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. 2020 Nov 2;5(45):29110–29120. doi: 10.1021/acsomega.0c03789

Production of Silicone Tetrachloride from Rice Husk by Chlorination and Performance of Mercury Adsorption from Aqueous Solution of the Chlorinated Residue

Yuuki Mochizuki 1, Javzandolgor Bud 1, Jiaqian Liu 1, Naoto Tsubouchi 1,*
PMCID: PMC7675561  PMID: 33225142

Abstract

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The production of silicone tetrachloride (SiCl4) from rice husk char by chlorination was investigated, and the effect of the char preparation temperature on SiCl4 volatilization and the coexisting element species in the char was examined. The behavior of chlorine (Cl) and the change in pore properties during char chlorination were analyzed, and the reaction mechanism was discussed. The performance of Hg ion removal of the chlorination residue was also investigated. At 1000 °C chlorination, the optimum rice husk pyrolysis temperature for attaining high ash-release extent was 800 °C. Ash volatilization during char chlorination with heat treatment mainly occurred at >300 °C and reached a release extent of ∼75% by 1000 °C. Si and P volatilization started at >300 °C and reached 70–75% by 1000 °C. In contrast, Na and K the volatilization occurred at >700 °C, with a 50% volatilization extent by 1000 °C. Mg and Ca had a volatilization rate of <20% by 1000 °C. When the char was held at 1000 °C, the release extent of Si and P reached 75–80% by 10 min. Na and K volatilized almost completely by 10 min, and the release extent of Mg and Ca increased with increasing holding time and became 10–50% by 60 min. The Cl content in the residue obtained at each chlorination temperature increased from 300 to 700 °C and then decreased with increasing temperature. The majority of Cl taken up in the residue was an H2O insoluble form. The surface area and pore volume of the chlorinated residue tended to increase with increasing chlorination temperature, with the former increasing to 335 m2/g at 1000 °C and 10 min holding. The maximum mercury adsorption amount of the chlorinated residue obtained at 1000 °C, 10 min holding was 620 mg/g, indicating the mercury ion adsorption performance of the chlorinated residue.

1. Introduction

The amount of rice husks (RHs) generated in Japan is approximately 2 million tons/year, of which approximately 65% (1.32 million tons) is used for mulch, barn bedding, compost, smoked charcoal, culvert material, and floor soil replacement. In addition, 15% (300,000 tons) is disposed of as fuel or incineration. However, the remaining 20% of the RHs is either plowed into the fields or neglected, and the use of RHs is not always sufficient. The RHs are composed of woody, silica, and cuticular layers. Because the presence of the former two species makes natural decomposition difficult, RHs are often disposed of by open burning. However, open burning of RHs is currently restricted in Japan because of the resulting air pollution; thus, a safe disposal method is needed for the excess RHs.

RHs are composed of 13–29% inorganic and 71–87% organic matter (cellulose, etc.), and the composition depends on the type of rice plant, climate, and soil conditions. In general, silica (SiO2) is the main constituent (87–97%) of the inorganic material in RHs, and the rest is trace elements such as alkali metals. High- and low-temperature combustion power generation is currently being considered as a treatment method for RHs; however, this method has several disadvantages such as low power generation efficiency and a tendency for the SiO2 in RHs to cause a clinker problem in the combustion furnace. Therefore, the use of RHs as a combustion fuel is not necessarily effective. On the other hand, SiO2, which is contained in large amounts in RHs, has the advantage of being highly reactive, easy to crush, and produced annually compared to natural silica minerals (quartz, silica, quartz, etc.).1 Currently, SiO2 is positioned as an important raw material for cement, glass, semiconductor, and optical communications industries.2−11 Silicon tetrachloride (SiCl4) produced from SiO2 has been conventionally used as a feedstock gas in the production of fine ceramics such as high-purity SiO2, SiC, and SiN4 by chemical vapor deposition.1 Traditionally, SiCl4 has been produced by reacting chlorine gas with SiC, produced from a mixture of SiO2 and carbon, at a temperature of approximately 600 °C.12 However, the process of producing trichlorosilane (SiHCl3) from metallic Si is currently the dominant method used to produce high-purity silicon compounds.1 The disadvantage of this process is that it requires high-purity silica (>99%) for production silicon metal and 16 MW/ton-Si of power for the refining silicon metal.1 Therefore, the development of resource and energy-saving processes for the production of high-purity silicon compounds is desired.

