It is often asked whether the use of CO₂ in the sodium silicate sand process in casting can be considered a form of carbon neutrality. Today, we will briefly analyze the carbon neutrality potential of the CO₂ hardening process for sodium silicate sand in casting.
1. Principle of Carbon Utilization in the Sodium Silicate Sand Process
The basic principle of sodium silicate hardening involves the hydrolysis of sodium silicate and its reaction with CO₂, which indeed consumes carbon.
Currently, most foundries use industrial-grade CO₂ from cylinders. Under carbon neutrality requirements, this CO₂ can be replaced with CO₂ generated from the foundry’s own furnace combustion or captured from emissions of other enterprises or industries through carbon capture technology, thereby participating in carbon utilization and carbon neutrality.
After CO₂ enters the sodium silicate-containing sand, it quickly forms relatively stable carbonates. These carbonates remain in the solid form in the used sand after pouring and shakeout. Through sand regeneration, this carbon is either removed as dust by collectors or remains in the discarded waste sand, exiting the sand circulation system. The dust and waste sand from sodium silicate sand can be recycled for use in cement, brick-making, or other new building materials, ultimately storing the carbon permanently in the form of solid carbonates in these materials. Carbonates are relatively stable, and the likelihood of carbon re-entering the atmosphere as a gas is low, thus achieving carbon fixation.
2. Carbon Neutrality Capacity
According to the chemical reaction principle, the amount of CO₂ consumed in the sodium silicate hardening process is fixed relative to the Na₂O content, allowing for theoretical calculations. When considering the carbon neutrality capacity per unit mass of sodium silicate sand, it is determined by the Na₂O content of the sodium silicate used and the amount of sodium silicate added in the process.
By making certain assumptions within common ranges for these parameters and performing calculations, we can determine the amount of CO₂ neutralized per ton of castings produced using the sodium silicate sand CO₂ hardening process. On average, this value is 0.0349 tons. Thus, a foundry producing 10,000 tons of castings per year can neutralize a total of 0.0349 × 10,000 tons of CO₂.
Meanwhile, we can estimate the total carbon emissions of the enterprise. Assuming that 90% of the total carbon emissions of a foundry producing 10,000 tons of steel castings per year come from energy sources, and assuming that all energy used is electricity, the estimated annual carbon emissions of the enterprise are approximately 29,500 tons of CO₂.
By comparison, the amount of carbon neutralized by the sodium silicate sand process accounts for only 1.2% of the enterprise’s total emissions, which can be described as “a drop in the bucket.”
3. Feasibility of Participating in CCUS
CCUS technology (carbon capture, utilization, and storage) refers to the collection, utilization, and storage of carbon. Although the sodium silicate sand process can indeed fix a certain amount of carbon, its carbon neutrality capacity is low and insignificant compared to the total carbon emissions of a foundry. At this stage, it is not yet capable of participating in CCUS. To promote the participation of the sodium silicate sand CO₂ hardening process in CCUS, the following issues need to be considered or resolved:
Define a Reasonable Carbon Utilization Amount
In actual production, it is absolutely not advisable to increase the amount of sodium silicate or sand used solely to absorb more carbon. Lean production improvements in casting require continuously pursuing production with less sand and lower sodium silicate usage, which reduces more pollutants and indirect energy consumption at the source and should be prioritized. Therefore, reasonable process parameters for sodium silicate sand and carbon utilization must be determined based on the actual production conditions of each enterprise, following a “one enterprise, one policy” approach.
Clarify the Carbon Mineralization Pathway
The current recycling pathways for foundry waste sand and collector dust are not yet well-established. The specific mechanisms and pathways of carbon mineralization (i.e., the formation of stable solid carbonates such as calcium and magnesium) in common disposal methods such as cement kiln co-processing, concrete mixing, or brick-making仍需确认。Whether there are higher-value utilization pathways for waste sand and collector dust from sodium silicate sand also requires further research.
Establish Feasible Carbon Utilization Processes and Facilities
At this stage, foundries do not have direct access to carbon sources, and feasible pathways need to be further established. Another carbon utilization approach is to use self-generated carbon. For example, foundries with direct combustion furnaces using natural gas can treat and introduce CO₂-rich exhaust gases into the sodium silicate hardening process to achieve carbon utilization. However, this idea has not yet been practically applied, and the required facilities and equipment need further research.
Research Ways to Avoid “Secondary Pollution”
Traditional sodium silicate sand hardening operations often use cylinder aeration, focusing only on mold hardening effects without considering whether CO₂ gas leaks or escapes. However, if this process is to be used as a carbon fixation pathway, it is essential to strictly avoid “secondary pollution” where unreacted CO₂ re-enters the atmosphere.
Calculate and Control Economic and Carbon Costs
The addition of new equipment and facilities will inevitably increase operational energy consumption and slightly raise carbon emissions. Whether these economic and carbon costs can be controlled within the benefits, especially whether the increase in carbon emissions can be limited to the amount of carbon neutralized by the project.
Develop Technical Guidelines and Incentive Policies
Currently, there are no clear standards for carbon emissions in the foundry industry, and guidelines for carbon accounting methods have not yet been issued. The industry’s carbon reduction efforts are in their early stages, with little progress in carbon neutrality work and insufficient motivation for enterprises, awaiting policy guidance.
However, in the long run, future requirements for carbon emissions will undoubtedly become stricter. For enterprises, the currently insignificant 1.2% neutrality capacity is still worth striving for.
4. Conclusions and Outlook
The CO₂ hardening process for sodium silicate sand has the potential to become a CCUS technology in terms of chemical principles. However, estimates show that the amount of CO₂ neutralized per ton of castings produced accounts for only 1.2% of the enterprise’s total emissions, making its practical application value limited. Additionally, significant work is needed to advance the participation of the sodium silicate sand CO₂ hardening process in CCUS, and its feasibility at this stage is low.
Nevertheless, it is foreseeable that as carbon neutrality requirements deepen, any process capable of consuming carbon is worth anticipating. The sodium silicate sand CO₂ hardening process still holds potential for participating in CCUS and contributing to carbon neutrality. We look forward to more related research in this area.