bunch (EFB) fiber. The process began with extensive washing of fiber at 3% consistency and a temperature of 50°C using warm water before alkaline treatment to eliminate external silica, reducing the concentration from 7683 ppm to 5294 ppm. This initial step caused a decrease in total yield by removing non-fiber material and short carbohydrates. Despite the significant material loss, the byproduct showed promising potential for further use as fertilizer. The washing stage was followed by alkali treatment when different dosages of caustic and temperature were optimized based on design experiments using 22 factorial designs and regression modeling. The optimum alkali treatment condition was set at 60°C and NaOH concentration of 0.05 M, resulting in EFB-treated yield of 90.6 % with silica content of approximately 3000 ppm as acid-insoluble ash.

Alkali treatment; EFB; Factorial design; Silica removal


Indonesia is the largest country of oil palm tree plantations globally, covering 14.59 million hectares in 2020 (Badan Pusat Statistik, 2021), establishing the position as the leading producer of crude palm oil (CPO) since 2006. The major biomass by-product from oil palm fruit total production (25 - 26%) is oil palm empty fruit bunch (EFB) (Srinophakun and Suwajittanont, 2022). Presently, approximately 10% of EFB has been used for boiler fuel and natural fertilizer materials, while the rest is discarded waste (Dewanti, 2018).

       The suboptimal use of EFB, particularly as a raw material in the pulp and paper industry, is attributed to the high silica content of approximately 1 % dry weight as acid-insoluble ash. Silica is an inherent oil palm tree that spreads to all parts, including EFB fiber and leaf (Yudha et al., 2023). The maximum acceptable silica content in raw material for kraft pulping is 3000 ppm. However, high silica content in the pulping system leads to complications in black liquor recovery problems, necessitating lime mud purging to remove silica from the system (Runge and Paul, 2015).

EFB is biomass obtained during CPO production, consisting of fiber composite in palm bunch form without fruits after the extraction from fresh fruit bunch. This extraction process is carried out through sterilization using steam at 294 kPa for 1 hour (Shinoj et al., 2011). The output of this process is palm fruit easily separated from bunch in the stripper stage through the threshing process (Omran, Sharaai, and Hashim 2021). EFB fiber is produced as an additional stage of palm oil extraction process, where oil is extracted through mechanical pressing and steaming, converting it physically to fiber.

EFB fiber has a rough body surface and a diameter size of 343-365 µm. This rough morphology impacted surface components such as wax, fat substance, silica, and other impurities (Rahmasita, Farid, and Ardhyananta 2017). Furthermore, EFB consists of nanofiber cellulose (NFC) with a diameter between 5 nm and 60 nm (Harahap et al., 2023). Several research have been conducted on removing silica from EFB and other biomass, with pre-treatment through mechanical hammering for alkaline peroxide pulping (Ghazali, Rosli, and Law, 2009). Sonication was also carried out to remove silica bodies on EFB surfaces for biogas production applications (Salleh, Hamid, and Hussain, 2013). Oil and other impurities, including silica, were removed by 0.4 NaOH and 0.6% acetic acid (Ibrahim et al., 2015). Approximately 90% of silica from rice straw biomass has been eliminated through the use of 0.5 M sodium carbonate or sodium hydroxide 2 M at 93°C (Khaleghian, Molaverdi, and Karimi, 2017). In the case of rice husk, containing silica, 15-20% can be removed through NaOH leaching system (Liu, Zhang, and Huang, 2019). The process of removing silica from rice husk biomass starts with a reaction between NaOH and silica, forming Na2SiO3, which is more water-soluble than pure silica (Bazargan et al., 2020). This previous research showed that caustic leaching potential removes silica from EFB fiber through two methods. First, NaOH reacts with organic matter and opens gaps or cutting silica-organic bonding, releasing into the environment. Second, NaOH reacts with SiO2 to form Na2SiO2, which dissolves into the liquid phase.

This research aimed to determine the optimum temperature and NaOH concentration in silica removal from EFB using NaOH solution to maximize fiber yield with acceptable silica content for Kraft pulping. The results are expected to enhance EFB fiber for the pulp and paper industry in kraft cooking. Integrating CPO plant and pulp mill could potentially increase EFB economic value as paper production raw material. Furthermore, wood feedstock consumption can be reduced by replacement with qualified EFB fiber.

