Life Cycle Assessment and Process Optimization of Precipitated Nanosilica—A Case Study in China

Abstract

To mitigate environmental emissions in the industrial nanosilica sector and promote its sustainable development, the life cycle assessment (LCA) method is employed to evaluate the environmental impacts throughout the life cycle of industrial precipitated nanosilica. This LCA spans from the acquisition and transportation of raw materials to the production of nanosilica. By identifying the critical contributing factors, effective optimization strategies have been proposed to enhance the environmental performance of the nanosilica life cycle. The effects of electricity, alkalis, acids, and steam on the life cycle emission factors of nanosilica were examined. The results indicate that substituting traditional coal power and steam with cleaner alternatives like wind energy, hydroelectric power, and solar power (both photovoltaic and thermal), as well as biogas steam, can lead to a significant reduction in the life cycle emission factors of nanosilica, ranging from 50% to 90%. Notably, the types of acids and alkalis used only significantly reduce certain environmental factors. These findings provide valuable theoretical insights and practical guidance for the industrial nanosilica sector, particularly in the areas of energy conservation, emission reduction, and the transition towards a lower-carbon economy.

1. Introduction

Advancements in nanotechnology have catalyzed a revolution across numerous domains of human endeavor. Nanosilica, a type of nanoscale silica particle, distinguishes itself through its exceptional physical and chemical properties that arise from its nanoscale dimensions. This material is highly valued for its diverse range of applications spanning multiple industries. Nanosilica is versatile, serving as a reinforcing agent and fortifier within the rubber and concrete sectors [1], and is also commonly used as an additive in the production of coatings and inks [2]. Its porous structure, coupled with its excellent dispersibility and high specific surface area, grants it remarkable selectivity and effectiveness in the adsorption and separation of a variety of water pollutants [3]. This includes not only trace metals and organic compounds but also dyes, antibiotics and emerging contaminants. Amorphous nanosilica is particularly advantageous for a wide array of nano-biomolecular applications, having substantial impacts on manufacturing, material science, the food industry, biotechnology, medicine, diagnostics and healthcare [4]. Dendritic fiber nanosilica, distinguished by its unique fiber morphology and exceptionally high specific surface area, is highly sought after for its extensive applications in fields such as catalysis, photocatalysis, energy harvesting and storage, magnetism, composite materials, carbon dioxide emission reductions, and biomedicine [5].

A variety of techniques, including the sol–gel process [6], chemical precipitation method [7], microemulsion processing [8], plasma synthesis [9], chemical vapor deposition (CVD) [10], combustion in a diffusion flame [11], and pressurized carbonation [12], are utilized in the production of nanosilica. Among these, chemical precipitation and chemical vapor deposition are particularly prevalent in industrial applications due to their reliability and the scalability of these processes. While precipitated silica may not eclipse fumed silica in all performance benchmarks, especially within the sophisticated domain of high-performance composite materials, its notable cost advantages, coupled with the flexibility to tailor its properties for specific industrial needs, positions it as a strategic and cost-effective alternative. Moreover, its environmentally benign attributes align with the current global shift towards sustainable practices, making it not just a viable choice, but also a preferred choice for a diverse spectrum of applications. A report by Global Industry Analysts [13] suggests that the global market for precipitated silica, valued at USD 2.3 billion in 2023, is anticipated to expand to a revised total of USD 4 billion by the year 2030. This growth is projected to occur at a compound annual growth rate (CAGR) of 8% throughout the period spanning 2022 to 2030, indicating a robust and sustained expansion in the market for this versatile industrial material.

The industrial production technology for precipitated nanosilica from water glass has reached a high level of maturity. However, research on the process flow of traditional precipitation methods of nanosilica remains rather limited. A minority of studies have aimed at pioneering new applications for such techniques, such as using precipitated nanosilica for fluorine removal from electrolytes in zinc electrowinning [14]. Current scholarly efforts are increasingly directed towards the utilization of environmentally friendly raw materials in place of conventional sources like quartz sand and sodium silicate. These materials are often derived from agricultural waste, using an improved precipitation method to prepare nanosilica. The types of agricultural waste explored in these studies encompass a wide range, including sugarcane bagasse [7], bamboo leaves [15], corn cob [16], rice husks and straws [17], sorghum leaves [18], wheat straws and husks [19], sorghum vulgare seed heads [20], palm oil residues [21], etc. While the modified precipitation method utilizing agricultural waste offers greater environmentally advantages, it may not be compatible with the existing infrastructure used by numerous traditional precipitated silica producers. The shift in raw material sources necessitates significant upgrades to current manufacturing facilities and entails operational realignments, potentially leading to increased costs.