In contrast, SiCl4 has been produced by reacting SiO2 directly with chlorine gas in the presence of carbon.1,13−19 This method is a resource- and energy-saving process by using low-grade silica ore and chemical energy. However, reactor materials must be resistant to Cl2 at high temperatures because 1300 °C is needed to produce SiCl4, and such materials are difficult to produce or receive; thus, this method has not been put to practical use.1 In Japan, the harvested rice is polished at a rice center. In other words, the RH is a Si resource that solves the problem of concentration when using biomass because it is produced intensively at the rice center. However, RHs have the disadvantages of low specific gravity, as well as low transportation storage efficiency. Therefore, attempts have been made to use RHs as a combustion power generation fuel for the power source of the rice center and to produce SiCl4 from the generated ash after combustion by the chlorination method.14−18 A a lower reaction temperature (<1000 °C) can be achieved using RH ash containing highly reactive SiO2 as the feedstock.14−18 Chloride smelting of titanium (TiO2 + 2C + 2Cl2 → TiCl4 + 2CO and TiCl4 + 2Mg → Ti + 2MgCl2) is a well-known industrial process that uses chlorine at high temperatures. In the former reaction, a fluidized bed is formed by chlorine gas at a temperature of about 1000 °C. The latter process is called the Kroll method, in which metallic Ti is produced from TiCl4. Therefore, if SiCl4 production from SiO2 is possible at temperatures below 1000 °C, this process can be applied. In other words, the safety issue of using chlorine gas in the chlorination process of RH is solved. However, the physical addition of carbon to the RH ash is not effective for the production of SiCl4 because the contact of SiO2 with carbon affects the chlorination reaction rate. As is well known, when RH is pyrolyzed, char is obtained. It has been reported that the SiO2/C composite with SiO2 finely dispersed in the carbon matrix can be prepared by pyrolysis of RH.20 It has also been reported that the rate of SiCl4 formation from the composite is greater than that from the physical mixture sample.1,16 In other words, it is possible to separate SiCl4 with greater energy savings by chlorinating the pyrolyzed char compared to the mixture of carbon and combustion ash of RHs. In addition, it is possible to design a freestanding reactor by burning the gas and tar generated during the pyrolysis process of the RH,21 and it also solves the abovementioned handling problems (low density and low transport ant storage efficiency). According to previous reports on the pore properties of RH under different pyrolysis conditions (rapid or slow heating rate), the specific surface area of char prepared with slow pyrolysis is larger than with rapid pyrolysis, and these values decrease with increasing temperature in both pyrolysis conditions. In other words, it can be said that slow pyrolysis is advantageous for the production of char with high reactivity with chlorine gas.22,23 However, the effect of the pyrolysis temperature on the volatilization of SiCl4 has not been investigated in previous reports.1,16

To produce high-purity SiCl4 from RH ash or char by the chlorination method, it is important to clarify the volatile behavior of trace elements in the starting material (ash or char) for the distillation and purification of SiCl4, but the trace elements have not been investigated so far. In addition, in chlorination reactions, carbon is generally added at above the stoichiometric ratio to advance the reaction,14−18 but Cl may be adsorbed on the residue after chlorination, and the treatment of unreacted carbon becomes a problem. Cl atoms have high affinity with mercury24 and have been used as adsorbents for gaseous mercury.25 If the residue generated from the production of SiCl4 from the RH char by the chlorination method can be used as an Hg adsorbent, the chlorination of the RH can be a coproduction method of SiCl4 production and Hg adsorbent.

In the present study, we investigated the production of SiCl4 from RH char by the chlorination method and examined the effect of char preparation temperature on SiCl4 volatilization and the coexisting element species in the char. The behavior of the Cl and the change in pore properties during the chlorination of char were analyzed and the reaction mechanism was discussed. In addition, the basic performance of Hg ion removal of the chlorination residue was also investigated.

2. Results and Discussion

2.1. Effect of Pyrolysis Temperature on Release Extent of Ash during Chlorination

Figure 1 shows the temperature change in the char yield when the RH was pyrolyzed from 400 to 900 °C. The weight loss of the RH started to be large above 200 °C and became 40 wt % dry at 500 °C. Although a slight decrease in the yield was observed at 500–800 °C, the yield was almost constant at 800–900 °C. This result indicates that the pyrolysis of RH occurs mainly at 200–400 °C. Figure 1 also shows that the ash content in the char recovered after pyrolysis. The ash content in the char increased from 20 to 40 wt % by 500 °C because of the desorption of the volatile matter in the RH during pyrolysis. A slight increase in the ash content was observed at 500–700 °C, and it was almost constant (50 wt %-dry) above 800 °C.

Figure 1.

Figure 1

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Changes in char yield and ash content in prepared char with pyrolysis temperature of RH. Heating rate; 10 °C/min, holding time; 0 min.

Figure 2 shows the effect of the char pyrolysis temperature on the release extent of ash when char prepared at different temperatures was chlorinated at 1000 °C. The release extent of ash (60%) from RC400 tended to increase with increasing char preparation temperature up to 800 °C (RC800), which reached 75% by 800 °C. In contrast, there was no change in the release extent at RC800–900, and the low release extent at RC400–600 was due to the incomplete pyrolysis of RH, as shown in Figure 1. Therefore, RC800 was used for the following studies because of the high ash volatility observed for the chlorination treatment. Tables 1 and 2 show the RC800 analysis values. C accounted for 89 wt % of the composition, and the ash content was 50 wt %. In addition, 22% of Si was found in the ash content of the char. According to a previous report, Si, which is present at the nanoscale in the silicate polymerized form in plant organic matter, is also present in the highly dispersed state in char after pyrolysis.20 In other words, the char is considered to be a complex in which Si exists in a highly dispersed state.

Figure 2.

Figure 2

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Change in release extent of ash from char prepared at different temperatures at a chlorination temperature of 1000 °C. Heating rate; 20 °C/min, holding time; 0 min.

Table 1. Composition of Slag Samples Used in This Study.

    elemental analysis, wt %-daf
  
sample code C H N S + Oa ash, wt %-dry
rice husk RH 46.9 5.9 0.53 46.7 20.4
800 °C char RC 89.1 2.0 0.51 8.4 49.5
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a

Estimated by difference.