Experimental Methods

    The treatment of EFB fiber was divided into two stages. The first stage was washing to pre-cleaning EFB from impurities such as oil and external silica content. The second stage was the alkali treatment stage to remove internal silica content. Process conditions for the washing and alkali treatment stage are presented in Table 1. The main analyses of this research were silica content, as acid-insoluble ash in each stage followed standard TAPPI T 244 (TAPPI, 2023), and soluble silica in water was based on standard APHA-AWWA-WEF standard method 23rd (Baird, Eaton, and Rice, 2017). The important parameter in this research is yield calculation, defined on a molar basis as the ratio of moles of the desired product formed after treatment to the number of moles of key reactants consumed (Fogler, 2016). On a weight basis, the yield can be calculated by Equation (1).

Table 1 Process condition of washing and alkali treatment stage




Alkali treatment




30 and 60





Liquor circulation speed




Mixing rotation speed




Process consistency




NaOH concentration



0.01 and 0.10

        The equipment used in the washing and alkali treatment stage is a pulp screening washer machine, with the working principle schematically shown in Figure 1. EFB fiber material mixed in the main tank follows the liquor mixer flow direction. In this equipment, material with dimensions below 10 mm hole is passed through the top screen and moved to the bottom tank area. Material with dimensions higher than 2 mm is trapped in the bottom tank. Other smaller materials will continue and be trapped in a filter bag 150 mesh sieve, while the rest of the filtrate is continuously sent back to the main tank.

Figure 1 The schematically working principle of pulp screening washer machine

        In the alkali treatment stage, NaOH 40% (pulp and paper industrial grade) was used to control alkali concentration in the liquid phase. Temperature and NaOH concentration optimization followed the 22 factorial design experiment method (Montgomery, 2013). This research was carried out based on the assumption that temperature in the range of 30 – 60oC and NaOH concentration at 0.01 – 0.1 M would have a linear effect on the yield and silica content in EFB fiber. The regression model was used to describe the impact of the factors following Equation (2). This equation facilitated the creation of chart, such as surface response and contour, to describe the impact of the factors (Montgomery, 2013).

Where  is the average of all data of measuring variables such as yield, silica, or others,  is half of A effect,  is half of B effect,  is high and low levels of A effect, is high and low levels of B effect. A and B effects can be calculated by the equation in Table 2.

Table 2 Equation for Effect of variable in 22 design experiment

Effect of




n = effect = 3

Value of come from å repetition data in low, high, and combination level



Results and Discussion

3.1. EFB Fiber Raw Material Preparation

        As shown in Figure 2, EFB sample raw material considers oily when touched and appears unclean. The dry matter, including ash and silica content of EFB raw material in the range 48.7 – 53.0%, 23500 – 39700 ppm, and 6082 – 10340 ppm, respectively, with average data 50.43% consistency, 3.14 ash content, and 7683 ppm silica (as acid-insoluble ash). The high deviation of silica content in EFB fiber raw material must be controlled before feeding to the alkali treatment stage. Consequently, the chemicals in the alkali treatment stage can be focused mainly on removing silica from the internal EFB or silica body.

Figure 2 EFB fiber raw material

3.2. Washing Stage

        Based on Figure 3, the washing stage solid product was divided into three types, namely EFB fiber, mud, and reject (sand and kernel). When observing the washed and dried fiber in Figures 3 (a) and (b), it appeared brighter than the raw material from Figure 2. Additionally, compared to EFB raw material, which was slippery, EFB-washed was rough. Oil content from raw material was observed in a filter bag 150 mesh sieve and mixed as mud material. Material with dimensions larger than the bottom hole strainer diameter (2 mm), such as kernels, and high-density material, including sands, remained in the bottom tank, as shown in Figure 3 (d).

        The solid material mass balance in the washing stage is presented in Table 3. The use of 100 ton/h EFB fiber raw material as a basis with a consistency of 50.7% produced the main product of washed EFB fiber of 38.46 dry ton/h or yield of 75.83 % dry basis. Similarly, 9.54% would be designated as by-product mud and 0.81% as a reject (kernel and sand). The remaining solid, around 13.82%, was estimated to be lost in filtrate flow. In this washing stage, 30-ton of mud were produced per 100-ton EFB raw material, presenting a potential material for fertilizer applications in plantation areas.