Innovation in raw materials can significantly enhance the environmental benefits of production processes. However, for existing manufacturing entities, it may be more practical to focus on reducing environmental emissions within their current production processes without altering their fundamental methodologies. Life cycle assessment (LCA) is a potent tool that enables businesses to comprehend the environmental impacts of their products and operations, guiding them to make informed decisions aimed at mitigating these impacts. It is a comprehensive method for evaluating the environmental impacts associated with all the stages of a product’s life from the extraction of raw materials, through production, use, and disposal or recycling [22]. LCA has gained widespread adoption across a spectrum of industries. For instance, Sackey et al. [23] assessed the emissions from material production in nanosilica-modified asphalt mixtures using the LCA methodology, comparing the results to those of a conventional asphalt mixture to understand the impact of nanosilica in asphalt mixtures. Fallah-Valukolaee et al. [24] investigated the environmental impacts of concretes containing silica fume and nanosilica and conventional concrete. It was found that concrete containing 12% silica fume has the greatest environmental damage in terms of acidification, human toxicity, and eutrophication. Joglekar et al. [25] performed an LCA of rice husk nanosilica and observed a total climate change of 7.26 kg CO₂ equivalent per kg of silica production. Dominic et al. [26] synthesized nanosilica from millet husk and indicated that millet husk nanosilica has a lower environmental impact compared to rice husk nanosilica. Farjana et al. [27] identified the sustainable circular economy pathways for waste medium-density fiberboards (MDFs) and particleboard management by comparing different recycling methods using LCA methodologies.

However, to date, no comprehensive studies have been published on the LCAs of industrial nanosilica production, particularly focusing on conventional precipitated nanosilica produced using water glass as the raw material. To alleviate environmental emissions in the field of industrial nanosilica and foster its sustainable development, this paper undertakes a comprehensive life cycle analysis (from cradle to gate) of industrially precipitated nanosilica. This analysis covers the acquisition and transportation of water glass raw materials to the nanosilica production stage, employing the LCA approach. The objective is to enhance the environmental sustainability of nanosilica at the lowest possible cost, without requiring modifications to the existing manufacturing processes. This study provides a theoretical foundation for the sustainable development and low-carbon evolution of enterprises involved in the production of precipitated nanosilica and water glass raw materials.

2. Methodology

2.1. Goal and Scope Definition

An LCA was performed with Gabi software 10.8.0.14 based on ISO 14040 [28] and 14044 [29]. Ecoivent 3.8 databases were employed. This study aims to evaluate the environmental performance of nanosilica production using the industrial precipitation method from a life cycle perspective (cradle to gate). The goal is to identify the hotspots where improvement efforts should be focused on making the systems more efficient from environmental points of view. The system boundary of life cycle systems is shown in Figure 1. The unit processes that make up the system are feedstock acquisition (stage #1), transportation to the plant (stage #2), and nanosilica synthesis (stage #3). The usage and the post-treatment of nanosilica were not included in the scope of the evaluation. The functional unit considered was the preparation of 1 kg of nanosilica products.

2.1.1. System Description: Water Glass (Sodium Silicate) Acquisition

The case study plant was Shanxi Shengyou Technology Building Materials Co., Ltd., in Lvliang City of China. Inventory data were sourced from the environmental impact assessment report of the company’s water glass production line construction project [30]. The project adopted soda ash and quartz sand as raw materials to produce solid sodium silicate using a dry process, with an output of 60,000 tons per year. The data of the soda ash and quartz sand were taken from the Ecoinvent 3.8 database. Coke oven gas was used as fuel for heating kilns in the production process. The chemical composition of the coke oven gas is shown in

Table 1. Chemical composition of coke oven gas for water glass production [30].