Table 2. Ash Composition of RH and RC Used in this Study.

    ash composition, wt %-ash content basis
sample code Si Ca Mg K Na P Mn Fe Al
rice husk RH 9.123 0.110 0.049 0.284 0.015 0.077 0.033 0.012 0.006
800 °C char RC800 21.61 0.312 0.130 0.726 0.726 0.201 0.088 0.051 0.020
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2.2. Temperature Dependency of Release Extent of Ash in RH Char during Chlorination

Figure 3 shows the temperature dependence of the yield and ash content with chlorination temperature. Here, yield and ash content were decreased as RC basis. The yield of RC800 tended to increase above 300 °C, reaching 110 wt % at 500 °C and then dropping significantly with increasing temperature to 50 wt % at 1000 °C. The increase in yield between 300 and 500 °C may be due to the formation of C–Cl bonds by the reaction of chlorine gas and carbon (details are described later). The decreasing ash content in RC800 (50 wt %) started above 300 °C, where the above yield increase was observed, and reached 40 wt % at 600 °C. The ash content at 600–800 °C, where a significant yield decrease occurs, decreased sharply from 40 to 20 wt % at 1000 °C, the ash yield was 10 wt %. The decrease in yield and ash content above 500 °C means consumption of C in char for ash volatilization by the reaction.

Figure 3.

Figure 3

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Change in yield of chlorinated residue with temperature during chlorination of RC800. Heating rate; 20 °C/min, holding time; 0 min.

2.3. Temperature Dependency of Volatile Extent of Elements during Chlorination

The results of the release behavior of each element are shown in Figure 4 in order to investigate the detailed temperature change in the ash content in RC800 during chlorination, as observed in Figure 3. The elements (Al, Fe, and Mn) for which the content in RC was less than 0.1% were excluded from this study, as listed in Table 2. Figure 4a shows the change in the release extent of ash with the chlorination temperature, corresponding to the results shown in Figure 3. As mentioned above, ash release began above 300 °C and increased with increasing temperature. Figure 4b illustrates the release behavior of each element during chlorination. The release of Si, the main component of the ash, was observed above 300 °C, and the extent up to 600 °C reached approximately 20%. At 600–700 °C, the volatilization of Si progressed rapidly and then increased with increasing temperature. The release behavior of Si was almost consistent with that of ash (Figure 4a). A similar release behavior was also observed in P. The volatilization of alkali metals (Na and K) occurred rapidly from above 800 °C, with 40–50% volatilization by 1000 °C; Mg volatilization was also observed from around 700–800 °C, but the release extent by 1000 °C was small at less than 20%; Ca almost did not volatilize by 1000 °C.

Figure 4.

Figure 4

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Temperature dependency of volatile extent of ash (a) and elements (b) during chlorination of RC800.

The results of thermodynamic equilibrium calculations using HSC Chemistry 5.1 (Outokumpu Research Oy.) to investigate the volatile forms of each element are shown in Figure S1 in Supporting Information. This calculation is based on the compositional values of each element in RC800, as listed in Table 2, and the assumption that it exists as an oxide. The carbon content of the amorphous form was determined by referring to the amount of carbon in the char, as listed in Table 2. The results showed that Si, P, K, Na, and Mg could volatilize as SiCl4(g), PCl3(g), KCl(g), NaCl(g), and MgCl2(g), respectively, according to the general reaction eq 1 (MO means metal oxide species) shown below, while Ca was solid and stable as CaCl2.

2.3. 1

Here, assuming that the volatile reaction kinetics is first-order against the elemental content in the solid phase and that the temperature dependence of the reaction kinetics constant follows the Arrhenius formula, the release behavior of each element (Na, K, and Mg), as shown in Figure 4b, can be expressed by the equation of 1 step reaction. The results of fitting the data of the relationship between the reaction temperature and the release extent using the frequency factor and the apparent activation energy as parameters are shown as dashed lines in Figure 4 and each parameter is shown in Table 3. As shown by the dashed lines in Figure 4, the release behavior of Na, K, and Mg can be expressed as E1 = 160 kJ/mol and k01 = 2.0 × 104 to 2.8 × 103 s–1. However, the high-temperature region of ash, Si, and P could not be expressed in the equation of 1 step reaction. As can be seen from Figure 4, the volatility of the ash, Si, and P progressed rapidly at 600–700 °C, followed by a moderate increase above 700 °C. According to previous reports, the reaction of SiCl4 formation from amorphous silica is reported to have a diffusion rate of the reaction gas above 800 °C.13 In other words, as the temperature increases, the rate-limiting process shifts from a reaction to a gas diffusion process. Although the details are described later, significant pore development was observed above 700 °C. There is a difference between amorphous silica and SiO2 in the RC800; however, the diffusion rate of the reaction gas may occur in the present chlorination as well. Therefore, considering that the whole reaction occurs as a two-step continuous reaction, the following reaction rate equation of equation of 2 step reaction was used. As shown by the dashed lines in Figure 4, the release behavior of ash and Si could be fitted with the same parameters of E1 = 40 kJ/mol, E2 = 80 kJ/mol, k01 = 7.5 × 10–1, and k02 = 6.5 × 101 s–1. The volatilization of P could be represented by E1 = 45 kJ/mol, E2 = 80 kJ/mol, k01 = 1.5 × 101, and k02 = 6.5 × 101 s–1. The release behavior of Si and P could be represented by nearly similar activation energies and frequency factors, so their positions in the char may be similar. These results may indicate that P can also be volatilized from unused biomass containing a large amount of P (such as sewage, etc.) by the chlorination method. It is well known that the phosphorus ore is being depleted worldwide and is positioned as a strategic element in many countries. Here, E2 was larger than E1 for ash, Si, and P. This may suggest that unlike the case of the amorphous SiO2 and activated carbon mixtures,13 the reaction rate-limiting rate is dominant for RC800 (SiO2/C composite) even above the chlorination temperature of 800 °C.

