CHALLENGE TO LOW-CARBON ECOTECHNOLOGY FOR CALCIUM RESOURCES

Calcium-based materials represent a critical industrial resource, particularly for the cement and steel 
manufacturing sectors, as well as for the development of various functional materials. The primary 
constituents of these resources are calcium oxide and calcium hydroxide—collectively referred to as lime—
 which are conventionally produced through the thermal decomposition of limestone (calcium carbonate). 
This calcination process typically requires temperatures around 1000 °C and relies heavily on fossil fuels 
such as heavy oil, resulting in substantial carbon dioxide (CO₂) emissions. In the context of advancing 
toward a low-carbon society—a key objective for sustainable development—the environmental impact of 
lime production has emerged as a significant challenge. Our investigation into the carbon footprint (CFP) 
of lime production at a small-scale industrial facility in Japan revealed that approximately 1.2 metric tons 
of CO₂ are emitted per ton of lime produced. Notably, 60% of these emissions originate from the 
decomposition of the limestone itself. These findings underscore the necessity of reducing reliance on 
natural limestone to mitigate carbon emissions in the lime industry. To address this issue, we have explored 
innovative technologies aimed at producing lime with reduced carbon emissions. One such approach 
involves mechanochemical reactions, which utilize frictional energy generated between milling media and 
the vessel wall in a rotating system. This high-energy environment enables chemical transformations at 
ambient temperature. Our research has focused on converting underutilized calcium-containing waste 
materials into valuable lime products. For instance, we successfully transformed calcium scale—obtained 
from water softening processes—into calcium hydroxide (Ca(OH)₂). Another promising strategy involves 
leveraging water treatment processes for low-carbon lime production. Specifically, we examined 
wastewater from the cleaning of returnable glass bottles, which contains a few percent sodium hydroxide 
(NaOH). As a calcium source, we selected gypsum (calcium sulfate dihydrate) recovered from building 
demolition waste. Our experiments demonstrated that gypsum can be effectively converted into Ca(OH)₂ 
in aqueous solutions with low NaOH concentrations. Our research integrates both academic inquiry and 
industrial collaboration, with the goal of scaling up the production of low-carbon lime. We anticipate that 
these technologies will contribute to the commercial availability of environmentally sustainable lime 
products in the near future.