	Symbol	Unit	Value
Hydrogen	H2	%	58.4
Oxygen	O2	%	0.40
Methane	CH4	%	24.0
Carbon Monoxide	CO	%	9.0
Carbon Dioxide	CO2	%	3.0
Nitrogen	N2	%	3.6
Hydrocarbons	CmHn	%	1.6
Low Calorific Value	Qnetar	kJ/Nm3	17,362.53
Hydrogen Sulfide	H2S	mg/Nm3	18.40
Ammonia	NH3	mg/Nm3	41.60

. The consumption of coke oven gas in the sodium silicate project typically ranges from 300 to 360 m³/t, with 320 m³/t adopted for this calculation.

The process flow of water glass acquisition is shown in Figure 2a. Soda ash and quartz sand were, respectively, measured using a measuring scale in a 2:1 ratio and then mixed in a mixing machine. The mixed material was fed into the horseshoe flame kiln though the conveyor belt for heating. A high temperature of approximately 1400 °C was generated by the combustion of the coke oven gas. The mixed material reacted in the high- temperature environment. A high-temperature liquid material of 1100–1200 °C was formed. The molten glassy melt flowed out from the other end of the kiln and was cooled to a solid glassy product of water glass through a water skimmer and a dry chain plate discharger.

2.1.2. System Description: Feedstock Transportation

It is assumed that road freight transport was used, with a transport radius of 50 km, indicating short-distance transport. Therefore, short-haul trucks were used as freight vehicles, with diesel used as the energy source. The environmental emission factors of short-haul trucks are calculated directly using the basic material database of GaBi software 10.8.0.14.

2.1.3. System Description: Nanosilica Synthesis

The case study plant was Nanxian Changzheng New Material Technology Co., Ltd., in Guigang City of China. Inventory data were sourced from the environmental impact report of the company’s nanosilica production project [31]. The process flow of precipitated nanosilica synthesis is shown in Figure 2b. Solid water glass and water were added to the solution kettle in equal proportions. Steam was used to maintain pressure and facilitate dissolution. The concentrated liquid water glass was obtained after precipitation in the buffer tank. Dilute water glass was prepared by adding water to the concentrated liquid water glass, which was filtered for use. Concentrated sulfuric acid was prepared into dilute sulfuric acid in the preparation kettle and was mixed with the prepared dilute water glass. After a period of reaction, the mixed material was pumped into the aging tank to age. High-dispersion silica products were obtained after the aging material was filtered, washed, pulped, modified, and dried. The products were then crushed and packed for storage.

2.2. Life Cycle Inventory (LCI)

The main inputs used for environmental and economic assessment in the industrial nanosilica production cases are shown in

Table 2. Inventory analysis of stages of nanosilica production.

	Stage 1		Stage 2		Stage 3
Input	Quartz sand	0.76 kg	Diesel	0.002 kg	Water glass	1.245 kg
	Soda	0.41 kg	Water glass	1 kg	Concentrated sulfuric acid	0.445 kg
	Coke oven gas	0.32 m3			Steam	0.8 kg
	Electricity	5 kWh			Water	26.689 kg
	Water	0.0654 m3			Electricity	0.18 kWh
	Air	0.45 m3
Major Output	Water glass	1 kg	Water glass	1 kg	Nanosilica	1 kg
	Carbon dioxide	0.166 kg			Sodium sulfate	0.6962 kg
					Wastewater	22.894 kg

Note: The table lists only the major outputs or emissions, but not all.

. Data on the water glass and nanosilica synthesis process were obtained from Refs. [30,31].

2.3. Life Cycle Impact Assessment (LCIA)

The environmental impact was characterized using the CML 2001 method provided in Gabi software, and the various environmental categories were analyzed. The limitations of the study include the exclusion of real-time transport data. The data were taken from the Ecoinvent 3.8 database, and the system boundary for nanosilica powder was defined from cradle to gate.