Table 3. Kinetic Parameters for Release Behavior of Elements during Chlorination of RC800.

  kinetic parameter
 physical properties
element K01, 1/s E1, kJ/mol K02, 1/s E2, kJ/mol α chloridea m.p.b b.p.c
ash 7.5 × 10–1 40 6.5 × 101 80 0.45      
Si 7.5 × 10–1 40 6.5 × 101 80 0.45 SiCl4 –70 57
P 1.5 × 101 45 6.5 × 101 80 0.40 PCl3 –112 76
Na 2.0 × 104 160       NaCl 800 1465
K 2.0 × 104 160       KCl 776 1420
Mg 2.8 × 103 160       MgCl2 714 1412
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a

Volatile forms estimated by thermodynamic equilibrium calculation.

b

Melting point (°C).

c

Boiling point (°C).

The results in Figure 4 show that the chlorination method can separate most of the Si in the RC800 as SiCl4, but the volatilization of the coexisting elements in the char also occurs; Table 3 shows the boiling and melting points of the chlorides for each element as calculated in HSC Chemistry. Chloride species volatilized by the chlorination method can be deposited by controlling the temperature at the reactor outlet.27 Here, based on a comparison of the boiling and melting points of each chloride species, these temperatures of Na, K, and Mg, which volatilize at high temperatures, are greater than 600 °C. This means that the volatile Si and P and the above three chloride species can be separated by the deposition method. On the other hand, the boiling point and melting point of Si and P are quite close, −120 to 70 and 57–76 °C, respectively. Therefore, the separation from PCl3 will be a problem when SiCl4 is produced from RC char by the chlorination method. Although separation of Si and P in products is a subject for future work, for example, it is proposed that Si and P are completely absorbed into the trap solution equipped at the reactor outlet, and then only Si and/or P is separated/recovered by adsorption or precipitation method. Recently, we have been developing adsorbents with high adsorption performance for P ions from aqueous in the acidic region. Such an adsorbent may be used for the separation of Si and P.

2.4. Effect of Holding Time on Release Extent of Elements

To clarify the effect of holding time on the release behavior of the ash and ash constituent elements, the release extent of each element at 1000 °C for 0–60 min holding was investigated and the results are shown in Figure 5. The release extent of ash at 1000 °C increased by 5% and reached 80% after 10 min of holding, but no further change was observed when the time was prolonged (Figure 5a). Figure 5b shows the results for each element; the release extents of Si and P at 1000 °C increased by <10% during 10 min of holding; however, the values did not increase after that. On the other hand, the release extents of Na and K at 1000 °C increased significantly, reaching almost 100% by 10 min of holding. The volatility of Mg and Ca for alkaline earth metals increased with increasing time up to 20–30 min holding, but remained almost constant after 30 min. The extent of the increase in volatility was greater for Mg compared to that for Ca. Based on these results, it can be said that retention is not effective for the separation of Si from RC and that the temperature is an important factor. In addition, it was impossible to separate Si from the other elements in the retention process. The effect of heating rate during chlorination on volatile behavior of elements will be the subject in our future work.

Figure 5.

Figure 5

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Effect of holding time on the volatile extent of ash (a) and elements (b) at 1000 °C during chlorination of RC800.

2.5. Change in Cl Content in Chlorinated Residue with Temperature

As shown in Figure 3, an increase in the yield of RC during chlorination was observed at 300–600 °C. In contrast, in the temperature range of 300–600 °C, the release extent of ash was approximately 20% (Figure 4), and a discrepancy between the two data was observed. Therefore, the Cl content and the form of the chlorinated RC prepared at each temperature were investigated and the results are shown in Figure 6. In the present study, the Cl chemical forms were classified into H2O-soluble and -insoluble forms. Figure 6a shows the yields of H2O-treated chlorinated RC with the chlorination temperature as the RC basis. When the chlorinated RC obtained at any chlorination temperature was treated with H2O, the yields decreased at all temperatures. This result suggests that a part of Cl is adsorbed on the RC surface as a H2O-soluble form during the chlorination process. However, the yield of H2O treated chlorinated RC obtained at 500–600 °C also exceeded 100%. Figure 6b shows the temperature change in the Cl content in the chlorinated RC and H2O-treated samples. The Cl content in the chlorinated samples increased significantly from 300 to 500 °C and reached 22 wt % by 700 °C. It then decreased with increasing treatment temperature to 8% by 1000 °C, 20 min holding. Although the change in behavior of the Cl content in the H2O-treated samples showed a similar trend to that of the chlorinated samples, the amount of H2O-soluble Cl was smaller than that of the insoluble Cl. In other words, while some of the Cl2 gas is adsorbed on the RC surface as a H2O-soluble Cl form, most of the Cl2 gas is adsorbed on the carbon at the RC surface with an H2O-insoluble Cl form. This indicates that the increase in yield observed at 400–600 °C is mainly due to the adsorption of Cl species on the RC surface. According to previous reports on the degradation of Cl2 gas on pyrolysis carbon surfaces, Cl2 is thought to dissociate on carbon at an activation energy of approximately half of its binding energy,28 forming an active intermediate through C–Cl2 interactions.28 In other words, it is speculated that active Cl atoms are formed on the carbon and the reaction of SiO2 is caused by the species.17 Therefore, it has been concluded that carbon plays a dual role as (a) a catalyst for the formation of active Cl species and (b) a reducing agent for the removal of oxygen from SiO2.17 Because the RC was prepared at a pyrolysis temperature of 800 °C in the present study, there is no (or very little) residue of RH-derived carboxyl or hydroxyl groups. Therefore, the site for Cl to react with carbon may be the active site at the edge of the carbon structure in RC.29 The elucidation of the reaction site is an issue for the future.