Figure 3 (a) EFB washed, (b) EFB dry, (c) mud, and (d) reject material

Table 3 Solid material mass balance in the washing stage


Inlet (dry ton)

Outlet (dry ton)

Yield (%)

EFB fiber








Sand & kernel




Solid in liquor








        Ash and silica reduction of EFB fiber, silica in mud, and soluble silica in the washing stage are presented below. Figure 4 (a) showed that ash and silica content decreased from 31425 ppm and 7683 ppm to 8375 ppm and 5294 ppm on average, while Figure 4 (b) indicated that soluble silica in liquor increased from 6 ppm to 14 ppm. Increasing soluble silica in liquor was significantly lower compared to decreasing trend as acid-insoluble ash in EFB. Filter bag 150 mesh sieve facilities effectively functioned in silica removal. Silica from EFB fiber raw material is more dominant moved to mud still in solid phase than soluble to the liquor. This data explained that 52% of silica remained in EFB, 46% moved to mud in the filter bag, and the remaining 2% was transferred to the reject and filtrate line. Furthermore, 11 kg soluble and 185 kg insoluble silica as acid-insoluble ash moved to mud per 100-ton EFB raw material.

Figure 4 (a) Ash and silica reduction of EFB fiber, (b) silica in mud, and soluble silica in the washing stage

3.3.  Alkali Treatment Stage

        After washing, EFB fiber passed through the alkali treatment to remove further internal silica or silica body on EFB. This stage was carried out to obtain silica as acid-insoluble ash content below 3000 ppm, meeting the cooking stage requirement. To achieve this target, the optimization was performed by experimenting with temperatures ranging from 30 to 60°C and NaOH concentration 0.01 - 0.1 M. Figure 5 shows silica content and EFB yield trend after alkali treatment. As presented in Figure 5(a), increasing NaOH concentration impacted the decreasing silica content in EFB fiber at different temperatures. This NaOH effect trend was almost similar to yield, even higher by temperature increase, as shown in Figure 5(b). Target silica content 3000 ppm was achieved at 60°C with NaOH 0.1 M, resulting in an acid-insoluble ash of approximately 2449 ppm and EFB treated yield of 88.0% dry basis. Although the desired content was achieved, the yield value could still be further improved as silica content was 3000 ppm. The estimation of optimum temperature and NaOH concentration was further discussed. At NaOH 0.01 M, the yield was not impacted significantly by increasing temperature. This result differed with NaOH 0.1 M, where increasing temperature from 30 to 60°C significantly impacted the yield from 91.3% to 88.0%.

Figure 5 (a) Silica remained in EFB and (b) EFB fiber treated yield after alkali treatment stage

        The solid material mass balance in the alkali treatment stage at 60°C and NaOH 0.1 M is shown in Table 4. Additionally, soluble silica in alkali-stage liquor before and after treatment and silica in mud are presented in Figure 6. Based on the result, using 100 ton/h (50.72 dry ton/h) raw material before the washing stage as a basis, alkali treatment produced the main product EFB fiber treated at 38.46 dry ton/h. The total yield of washing and alkali treatment was approximately 66.74% dry. Approximately 2.44% would be transferred as by-product mud and 0.92% as a reject (kernel and sand), while the remaining solid of 8.63%, was estimated to be lost in filtrate flow.

Table 4 Solid material mass balance in alkali treatment stage at 60°C and NaOH 0.1 M


Inlet (dry ton)

Outlet (dry ton)

Yield (%)

EFB fiber








Sand & kernel




Solid in liquor








                        Note: *from data Table 3.

Figure 6 Soluble silica in alkali stage liquor before and after treatment and silica in mud at 60°C and NaOH 0.1 M

        Based on the solid material mass balance data in Table 4, silica in EFB at 60°C and NaOH 0.1 M, and soluble silica presented in Figure 6, the alkali treatment stage estimated 41% of silica moved to mud in a solid phase, 4% dissolved to filtrate in a liquid phase, 46% remained in EFB, and the rest 9% was left reject part in the solid phase. These data were obtained from the assumption that silica in acid-insoluble ash measurement in alkali treatment was almost the same as soluble silica in liquor. The dominance of silica in the solid phase after alkali treatment showed that a small portion of SiO2 reacted with NaOH to form Na2SiO3 soluble in water. The domination was that silica moved from EFB fiber to mud trap in a solid phase. Consequently, silica removal mechanism in alkali treatment probability occurred following the steps presented in Figure 7.

·           Silica is dominant in crystal form and located in the wall of EFB fiber that bonds with organic matter.

·           NaOH dominant reacts to organic matter such as lignin and carbohydrates at a specific temperature, making silica crystal bonding in saggy conditions.

·           Additional mechanical force removes silica from organic matter net bonding and follows the liquor flow.

Figure 7 Potential mechanism of silica removal on EFB fiber

3.4.  Microscopic Visualization

        Based on Figure 8, microscopic visualization of EFB fiber uses scanning electron microscope (SEM) from raw material, after washing and alkali treatment. Figures 8 (a) and (b) show no significant difference between silica body in raw material and after the washing stage. This phenomenon shows that silica body cannot be removed using warm water in the washing stage. Meanwhile, Figure 8 (c) shows a significant number of empty cavities in the wall of fiber, resembling an ex-silica body place. This shows that silica body is loose from the cage after additional alkali, temperature, and mechanical force, as explained in silica removal mechanism.

