Using Magno and Respin Waste Cement to Trap Carbon Dioxide And Produce Calcium Carbonate

Introduction
Global warming due to greenhouse gas emissions has been one of the most challenging issues of the last century and many warnings have been issued to prevent it. On the one hand, the development of industries and cities will increase the emission of greenhouse gases; On the other hand, increasing regional greenhouse gases will pollute the environment and increase the mortality of living things.
In general, it can be said that with extreme emissions, pollutants accumulate in the earth's atmosphere and affect its ecosystem. According to studies, the accumulation of some gases such as carbon dioxide and methane in the atmosphere catches a fraction of the heat light reached to the earth from the sun. Although the rising temperature of the planet is slow, it will have a significant impact on future human life in different regions. Many solutions have been proposed to reduce greenhouse gas emissions. Meanwhile, some solutions have overcome technical challenges and can be implemented to reduce emissions. But often due to high costs, they have not been able to develop in the market. One of these strategies is carbon sequestration after capturing from factory exhaust. In this process, carbon dioxide is captured and consumed to produce useable products. One way conversion of carbon dioxide to mineral carbonates. For this purpose, minerals with compounds such as calcium, magnesium, and iron are used to produce mineral carbonates.
The examination of different sources shows that the use of calcium for the sequestration of carbon dioxide has received much attention.
Because calcium carbonate is a feed of the cement production process that is most widely used in industrials and urban areas. On the other hand, studies have shown that cement waste can be used as a rich source of calcium, and calcium carbonate produced in this process can be made an integrated cement process [7]. This process is called the calcium cycle in cement production, which is technically really feasible; But economically, parameters such as the energy required in the crushing stage of waste cement and low conversion efficiency in carbonation reaction are the most important economic barriers in this process. Natural limestone found in nature is currently the main source of calcium carbonate in the cement industry. Moreover, cement waste is often disposed of in the environment and is sometimes reused as a building material. The addition of calcium cycle to the cement industries, in addition to integrating the cement industry and eliminating the limestone extraction, reduces the emission of concrete waste, and prevents the release of carbon. It's well-known that cement production is one of the most polluting industries. It should be noted that according to the information provided in the references, the cement industry produces more than 5% of atmospheric carbon dioxide. According to the International Energy Agency (IEA), the volume of carbon dioxide emissions in the cement industry should be reduced by about 33%. This reduction in greenhouse gas emissions is only possible by using carbon sequestration. Carbon sequestration is applicable in industries such as cement production and power plants.
Materials and methods
Hydrochloric acid (HCl), sodium hydroxide (NaOH), carbon dioxide gas, and distilled water were used to prepare the solutions. All of these materials were laboratory grade. The required sodium carbonate (Na2CO3) is obtained from the direct adsorption of carbon dioxide gas in a caustic solution inside a laboratory scrubber. Also, Respin and Magno mineral residues have been used as a source of calcium in this research. A laboratory centrifuge equipment, vortex, heater stirrer, pH-meter, oven, and laboratory vessels were used in different steps of the experiment.
Respin and Magno are two types of calcium-rich wastes produced in the cement industry. In this research, Respin and Magno samples were achieved from the "Gilan Sabz Cement Industries". The samples were grinded for testing by a ball mill and sieved with a mesh size of 50 (equivalent to 300 μm).
A stirred balloon containing hydrochloric acid was used to extract calcium. The balloon was equipped with a condenser due to the concentration of material in the balloon did not change during the experiment. 25 g of sample powder (Magno or Respin) was extracted by 500 ml of 2 M hydrochloric acid into the balloon at a temperature of 80℃ and 500 rpm.
The solution was then poured into a beaker and its undesirable metal ions (iron, aluminum, titanium, etc.) were precipitated in a two-step pH-swing process. The filtration solution was then titrated with 1 N sodium carbonate solution until the pH of the solution reached 11. During the titration, calcium ions precipitated in solution as calcium carbonate. At the end of the reaction, the resulting precipitate was washed twice, centrifuged, and dried in the oven. Finally, it was found that from 25 gr solid Magno feed (or Respine), can produce 6.4 gr calcium carbonate-rich product (white precipitate).
Discussion of results
The calcium carbonate-rich precipitates obtained from Magno and Respin were examined by SEM-EDX, XRF, XRD, and TGA analysis. As shown in EDX analysis, magnesium, aluminum, silicon, phosphorus, chlorine, potassium, calcium, chromium, manganese, and iron are present in the Magno white precipitate feed.
Also, XRF analysis was used to obtain the weight percentage of the components in the white precipitate from Magno and Respin. XRF analysis proved that the precipitate obtained from Magno has the lowest impurities and the highest amount of calcium compounds. Also, magnesium compounds are the highest amount of impurity in this product. On the other hand, manganese compounds are the highest impurity in white precipitate from Respin.
The morphology of white precipitate obtained from Magno and Respin feed was investigated by SEM analysis. Three types of morphology including porous spherical, amorphous, and rod-type are observed in the Magno product. On the other hand, there is a porous spherical and rod-type morphologies are recognized in white precipitate from the Respin. As mentioned above, the spherical morphology is related to the vitrite crystals and rod-type morphology represents aragonite crystals. Comparing the SEM images of Magno and Respin white precipitate, it can be stated that crystals have appeared more in the Respin product.
The crystalline phase of the white precipitate composed of the Magno and Respine product was examined by X-ray diffraction. Short peaks on the XRD patterns indicate the amorphous nature of products.
TGA analysis was used to examine the chemical structure and its stability of products. The graph TGA showed that the Respine product is more stable in thermal reduction. This analysis showed that the thermal reduction of the products up to 800 reduced 65.12 and 63.85% of the weight of Magno and Respin products, respectively.
Conclusions
In this study, two types of waste from wastes of cement production were used to trap carbon dioxide as mineral compounds. After extracting calcium from cement wastes, the pH-swing method was used to produce calcium carbonate. The results showed that cement waste can trap carbon dioxide and produce a stable component. In this process, calcium carbonate-rich precipitate with an average efficiency of 25.6% was produced.