Taiheiyo’s bottom ash washing and desalination system

Taiheiyo Cement has installed a full-scale incineration bottom ash washing facility at its  Kumagaya cement plant, Japan, for desalination and recycling the ash as a cement raw material. The facility is capable of removing insoluble salts while reducing the amount of wastewater. By Taiheiyo Cement, Japan.

As a contribution to the formation of a zero-waste society, Taiheiyo Cement Corp has developed a technology to recycle municipal waste incineration residue (incineration bottom and fly ashes) for use in the cement production process. This technology has been put into actual operation, with no effects on the health and safety of employees, or the quality of cement.

Chlorine content – which is harmful to both kiln operation and cement quality – is as high as 10-20 per cent in incineration fly ash. Recycling technology turns the fly ash into a cement raw material by washing the ash and removing the chlorine (Figure 1). In contrast, incineration bottom ash, which contains a chlorine content as low as 1-2 per cent, only requires metal removal for use as a cement raw material.

The need to recycle incineration bottom ash for use in the cement production process has rapidly increased due to the decline in landfill capacity and the recent development of resource circulation. To address this, it has become necessary to develop an efficient technology for the desalination of low-chlorine bottom ash.

Incineration bottom ash is known to contain insoluble salts including Friedel’s salt which requires decomposition by acid for removal.¹ In addition, the amount of incineration bottom ash generated, and thus to be processed, is significantly larger than that of incineration fly ash. This means a considerably large amount of wastewater would be produced when the ash is treated by simple washing. Described below is a new incineration bottom ash washing and desalination system (see Figure 4) that is capable of removing insoluble salts while reducing the amount of wastewater.

Outline of development

Characteristics of incineration bottom ash
Figure 2 shows the chlorine and weight distributions of incineration bottom ash by particle size group. Figure 3 shows powder X-ray diffraction (XRD) measurement results on coarse particles (0.7mm or larger) and fine particles (smaller than 0.7mm). The chlorine distribution in incineration bottom ash was found to be uneven, being more concentrated in fine particles (smaller than 0.7mm), as shown in Figure 2. It was also found that insoluble Friedel’s salt was present specifically in fine particles (smaller than 0.7mm), as shown in Figure 3.

These suggest that efficient desalination will be possible by selectively recovering fine particles not larger than 0.7mm and applying acid treatment to them.

Incineration bottom ash transported to cement plants is usually moistened for cooling and dust generation control. The moistened ash particles form clumps and cannot be sorted properly as they are. To separate fine particles efficiently, dispersion treatment is required prior to sorting.

Incineration bottom ash washing and desalination system

The incineration bottom ash washing and desalination system developed by Taiheiyo Cement consists of the processes illustrated in Figure 4. 

Preprocessing process
Incineration bottom ash is mixed with water and the dispersion of the clumps of fine particles and the washing of coarse particles is performed simultaneously, thereby improving the efficiency of separation in the process. Soluble chlorine contained in the coarse particles is also dissolved in this process.

Separating process
The ash particles are classified at a particle size threshold of 0.7mm and separated into a slurry of fine particles, which are rich in Friedel’s salt or coarse particles. The chlorine-lean coarse particles are fed to cement kilns as raw material, while the slurry of chlorine-rich fine particles is sent to the dissolving process.

Dissolving process
After pH adjustment of the slurry of fine particles, Friedel’s salt is decomposed to improve the desalination rate.

Dehydration process
The slurry of fine particles undergoes solid-liquid separation using a filter press. The solid fraction separated by dehydration is fed to cement kilns as raw material, while the filtrate is discharged after wastewater treatment.

Demonstration test using small-scale test equipment

To verify the effect of the proposed system, a demonstration test was carried out using small-scale equipment installed at a Taiheiyo cement plant.

Table 1 shows the weight distribution after the preprocessing and separating processes. Figure 5 compares the XRD patterns of the solid fraction before and after the dissolving process. Figure 6 compares the desalination rate between the simple and proposed treatment systems. In the simple full washing system, incineration bottom ash (as-is) was mixed with water and stirred for a certain period of time before undergoing solid-liquid separation, without undergoing any preprocessing, sorting or pH adjustment.

It was found from Table 1 that the ratio of the particles smaller than 0.7mm, after preprocessing and sorting, was larger than that of the raw incineration bottom ash (as-is). This shows that the proposed system is capable of selectively recovering the fine particles. The peak intensity of Friedel’s salt in the fine particles decreased after the dissolving process, showing decomposition of the insoluble salt (Figure 5). The demonstration test using the small-scale test equipment revealed that the desalination rate was improved by 20 per cent with the proposed system (desalination rate: 64 per cent), compared to the simple washing system (desalination rate: 44 per cent), as shown in Figure 6.

Figure 7 compares the amount of wastewater from the proposed system with that of the simple full washing system. The amount of wastewater from the proposed system was 1.3x the volume of incineration bottom ash, which is equivalent to a reduction of 65 per cent compared to the simple full washing system.

Full-scale installation at a Taiheiyo cement plant

Based on the data from the demonstration test, Taiheiyo Cement completed a design of the full-scale incineration bottom ash washing equipment and built it at its Kumagaya cement plant. Commercial operations started in July 2019 (see Figure 8). The equipment has a processing capacity of 48,000tpa, and is capable of washing and desalinating the whole volume of the incineration bottom ash accepted at the plant. The desalination rate of the full-scale equipment was found to be equivalent to or even better than that of the small-scale test equipment, as shown in Figure 9.

The more moisture contained in the incineration bottom ash after washing, the larger the amount of energy that is required for drying and other processes, which is not preferable. With the proposed system, the moisture increase in incineration bottom ash after washing and desalination was found to be as small as approximately two per cent from the level before washing (Figure 10) – meaning a very limited impact on the cement manufacturing process.

Consequently, by using the proposed equipment it is possible to remove chlorine related to incineration bottom ash and to recycle waste containing chlorine for use in the cement production process.

Conclusions

The incineration bottom ash washing and desalination system developed by Taiheiyo Cement can be summarised as follows:
• The desalination rate is improved by 20 per cent compared to the simple washing system by selectively recovering fine particles from the incineration bottom ash and decomposing Friedel’s salt contained in them.
• The amount of wastewater can be reduced by 65 per cent compared to the simple full washing system by selectively recovering and treating the fine particles smaller than 0.7mm.
• The moisture increase in the incineration bottom ash after washing and desalination is as little as about two per cent from the level before washing, meaning a very minor impact on the cement manufacturing process.

Taiheiyo Cement will continue its contribution to the formation of a resource-circulating society by promoting the recycling of more waste for the cement production process through this new technology.

References

¹ Takemoto, T, Etoh, J, Naruoka, T and Shimaoka, T (2008) ‘Mechanism of formation and decomposition of insoluble chlorine compounds in municipal solid waste bottom ash’ in: Journal of the Japan Society Waste Management Experts, 19(5), p293-302.

This article was first published in International Cement Review in July 2020.