Figure 6.

Figure 6

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Changes in the yield (a) and Cl content (b) in the chlorinated sample or H2O-treated sample with the temperature.

2.6. Investigation of Solid-Solid State Reaction during Chlorination Process

As observed in Figure 6, the Cl content in the chlorinated residue shows a maximum value at 600–700 °C. Here, we investigated the effect of Cl adsorbed on the solid phase on the volatile separation of ash. In other words, we considered the possible occurrence of a solid–solid state (between ash and Cl adsorbed on the carbon surface) reaction. As discussed in refs (17) and (26) mentioned above, if the Cl species dissociated on the carbon surface are associated in the chlorination reaction, the release of ash progresses by the adsorption of Cl in the solid phase beforehand, even in an inert gas. Figure 7a shows the change in RC yields. Here, the chlorinated sample and the sample heat-treated in N2 after chlorination up to 600 °C are denoted RC-Cl2 and RC-N2, respectively. The yields of RCs kept for 30 min in N2 after chlorination at 600 °C were almost unchanged compared to those at 600 °C chlorination alone. On the other hand, the yields of RC-N2 prepared at 700–1000 °C in N2 were larger than those of RC-Cl2 at 700–1000 °C. This indicates that ash volatilization in RC-N2 may not have progressed compared to that with Cl2 treatment. The ash content was investigated (denoted as ash-Cl2, ash-N2), and although a decrease in yield occurred over 700–1000 °C when heated in N2, there was almost no ash volatilization, which was constant even when heated at 1000 °C holding. These results indicate that the Cl species adsorbed on the RC do not contribute to the chlorination reaction of the ash in the char. However, because a decrease in yield was observed when the chlorinated sample was heated in N2, we performed a Cl analysis of the sample. The results are shown in Figure 7b. After chlorination up to 600 °C, the Cl content was significantly reduced from 22 to 13% by holding it at 600 °C in N2 for 30 min. As the heating temperature in N2 increased, the Cl content tended to decrease, falling to less than 5% at 1000 °C. The Cl content was almost unchanged at 1000 °C holding. Based on these results, it can be said that the Cl adsorbed on the char is desorbed by the heat treatment and does not contribute to the chlorination reaction of the ash. In other words, it can be concluded that there is no solid-solid state reaction between the ash and Cl adsorbed on the carbon.

Figure 7.

Figure 7

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Changes in yield and the ash content (a) and Cl content (b) in RC or RC chlorinated at 600 °C with chlorination or heat treatment in N2.

2.7. Change in the Pore Structure in Chlorinated RC with Temperature

As shown in Figure 4, the release extent of ash tended to increase sharply at 600–700 °C, and it became low above 800 °C. To clarify the reason for this, the pore structure of the chlorinated residue by the N2 adsorption method is shown in Figure 8a. Although the surface area of the RC was extremely small <10 m2/g, its value increased with increasing chlorination temperature to 40 m2/g at 600 °C. A rapid increase in the surface area was observed from 600 to 700 °C, where ash volatilization progressed, and the surface area increased linearly from 700 to 1000 °C, reaching 335 m2/g at 1000 °C. On the other hand, no change in the surface area was observed at 1000 °C holding. A similar trend was also observed for the Barrett, Joyner, and Halenda (BJH) pore volume. Figure 8b shows the results of the relationship between the ash release extent and the surface area or pore volume. A relatively good positive correlation was found between the ash release extent and surface area. The pore size distribution was examined by the BJH method based on the observed N2 adsorption/desorption isotherms curves, and the pore peaks observed in RC at approximately 3–4 nm increased with the chlorination temperature. These results indicate that SiO2 in RC is mainly present in the carbon matrix in the nanometer order and that their volatilization during chlorination led to the development of mesopores in the RC chlorinated residue. In addition, it is considered that the volatilization of Si during chlorination led to the development of pores, and the diffusion of Cl2 gas into the carbon pores became rate controlled, resulting in a decrease in the rate of Si chloride volatilization (Figure 4).

Figure 8.

Figure 8

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Change in the specific surface area of the chlorinated residue obtained after chlorination at different temperatures (a) and relationship between the release extent of ash and the specific surface area or pore volume (b).