The most common categories, abiotic resource depletion potential (ADP-fossil), acidification potential (AP), eutrophication potential (EP), freshwater aquatic ecotoxicity potential (FAETP), global warming potential (GWP), human toxicity potential (HTP), ozone layer depletion potential (ODP), photochemical ozone creation potential (POCP), terrestrial ecotoxicity potential (TETP), and marine aquatic ecotoxicity potential (MAETP) [32], were included. Normalization and weighting of the obtained results were applied to evaluate the comparative impact or benefits of the various processes within each impact category. This analysis enabled the identification of processes or materials that contribute significantly or marginally to environmental impact and benefits. By focusing on optimizing these critical contributing factors, effective strategies could be proposed to mitigate the environmental footprint of the process. These recommendations are intended to serve as practical guidance for industrial production, aiming to enhance sustainability and efficiency.

3. Results

The results of the life cycle impact assessment (LCIA) obtained using GaBi software 10.8.0.14 are reported in this section. The first section reports the results and analysis of LCIA for the life cycle of nanosilica production. The second section demonstrates the optimization strategies for the environmental aspects of the nanosilica production life cycle.

3.1. Life Cycle Environmental Impacts

The contribution of different life cycle inventories to the various impact indicators for the nanosilica production system is shown in Figure 3 and Figure 4. The electricity during water glass production contributed significantly to the emissions for all the environmental impacts of the nanosilica production life cycle (Figure 3). This is mainly attributed to the preparation process of water glass. This is because quartz sand is mainly used as the raw material during the preparation process of water glass. Due to the high bond energy of the Si-O chemical bonds within the quartz sand, a large amount of energy is required to break these bonds [33]. When preparing water glass, quartz sand and soda need to be melted under high-temperature conditions within a temperature range of 1300 °C to 1400 °C. In addition to the thermal energy provided by the fuel (coke oven gas), a significant amount of electrical energy is also consumed to maintain the operation of the kiln. Additional machinery, including quartz sand and solid water glass grinders, mixers, conveyors, dust removal blowers and ventilation fans, also demand considerable electrical energy.

As shown in Figure 4, eliminating the effect of electricity during water glass production reveals that most of the environmental impacts are attributed to soda (sodium carbonate), including EP (69.0%), AP (47.0%), POCP (44.0%), GWP (40.6%), ODP (27.6%) and the ADP fossil (26.4%). Sulfuric acid is the dominant contributor to the environmental impact of FAETP (38.8%). Sulfuric acid is the second largest contributor to AP (37.0%) and POCP (29.8%) after sodium hydroxide. Eliminating the effect of electricity during water glass production shows that steam contributes the most to HTP impact, with a contribution of 34.1%, followed by electricity, which contributes 24.4%. The main contributor to the MAETP impact is the electricity during nanosilica production, with a contribution of 27.6%, followed by sulfuric acid, steam, and soda, with contributions of 16.4%, 16.4%, 13.7%, respectively.

Based on the above analysis, elements of electricity, soda, sulfuric acid, and steam were optimized to reduce the environmental impacts of the nanosilica production life cycle.

3.2. Improvement Assessment

3.2.1. Electricity

To better demonstrate the influence of environmental factors during the production cycle of nanosilica, the electricity used in both the water glass production stage and the nanosilica production stage was uniformly analyzed. A comparison of the total emissions for the nanosilica life cycle under electricity generated from different technologies and 10% power saving is reported in Figure 5. In order to conduct effective comparative analysis, the single-factor variable control method was adopted, which involved adjusting only the type or consumption of electricity while keeping all the other parameters constant. Specifically, in the “10% power saving” case, this referred to a 10% reduction in the use of coal-fired power generation. For other cases, only the type of electricity is changed, with the electricity consumption remaining the same. The parameters of all the energy resources (electricity referred to in Figure 5 and steam referred to in Figure 6) are exhibited in

Table 3. The parameters of all energy resources (electricity referred to in Figure 5 and steam referred to in Figure 6). Unit: for electricity, kg/kWh; for steam, kg/MJ.

