The invention relates to a method of making a mineral melt comprising providing a circulating combustion chamber (1) which comprises an upper zone (2), a lower zone (3) and a base zone (4), injecting particulate fuel, particulate mineral material and primary combustion gas which has optionally an oxygen level of at least 25% by volume into the upper zone of the circulating combustion chamber so that the fuel undergoes pyrolysis in the upper zone to produce char, thereby melting the particulate mineral materials to form a mineral melt and generating exhaust gases, injecting secondary combustion gas which has optionally an Oxygen level of at least 25% by volume into the lower zone of the circulating combustion chamber so that the char combusts, thereby completing combustion of the fuel, and separating the mineral melt from the hot exhaust gases so that the hot exhaust gases pass though an outlet in the circulating combustion chamber and the mineral melt collects in the base zone. The melt is optionally fiberized. The invention also relates to apparatus suitable for use in the method.

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