2.8. Hg Adsorption Performance of Chlorinated Residue

As shown in Figure 6b, H2O insoluble Cl is present in the chlorinated residue. This section described the Hg adsorption performance of the chlorinated residue prepared at 1000 °C, 10 min holding (denoted as RC-Cl2), which is the residue after Si is separated by the chlorination process. Figure 9 shows the change in the amount of Hg adsorption on RC and RC-Cl2 residues in the HgCl2 solution at 4.8 × 103 mg-Hg/L as a function of the reaction time. The adsorption amount of RC increased gently with increasing reaction time and was almost consistent with that of RC for more than 8 h. In contrast, in the RC-Cl2 residue, adsorption occurred rapidly by a reaction time of 2 h and the subsequent increase in adsorption was small. From the relationship between the adsorption and the reaction time, as shown in Figure 9, the adsorption kinetics were calculated by Lagergren’s pseudo first-order and secondary-order reaction equations. The results are summarized in Figure 10 and Table 4. The correlation coefficients of the above relationships for the RC and RC-Cl2 residues were 0.985, 0.741, and 0.987, 0.999 for the pseudo first- and second-order reaction kinetics, respectively, and the correlation was high for the pseudo second-order reaction kinetics model. The results of fitting of experimental results using these values are shown by the dashed lines in Figure 9. Although there is some error, the data can be expressed by the pseudo second-order reaction rate equation. In accordance with previous reports investigating an adsorption rate of 4.8 × 103 mg-Hg/L in the HgCl2 solution using sulfur-loaded carbon, the qe was in the range of 223–253 mg/g30 and the qe of RC-Cl2 (330 mg/g) was greater than that of the sulfur-loaded carbon. In other words, the present adsorbent exhibited better performance compared to that of the sulfur-loaded carbon. It is well known, the form of mercury divalent in aqueous solutions depends on the pH of the solution, the coexisting halogen ion species and their concentrations, and the predominant Hg species in acidic solutions of HgCl2(II) are HgCl2, HgCl+, and Hg2+.31−33 Therefore, in this study, mercury species, such as HgCl2(II) are HgCl2, HgCl+, and Hg2+, in the liquid phase (pH 2–2.5) may be adsorbed by the chlorinated residue. Therefore, it is concluded that the present adsorbent is effective for removing mercury from wastewater in acidic regions. The effect of pH on adsorption performance is an issue for future study.

Figure 9.

Figure 9

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Mercury adsorption of RC and RC-Cl2 residues as a function of the reaction time.

Figure 10.

Figure 10

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Pseudo-second-order kinetic plots of mercury adsorption by RC and RC-Cl2 residues.

Table 4. Kinetic Parameters of Hg(II) Adsorption in RC and RC-Cl2 Residues.

  pseudo first-order kinetic model
 pseudo second-order kinetic model
adsorbent k1, 1/min R2 k2, g/mg·h R2
RC 0.167 0.985 0.0160 0.987
RC-Cl2 0.253 0.741 0.0029 0.999
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Next, the equilibrium adsorption of RC and RC-Cl2 was investigated by using HgCl2 solution with an initial Hg concentration of 0.01 × 103 to 20 × 103 mg/L, and the results are shown in Figure 11. The adsorption equilibrium amount of RC was greater than 5.0 × 103 mg/L and its adsorption was small. In contrast, the Hg adsorption amount of RC-Cl2 increased almost linearly up to 5.0 × 103 mg/L and reached almost equilibrium above 10 × 103 mg/L. The results were analyzed using the Langmuir and Freundlich adsorption isotherm models. In the Freundlich analysis, the log qe values were plotted against log Ce, and the adsorption rate constants (1/n) and KF were obtained from the slope and the intercept of the obtained linear relationship, as shown in Figure 12. The results of fitting the obtained equilibrium concentrations and adsorption amounts by the Langmuir adsorption isotherm models are shown as dashed lines in Figure 11, and the results are summarized in Table 5.

Figure 11.

Figure 11

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Mercury adsorption of RC and RC-Cl2 residues as a function of Hg equilibrium concentration.

Figure 12.

Figure 12

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Langmuir plots of mercury adsorption by RC and RC-Cl2 residues.

Table 5. Adsorption Constants for Hg(II) Adsorption by RC and RC-Cl2 Residues.

  Freundlich
 Langmuir
adsorbent KF, mg/g 1/n R2 KL, L/mg qm, mg/g R2
RC 0.267 0.625 0.984 0.217 216 0.990
RC-Cl2 0.577 0.8085 0.971 0.617 617 0.975
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Higher correlation coefficients were obtained for the Langmuir adsorption isotherm model compared to those for the Freundlich adsorption isotherm model, and a relatively good linear relationship between the equilibrium concentration and the Ce/qe value was obtained for the Langmuir adsorption isotherm model. The qm values obtained from the calculations were almost consistent with the adsorption capacity. However, a comparison of RC and RC-Cl2 showed that the correlation coefficient was higher in the former. The qm values of RC and RC-Cl2 were 216 and 617 mg/g, respectively, showing a larger value for the latter. It has been reported that OH and COOH groups on activated carbon are the adsorption sites for Hg adsorption.34 In addition to the two oxygen-containing functional groups mentioned above, adsorption by the π–Hg interaction also occurs.34 As mentioned earlier, because the pyrolyzed chars of RH are prepared at 800 °C, it is unlikely that oxygen-containing functional groups such as OH and COOH remain, so the active sites such as the edges and π-electron sites on the carbon structure might be the Hg adsorption sites. Also, it is well known that the surface area is a factor in the adsorption of heavy metal ions; the specific surface areas of RC and RC-Cl2 are <10 and 325 m2/g, respectively, and the porosity is well developed in the latter. Therefore, the surface area may have an effect on the Hg adsorption performance. To investigate the effect of the surface area, the Hg adsorption capacity of carbon (surface area 405 m2/g) obtained by demineralization of RC in 48% HF solution was examined, and it was found that the Hg adsorption capacity was 650 mg/g, which was almost similar to the Hg adsorption capacity of RC-Cl2. From these results, the reason for the greater Hg adsorption performance of RC-Cl2 compared to that of RC cannot be explained by the surface area alone. According to a report on the Hg adsorption performance of adsorbents impregnated with copper chloride on activated carbon, the loading of copper chloride the increased the Hg adsorption, which was attributed to the fact that Cl supported on activated carbon acted as an Hg adsorption site.35 The Cl content in RC-Cl2 is 8.9%, most of which is H2O insoluble Cl (Figure 6), but the involvement of Cl in Hg adsorption has not been investigated in detail. In other words, the H2O insoluble Cl adsorbed on the carbon during chlorination may also be an adsorption site, and the effect of Cl on the carbon and on Hg adsorption is a future issue.