	Electricity from Coal Power Generation	Electricity from Wind Power	Electricity from Hydropower	Electricity from Municipal Solid Waste Power Generation	Electricity from Solar Thermal Power Generation	Electricity from Photovoltaic Power Generation	Electricity from Biomass Power Generation	Steam from Hard Coal	Steam from Biogas	Steam from Biomass	Steam from Natural Gas
Resources	65.54	19.97	3121.15	29.25	6.60	15.31	102.28	1.74	35.19	4.44	0.90
Deposited goods	1.89	0.05	0.04	0.18	0.04	0.24	0.04	0.19	0.03	3.48 × 10−3	5.18 × 10−3
Emissions to air	7.84	0.20	22.92	17.84	0.24	17.28	38.08	0.58	27.77	2.79	0.47
Emissions to fresh water	41.26	19.73	3098.17	12.69	6.36	14.97	52.56	0.95	8.81	1.79	0.35
Emissions to sea water	14.54	0.09	1.16 × 10−3	0.32	0.02	0.07	13.48	0.01	0.03	7.74 × 10−3	4.10 × 10−3
Emissions to agricultural soil	2.60 × 10−8	5.25 × 10−8	1.11 × 10−9	2.23 × 10−7	4.21 × 10−8	1.84 × 10−7	5.59 × 10−7	2.68 × 10−9	−2.70 × 10−6	4.31 × 10−8	2.03 × 10−9
Emissions to industrial soil	9.47 × 10−8	2.06 × 10−7	2.67 × 10−9	5.59 × 10−4	1.33 × 10−6	3.04 × 10−6	1.89 × 10−7	9.59 × 10−9	4.70 × 10−8	1.48 × 10−8	1.71 × 10−7

.

The environmental impacts of electricity generated from wind, hydropower, solar thermal, and photovoltaic power generation were significantly lower than those from the coal power approach for all the impact categories. For instance, compared to the coal power approach, the environmental impacts of the nanosilica life cycle for cases using electricity generated from the wind, hydropower, solar thermal, and photovoltaic power generation were decreased by 40~90%. It is evident that the electricity generated from renewable energy sources such as wind, solar, and hydro power can significantly reduce pollution emissions during the electricity usage process, making it environmentally friendly. This is primarily because the power generation process does not produce greenhouse gas emissions such as carbon dioxide (CO₂), methane (CH₄), and nitrogen oxide (NOx). Consequently, this type of electricity is commonly known as green electricity. Green electricity has become an inevitable trend in reducing carbon emissions and combating global climate change [34]. To promote the development of green electricity, China formulated the “Implementation Plan for Promoting Green Consumption” in early 2022 [35]. This plan proposed the establishment of a green electricity trading and renewable portfolio standard linkage mechanism. Therefore, electricity consumers, such as producers of precipitated nanosilica and water glass, can fulfill their renewable energy quotas by purchasing green electricity.

However, biomass electricity, which is also considered as green power, does not exhibit significant environmental friendliness throughout the life cycle of nanosilica. In many policies and regulations, biomass utilization is considered to be “carbon-neutral” because the CO₂ released is refixed by vegetation in the next growth cycle [36]. Similarly, for the case of biomass power generation, the ADP fossil, GWP, and ODP exhibited reductions of 58%, 69%, and 88%, respectively. It can be seen that biomass electricity plays a significant role in reducing the consumption of fossil fuels, decreasing greenhouse gas emissions, and mitigating damage to the ozone layer. However, biomass electricity has a significant ecotoxicological impact on human, terrestrial, and aquatic ecosystems. Impact categories EP, FAETP, HTP, TETP, and MAETP increased to varying degrees. Notably, the TETP and MAETP increased by 92% and 87%, respectively. This is mainly attributed to the fertilizer required for biomass growth containing high levels of nutrients such as nitrogen and phosphorus. These nutrients may not be fully absorbed by the crops, instead entering water bodies through surface runoff and groundwater. There is room for improvement, and it is possible to improve the toxicity results through the rational application of fertilizers or the use of slow-release fertilizers.

