## 4 Discussion
### Circular Flows  
Enhancing the efficiency of combustion as well as mitigating the use of silica sand by utilizing waste streams are in themselves ways to make FBC more sustainable and resource-efficient. A problem that remains unsolved, however, is how to treat the bottom ashes without relying on landfilling as an alternative. Due to the composition of the materials discussed here, there's no reason to believe that they are not possible to handle in the  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Tierga (Iron Ore)** & **Sibelco (Mn-Ore)** & **Elwaleed C (Mn-Ore)** \\ \hline Raw material batch size (tons) & 25 & 25 & 20 \\ Raw material cost (\(\up\)/ton) & 310 & 850 & 200 \({}^{*}\) \\ Preparations etc. (\(\up\)/batch) & - & - & 4300 \\ Calculation (\(\up\)/ton) & 1900 & (pre-calciined) & - \\ Drying (\(\up\)/ton) & 750 & - & - \\ Crushing from large chunks (\(\up\)/ton) & - & 450 & - \\ Milling, sieving, dedusting, packaging (\(\up\)/ton) & 900 & 962.5 & 1080 \\ Declaration of analysis for waste disposal (\(\up\)/batch) & - & 350 & 350 \\ Disposal of residues (\(\up\)/ton residues) & - & 280 & 300 \\ Amount 100–400 \(\up\)m particles received (tons) & 15 (estimated) & 12.1 & 8.2 \\ Yield in production (\%) & 60 (estimated) & 48.4 & 41 \\ Final cost (\(\up\)/batch) & 96,500 & 60,525 & 33,790 \\ Final cost (\(\up\)/ton particles) & 6433 & 5002 & 4121 \\ \hline \hline \end{tabular} \({}^{*}\) Assumed representative price for low grade ore.  
\end{table}
Table 5: Costs for large-scale preparation of oxygen-carrier particles. Note that these are costs for preparation of one large batch for experimental purposes. Economics of scale and learning curve are likely to reduce these costs significantly for a dedicated manufacturing process.  
same way as conventional bed material. This type of end-of-life handling is, however, not desirable and should be considered unsustainable.  
Bottom ash treatment and utilization possibilities will have to be evaluated from case to case, and research is ongoing on how to turn ashes into a resource rather than a waste. This includes both using it as construction material and using it as feedstock to extracting certain valuable components [83]. Some options are here presented which could be a part of a solution that does not involve landfilling. Further discussion about these options is out of the scope of this study.  
* Bottom ash from OCAC with iron ore materials are recirculated to the steel industry for steel production.
* Bottom ash from OCAC with LD-slag is used for soil amendment. LD-slag has been seen to absorb alkali and phosphorous in OCAC [47; 79]. It has been approved to be used for soil amendment to counter forest and land acidification, although it is currently not used for this purpose. It's reasonable to believe that the absorption of nutrients in the boiler makes it even more suitable for this.
* Bottom ash from OCAC with LD-slag replaces fresh LD-slag as feedstock for current uses, such as mineral wool production.
* Valuable minerals like V and Zn are extracted from bottom ashes. The bed materials undergo changes in the boiler, potentially making certain valuable phases more available for extraction. A process for extracting Zn from fly ashes.
* Bottom ash from OCAC with ilmenite is used to produce TiO\({}_{2}\).