From these results, it was found that (1) more than 80% of Si could be separated from the RH pyrolyzed char by the chlorination method as SiCl4, and (2) the chlorinated residue had the Hg adsorption ability because of the porousness by demineralization and Cl adsorption on the char during chlorination process. Therefore, it was shown that the present method could be a potential coproduction method for SiCl4 and Hg adsorbents from RH.

Finally, Table 6 shows the previous studies on Hg adsorbent performance from agricultural residues. The Hg adsorption performance (617 mg/g) of RC-Cl2 prepared at 1000 °C, 10 min holding in Cl2 in this study was found to be higher than other agricultural residues. In addition, prepared RC-Cl2 shows high Hg adsorption amount comparing with the other hetero-loaded carbon (nos. 15–21 in Table 6). Therefore, it was found that the present method can produce SiCl4 and Cl-loaded carbon with excellent Hg adsorption performance.

Table 6. Previous Studies on Hg Adsorbent Performance from Agricultural Residues.

no. adsorbent pH temperature, °C maximum Hg adsorption amount (qm), mg/g refs
1 copper chloride-impregnated AC 6 25 167 (35)
           
2 corn straw biochar (BC) 6 25 3.2 (36)
3 Na2S modified BC (BS)     5.7  
4 KOH modified BC     4.2  
5 AC (KOH activated BC)     4.5  
           
6 RH 6.5 30 63 (37)
7 RH-NaOH modified     83  
8 rice straw (RS)     75  
9 RS-NaOH modified     58  
           
10 unmodified RH 6 50 1.3 (38)
           
11 biomatrix from RH 5.5 32 36 (39)
           
12 extracellular biopolymer poly(c-glutamic acid) 6 30 97 (40)
13 AC from fruit shell of Terminalia catappa 5 60 184 (41)
14 steam activated carbon (S-AC) 6 60 208 (42)
           
15 S-AC in presence H2S   60 217  
16 S-AC in presence SO2   60 222  
17 S-AC in presence SO2 and H2O   60 227  
           
18 H2SO4 treated RH (dry) 6 45 303 (43)
19 H2SO4 treated RH (wet)   45 384  
           
20 H2SO4 treated flax shive (dry) 6.5 45 385 (44)
21 H2SO4 treated flax shive (wet)   45 526  
           
22 RC 2–2.5 25 216  
23 RC_Cl2     617 in this study
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3. Conclusions

The production of silicone tetrachloride (SiCl4) from RH char by chlorination was investigated, and the effect of the char preparation temperature on SiCl4 volatilization and the coexisting element species in the char was examined. The behavior of chlorine (Cl) and the change in pore properties during char chlorination were analyzed and the reaction mechanism was discussed. The performance of Hg ion removal of the chlorination residue was also investigated. At 1000 °C chlorination, the optimum RH pyrolysis temperature for attaining high ash-release extent was 800 °C. Si volatilization reached 70–75% by 1000 °C. The Cl content in the residue increased from 300 to 700 °C and then decreased by 1000 °C. The majority of Cl taken up in the residue was an H2O insoluble form. The chlorinated residue became porous by chlorination treatment. The maximum Hg adsorption amount of the chlorinated residue obtained at 1000 °C, 10 min holding was 620 mg/g. Therefore, it is concluded that the present adsorbent may be possible to apply for removing mercury from wastewater in acidic regions from the industry.

4. Experimental Section

4.1. Sample

RH generated in Japan was used as a sample. The analysis values of the RH used in this study are listed in Tables 1 and 2. The values were determined by Japanese Industrial Standard (JIS M8813) and the previous report.45 The ash content in the RH was 22 wt %-dry, of which most of the ash was Si.

4.2. Pyrolysis

The pyrolysis of RH was carried out in a fixed-bed reactor. The apparatus image is shown in Figure S2 in Supporting Information. Approximately 1.0 g of RH was placed in a quartz boat and positioned in the center of the reactor. The samples were heat-treated in high-purity N2 (99.9999%) at 10 °C/min up to 400–900 °C and recovered after cooling to room temperature (20 °C). The pyrolyzed RH (char, denoted as RC) was crushed and sieved to −200 mesh, and then subjected to chlorination experiments. In the present study, the char that was pyrolyzed at 800 °C is referred to as RC800, as shown for the example.

4.3. Chlorination

Chlorination was carried out with a flow-type fixed-bed reactor. The temperature of the reactor was measured and controlled by vertically inserting a thermocouple from outside the furnace and making contact with the outer part of the reaction tube. For the experiment, 1.0 g of the sample was placed in an alumina boat, inserted into the center of the reaction tube, and then heated in Cl2 (99.99%) at 20 °C/min up to 200–1000 °C, and a holding time of 0–60 min. The Cl2 gas and volatile products were completely absorbed by a trap containing sodium hydroxide solution at the outlet of the reaction tube. After reaching the target temperature or time, the gas was switched to high-purity N2 and the furnace was cooled to room temperature. The sample was then removed from the reactor and the yield of the solid sample was calculated by eq 2 by measuring its weight.