There were significant reductions in ADP fossil, AP, FAETP, HTP, ODP and POCP for the case of the municipal solid waste power generation of 55%, 45%, 45%, 77% and 64%, respectively. The EP, MAETP, and TETP of the municipal solid waste case decreased by 22%, 26% and 35%, respectively. However, the GWP of municipal solid waste case decreased by only 2%, compared to the coal power approach. The use of municipal solid waste power has little effect on greenhouse gas emission reductions. When the process saved 10% of the electricity under coal power generation, all the environmental categories experienced a certain degree of decline. However, compared to the cases using other types of electricity without power saving, most of the impact categories changed slightly. It is evident from the results that green electricity sources like wind, hydropower, solar thermal, and photovoltaic power generation can significantly reduce the environmental impacts of the nanosilica life cycle, which is more environmentally friendly. Electricity generated from municipal solid waste and biomass power generation is friendly to certain aspects of the environment, the usage of which should be considered carefully, such as for the purpose of treating solid waste.

3.2.2. Alkalis and Acids

Soda ash and sulfuric acid are pivotal raw materials within the nanosilica life cycle, serving as principal ingredients in the creation of water glass and nanosilica, respectively. The effects of different alkalis and acids on the environmental factors of nanosilica’s life cycle are illustrated in Figure 7. The amounts of alkalis and acids are determined by the corresponding chemical reactions. The main input inventory of the cases mentioned in Figure 7 is outlined in

Table 4. Main input inventory of cases mentioned in Figure 7.

		Na2CO3 + H2SO4	NaOH + H2SO4	Na2CO3 + HCl	NaOH + HCl	10% Saving of Na2CO3	10% Saving of H2SO4
Stage 1	Quartz sand	0.76 kg	0.76 kg	0.76 kg	0.76 kg	0.41 kg	0.41 kg
	Na2CO3	0.41 kg	/	0.41 kg	/	0.684 kg	0.76 kg
	NaOH	/	0.31 kg	/	0.31 kg	/	/
Stage 3	Water glass	1.245 kg	1.245 kg	1.245 kg	1.245 kg	1.245 kg	1.245 kg
	H2SO4	0.445 kg	0.445 kg	/	/	0.445 kg	0.4005 kg
	HCl	/	/	0.37 kg	0.37 kg	/	/

Note that: Na₂CO₃ is soda (sodium carbonate), H₂SO₄ is sulfuric acid, NaOH is sodium hydroxide, and HCl is hydrochloric acid.

, ignoring the influence of acid and alkali types on the consumption of other parameters such as electricity, water, coke oven gas, air, steam, and diesel.

Given the significant impact of the quantities of alkali and acid on the quality of products, any substantial decrease in the use of soda ash and sulfuric acid would adversely affect both the production yield and quality of water glass and nanosilica. Consequently, Figure 7 illustrates the environmental impact of a 10% reduction in soda ash and sulfuric acid within the nanosilica life cycle. It is assumed that a 10% reduction in soda ash and sulfuric acid has no effect on the yield of the process.

Compared with soda ash, the usage of sodium hydroxide can reduce the impact of most of the environmental factors, such as the ADP fossil, AP, EP, GWP, POCP, and TETP, by 4.0%, 11.3%, 18.1%, 5.3%, 7.4% and 0.6%, respectively, as shown in Figure 7. However, the ODP and MAETP showed significant increases of 24.1% and 3.6%, respectively. Similarly, compared to sulfuric acid, the usage of hydrochloric acid can reduce all environmental factors except HTP and MAETP. It can be seen that sodium hydroxide and hydrochloric acid play a role in resource consumption and climate change. But the usage of sodium hydroxide will exacerbate the destruction of the atmospheric ozone layer. Chlorine-containing wastewater caused by hydrochloric acid is difficult to treat, which is not conducive to the stability of marine ecosystems.

A 10% reduction in soda ash resulted in a modest decrease across various impact categories: the ADP fossil, AP, EP, FAETP, GWP, POCP and TETP saw reductions of 1.8%, 2.6%, 4.3%, 1.3%, 1.9%, 1.8%, and 2.0%, respectively. Similarly, a 10% reduction in sulfuric acid led to a minimal decrease in environmental impacts, ranging from 0.1% to 1.7%. These findings indicate that while there is some reduction in environmental impacts with the lower consumption of soda ash and sulfuric acid, the overall effect is not substantial, compared to changing the types of alkalis and acids. This suggests that the environmental benefits of reducing these raw materials may be limited and should be considered in the broader context of maintaining product quality and yield.

