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In 1995, at the central heating plant of Kalmar, Draken, a project aiming to manufacture a granular ash product for nutrient recycling to forest soils instead of depositing it in landfills was started. Prototype equipment for granule manufacturing was developed and built by Kalmar Energi & Miljö in co-operation with Kalmar University College. Production of fertiliser from wood ash comprises aspects of technology, ecology and economy. The composition of granules mostly complies with the requirements of The National Swedish Board of Forestry for nutrient content and heavy metal levels. When constructing an automatic and continuous manufacture process, the on-line measurement of percentage of unburned carbon present in the wood ash is most important for the overall granule properties. Granulation is a more expensive method of treating ash, but granulated ashes give very slow ecological effects and could make ash recycling more acceptable in ecologically sensitive areas.

The central heating plant of Kalmar, Sweden produces 200–300 tons wood ash every year. A stabilised material for nutrient recycling is produced by adding water and dolomite to the wood ash and granulating the mixture. Combined mineralogy and chemistry can be used to interpret the transformation processes that occur during hardening and weathering of the granules, thus leading to a possibility to refine the production process and final characteristics of the granules. Mineralogy was separately studied in the wood ash, dolomite, self-hardened wood ash and granules by X-ray diffraction. Magnesium- and calcium-containing minerals are most common in the ash materials in the present study. The amounts of portlandite and calcite present in self-hardened and granulated ash samples are clearly higher than those in the untreated ash, showing that these minerals are formed during the treatments. Additionally, one potassium-containing secondary mineral, syngenite, is formed during the self-hardening of wood ash. Quartz, dolomite and the Fe–K–Mg-silicate in the granules originate from dolomite. The secondary minerals gypsum and calcium silicate hydrate are present in the granules. Portlandite occurs only in control granules in the field study. This suggests that hardening of granules continues in the field and portlandite is transformed into calcite. After up to 3 years on forest soil, the crystalline compounds dolomite, calcite, quartz, ankerite, albite and alumohydrocalcite are present in granules, of which alumohydrocalcite is formed as a secondary mineral in the field. These results suggest that the dissolution of granulated wood ash is strongly delayed compared with untreated wood ash and self-hardened wood ash because of the formation of less soluble compounds during the granulation process.