4.3. 2

The ash content in the solid sample (residue) recovered after chlorination was determined from the gravimetric measurement at 815 °C combustion in accordance with JIS M8812. The volatility rate of ash during chlorination was calculated by eq 3 as shown below. In this study, the reproducibility was within ±2%.

4.3. 3

4.4. Sequential Heat Treatment in N2 after Chlorination

To clarify whether the chlorination reaction occurs as a solid-state reaction, the chlorinated samples were continuously heated in N2. The flow-type fixed-bed reactor described above was used for the experiments. Using the same method as above, the sample was heated in Cl2 up to 600 °C, and then the Cl2 was switched to 100 mL STP/min N2 for 30 min to completely remove the Cl2 gas in the reactor while maintaining the temperature. The samples were then heated to 700–1000 °C at 20 °C/min in N2. The yield and ash content of the samples recovered after cooling the reactor were calculated by eqs 2 and 3. The reproducibility was within ±3%.

4.5. Characterization

The dissolution of ash in the prepared samples was performed with a mixed acid of HCl/HNO3/HF as previously reported.45 The content of each element in the solution was measured by induction plasma optical emission spectrometry. The volatility of each element during chlorination was calculated by eq 4. In this study, the ash analysis in the sample was performed until reproducibility was confirmed (mostly 2–3 times per one sample), and the reproducibility and analytical error [analytical range (sensitivity analysis) was 0.01–10 ppmw] were within ±2 and ±3%.

4.5. 4

The kinetics of release behavior of elements in ash was performed as follows; assuming that the volatile reaction kinetics is first-order against the elemental content in the solid phase and that the temperature dependence of the reaction kinetics constant follows the Arrhenius formula, the release behavior of each element can be expressed by eq 5 below

4.5. 5

In the case of considering that the whole reaction occurs as a two-step continuous reaction, the following reaction rate equation (eq 6) was used.

4.5. 6

Here, α indicates the maximum release extent at the first step. In this examination, given that the release extent of Si and P increases moderately above 700 °C, the release extent at 700 °C was assumed to be α at the first step. The reasons for selecting 700 °C were mentioned above.

The Cl content in the samples before and after chlorination was measured by ion chromatography equipped with an automatic combustion device. The Cl content of 0.1 g of the chlorinated sample treated with water in 100 mL of distilled water for 12 h at room temperature was also analyzed by ion chromatography equipped with an automatic combustion device in order to examine the Cl form present in the solid sample after chlorination and to investigate the chlorination reaction. The pore structure of the samples was determined by the N2 adsorption/desorption isotherms using the N2 adsorption method, and then the Brunauer, Emmett, and Teller and BJH methods were used to calculate the pore structure. In these analyses, the reproducibility and analytical error were within ±2 and ±3%.

4.6. Adsorption of Hg

The Hg adsorption of RC and chlorinated residues was investigated using the HgCl2 solution at 0.01 × 103 to 20 × 103 mg Hg/L (pH 2–2.5). A sample measuring 0.1 g was added to 10 mL of Hg solution and stirred at 150 oscillation per min at 25 °C and 0.1 mL was sampled at any given time. The concentration of Hg in the solution was measured with a mercury analyzer. Hg adsorption amount was calculated by eq 7

4.6. 7

where qe is the amount of Hg ion adsorption at equilibrium (mg/g), C0 and Ce are the initial and equilibrium Hg concentrations (mg/L), W is the adsorbent weight (g), and V is the volume of the solution (L). In this experimental, the reproducibility and analytical error were within ±5 and ±3%.

The adsorption kinetics were calculated by Lagergren’s pseudo first-order (eq 8) and second-order (eq 9) reaction equations.

4.6. 8
4.6. 9

where qe is the adsorption amount (equilibrium adsorption amount) (mg/g), qt is the Hg ion adsorption at time t (mg), and k1 (1/h) and k2 (g/mg·h) are the pseudo first- and second-order reaction rate constants, respectively. Each reaction rate constant, k1 and k2 was obtained from the slope of the linear relationship between the ln (qe – qt) and t/qt values against the reaction time from the pseudo first- and second-order reaction rate equations, respectively.

Mercury adsorption of adsorbents as a function of Hg equilibrium concentration were analyzed using the Langmuir (eq 10) and Freundlich (eq 11) adsorption isotherm models.

4.6. 10
4.6. 11

where, qm is the equilibrium adsorption amount (mg/g), KL and KF are the Langmuir (L/mg) and Freundlich constants, respectively. In the Langmuir analysis, the relationship between the equilibrium concentration Ce and the Ce/qe values was plotted, and the qm and KL were calculated from the slope and the intercept of the obtained linear relationship.

Acknowledgments

This study was supported in part by a Grant-in-Aid for Challenging Research (Exploratory) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan, and by the Steel Foundation for Environmental Protection Technology (SEPT).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c03789.

  • Equilibrium calculation results of Si (a), Ca (b), Mg (c), K (d), Na (e), and P (f) during the chlorination of RH char and apparatus for pyrolysis of RH (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c03789_si_001.pdf (616.8KB, pdf)

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