3.2.3. Steam

As detailed in Section 3.1, the type of steam used is also a critical factor affecting the environmental emissions throughout the nanosilica life cycle. Figure 6 encapsulates the environmental impacts of the nanosilica life cycle based on steam derived from various sources. In order to conduct effective comparative analysis, the single-factor variable control method was adopted, which involves adjusting only the type of steam acquisition process while keeping the steam consumption and all the other parameters constant.

Observing Figure 6, it becomes evident that the environmental impacts across cases utilizing steam from different processes do not follow a straightforward pattern. Consistent with the findings related to biomass electricity case, the ecological toxicity linked to the biomass steam case remains substantially high. Specifically, the FAETP, HTP, POCP, TETP, and MAETP of the biomass steam cases are the most significant when juxtaposed with other cases. Notably, the MAETP is as much as 1.4 times higher than that of the cases utilizing steam from hard coal. Given these considerations, the use of process steam from biomass is not advocated for nanosilica production due to its disproportionately high environmental impact in several critical areas.

The AP, EP, HTP, ODP, POCP, and MAETP of cases under steam from natural gas are the lowest compared those of the other cases. However, the ADP fossil and GWP are notably higher than those of the biomass and biogas cases, yet slightly lower than those of hard coal steam. In contrast to the case of hard coal steam, the ADP fossil, GWP, HTP, TETP, and MAETP in the case of biogas steam were significantly lower. However, the AP and EP were higher than in other cases. Given the substantial impact of fossil fuel consumption and greenhouse gas emissions on global climate change, the biogas steam cases demonstrated superior environmental performance in comparison to natural gas steam cases. Taking into account China’s limited natural gas reserves, the costs of natural gas steam are anticipated to be relatively higher. Therefore, biogas steam is more recommended for application throughout the nanosilica production life cycle due to its more favorable environmental profile and resource availability considerations.

3.3. Final Changes for Improvement

In conclusion, the selection of electricity sources substantially influences the environmental emissions throughout the nanosilica life cycle. Renewable energy sources, including wind, hydropower, and solar (both thermal and photovoltaic), can significantly curtail emission levels. Compared with sodium carbonate and sulfuric acid, sodium hydroxide and hydrochloric acid can reduce most environmental factors, but have significant adverse effects on the atmospheric ozone layer (sodium hydroxide) and marine ecosystems (hydrochloric acid). The reduction in materials such as soda ash and sulfuric acid has a minimal impact on outcomes and could potentially compromise product quality. Additionally, the steam source is a contributing factor to the overall results. Biogas steam has the potential to reduce emissions to varying degrees in specific regions. Consequently, based on the aforementioned analysis, this section focuses on four energy sources with relatively lower environmental impacts—hydropower, sodium hydroxide, sulfuric acid, and biogas—to refine life cycle emissions. The outcomes of this optimization are depicted in Figure 8.

As illustrated in Figure 8, the optimization process led to a substantial reduction across all the environmental impact categories. There was a significant decrease in FAETP and ODP by 51.2% and 64.9%, respectively. Other impact categories experienced an even more pronounced reduction, ranging from 70 to 90%. For instance, GWP was reduced by 84.0%, HTP by 85.5%, POCP by 85.4%, TETP by 79.7%, AP by 77.6%, the ADP fossil by 74.1%, EP by 72.2%, and MAETP by an impressive 90.5%.

It is worth noting that while the combination of hydropower, sodium hydroxide, sulfuric acid, and biogas has proven effective, it may not be the sole optimal solution. Alternative combinations involving green electricity and natural gas steam could potentially yield similarly reduced environmental emissions. Nonetheless, when juxtaposed with the pre-optimization scenario that relied on coal power and coal steam, the synergistic use of hydropower and biogas demonstrated a marked impact in diminishing environmental emissions.