3.5.  Optimum Condition Estimation

          The optimum process conditions for temperature and NaOH concentration during the alkali treatment stage were estimated using a 22 factorial design experiment. The repeatability data of yield and silica from a combination factor between temperature and NaOH concentration is shown in Table 5. Similar to Figure 5, the data obtained showed the impact of temperature, NaOH concentration, and the interaction with yield and silica, as shown in Table 6.

Figure 8 Microscopic SEM visualization of EFB (a) raw material, (b) after washing stage, and (c) after alkali treatment stage

Table 5 Replicate data of yield and silica, the impact of combination temperature (T) and NaOH concentration (C)


Yield (%)

Silica (ppm as acid-insoluble ash)

T (oC)

C (M)

















































          All data presented in Table 6 showed a negative value, implying that temperature, NaOH concentration, and the interaction negatively affected EFB-treated yield and silica content. This showed that an increase in these factors would impact the decrease of yield and silica, where NaOH concentration had the highest effect. Meanwhile, the interaction between temperature and NaOH concentration produced the lowest impact on both parameters.

Table 6 Effect of temperature and NaOH concentration to yield and silica

Effect of

to Yield

to acid-insoluble ash

Temperature (T)



NaOH Concentration (C)



Interaction (TC)



        The impact of temperature and NaOH concentration on EFB fiber silica content in alkali treatment was described by the regression model in Equation (3) and Figure 9. The regression model and plotting were calculated following Equation (2). Figure 9 (a) showed that increasing temperature and NaOH concentration reduced silica content in EFB fiber. Meanwhile, Figure 9 (b) showed that the area acid-insoluble ash target (<3000 ppm) could be achieved in range temperature 40 – 60°C and NaOH concentration 0.05 – 0.1 M.

Where  is temperature in °C, and  is NaOH concentration in M.

Figure 9 Effect of temperature and NaOH concentration on EFB fiber silica with (a) response surface plot, (b) contour plot

Figure 10 Effect of temperature and NaOH concentration on EFB fiber yield with (a) response surface plot, (b) contour plot

          A Similar approach using Equation (2), impact temperature and NaOH concentration to EFB fiber treated yield in alkali treatment can be described by regression model in Equation (4) and Figure 10. Figure 10 (a) shows that increasing temperature and NaOH concentration will reduce the EFB fiber-treated yield. From Figures 9 (b) and 10 (b), the optimum point to get maximum yield is at 60 °C and NaOH 0.05 M. At this point, it can be achieved a yield of approximately 90.6 %.


In conclusion, this research showed that the washing stage effectively removed silica from external and other impurities such as sand and oil, but there was no significant effect on silica body. Silica content in the washing stage reduced from approximately 7683 to 5294 ppm as acid-insoluble ash, with EFB-washed yield of 76%. Higher silica content in solid form, particularly in alkali stages, showed that the dominant alkali reacted with organic matter more than silica in a range temperature of 30 – 60°C. Silica level below 3000 ppm was achieved at 60°C, and NaOH 0.1 M. Silica content and yield at this condition was approximately 2449 ppm as acid-insoluble ash and 88.01 % as a dry basis, respectively. Total yield washing and alkali treatment was at 66.74% was in the acceptable limit The optimum alkali treatment condition estimated was at 60°C and NaOH 0.05 M to produce a 90.6% yield with silica content of 3000 ppm as acid-insoluble ash. At this optimum condition, the estimated total EFB yield from the washing and alkali treatment stage was 68.7 %. In the optimum alkali treatment stage, 51% of silica remained in EFB, 36% moved to by-product mud, 10% was in the rejected part, and 3% dissolved in liquor. To improve this process, future research should focus on reducing chemical (NaOH) usage in the alkali treatment stage by increasing the temperature beyond 60°C. Additionally, mechanical forces such as circulation and mixing rotation speed could be varied in the washing and alkali treatment stage to determine the best condition in silica removal from EFB fiber from mechanical effect. 


    This paper is part of a master's degree scholarship program from APRIL Indonesia for the employees at Universitas Gadjah Mada. The authors are grateful for the support received from Asian Agri to provide EFB fiber material, Nanyang Technological University (NTU) Singapore for offering scanning electron microscope (SEM) visualization data, and the PPP R&D Department laboratory team in research data production.


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