It is noteworthy that regardless of the combination of any type of green electricity (excluding biomass electricity) and clean steam (such as steam from biogas or natural gas), replacing traditional energy sources with clean energy will undoubtedly bring about significant changes in emission reductions in the field of nanosilica production. According to the “2024 China silica Market Research Report” released by the Industry Research Institute, China’s production of precipitated nanosilica was 1.83 million tons in 2023. It is assumed that the market share of nanosilica via the precipitation method is 50%. If the energy [37] used in the production process of these nanosilica was all replaced with clean energy (such as hydropower and steam from biogas), approximately 48,788 GJ of the ADP fossil could be reduced, the GWP could be reduced by about 4.81 million tons of CO₂ equivalents, and the MAETP could be reduced by approximately 494.1 million tons of DCB equivalents (based on the Na₂CO₃ + H₂SO₄ process). With China’s broad support for new energy power generation, the installed capacity of renewable energy has continuously achieved new breakthroughs. As of the end of September 2024, the installed capacity of renewable energy nationwide reached 1.73 billion kW, representing a year-on-year increase of 25%, and accounting for approximately 54.7% of China’s total installed capacity. The generation of renewable energy has steadily increased. In the first three quarters of 2024, the generation of renewable energy nationwide reached 2.51 trillion kWh, an increase of 20.9% compared to the same period last year, accounting for about 35.5% of total electricity generation [38]. Evidently, in the field of nanosilica production, the adoption of clean energy to replace traditional energy sources was highly likely to become a reality, and presented an extremely promising outlook.

4. Conclusions

Traditional industries are undergoing significant transformations in terms of energy conservation, emission reduction, and low-carbon initiatives. Life cycle assessments (LCAs) stand as a powerful tool for businesses to comprehend the environmental ramifications of their products and operations. However, comprehensive LCA studies on industrial nanosilica production, particularly focusing on conventional precipitated nanosilica, remain scarce to date. This paper systematically investigated the environmental emission characteristics of the life cycle of industrial precipitated nanosilica, employing the LCA methodology (cradle to gate). The key findings are as follows.

Compared to raw materials and steam types, the selection of electricity types holds the utmost significance in influencing environmental emissions throughout the nanosilica life cycle, compared to raw materials and steam types. Green electricity sources, such as wind, hydropower, solar thermal, and photovoltaic generation, can markedly diminish the environmental footprint of the nanosilica life cycle, achieving a reduction of 40–90% across all environmental impacts. For instance, the ADP fossil, AP, EP, FAETP, MAETP, and TETP for the above green electricity cases have reduced by about 40–60%. The GWP, POCP, and ODP for the above green electricity cases have reduced by about 69%, 73%, and 75–89%, respectively. Biomass electricity notably reduces the ADP fossil and GWP, but introduces a substantial increment in ecotoxicological impacts on humans, terrestrial, and aquatic ecosystems due to the use of chemical fertilizers. Conversely, municipal solid waste power exhibits minimal impact on greenhouse gas emission reduction.

Compared with soda ash and sulfuric acid, the utilization of sodium hydroxide and hydrochloric acid can alleviate most environmental factors. However, sodium hydroxide exacerbates the depletion of the atmospheric ozone layer. Additionally, chlorine-laden wastewater resulting from hydrochloric acid use is detrimental to marine ecosystem stability. Notably, altering the types of acids and alkalis has a more pronounced effect on environmental factors than reducing their quantities. Biogas steam demonstrates significantly lower ADP fossil, GWP, HTP, TETP, and MAETP compared to other steam options. Biogas steam is more highly recommended for application throughout the nanosilica production life cycle.

Based on the existing Na₂CO₃ + H₂SO₄ process, replacing conventional energy resources with clean alternatives (such as hydropower and steam from biogas) is expected to achieve ADP fossil savings of about 48,788 GJ/year, GWP emission reductions of about 4.81 million tons of CO₂ equivalent per year, and MAETP emission reductions of about 494.1 million tons of DCB equivalent per year. This estimate is based on the production of 0.915 million tons of nanosilica via precipitation in China in 2023. With China’s broad support for new energy power generation, the adoption of clean alternatives to replace traditional energy sources in the field of producing nanosilica is highly likely to become a reality, and it has an extremely promising outlook.