The BC dry-chemical extinguisher is filled with sodium or potassium carbonate. It isolates the burning material from oxygen, but enough heat can convert the solid carbonate into CO 2 gas and so this type of extinguisher should NOT be used for Type A or Type D fires. The ABC dry-chemical extinguisher is filled with ammonium phosphate. It isolates the burning material from oxygen, but burning Class D materials can liberate oxygen from the phosphate ion and so this type of extinguisher should NOT be used for Type D fires. The cheap version of this is a bucket of sand, which isolates the Type D combustible from oxygen.
Sand is silicon dioxide, which is too stable to liberate oxygen.
Commercial Type D extinguishers are filled with materials which are even more stable than sand. For more, consult the Fire Safety ABCs , from the University of Oklahoma, or the Fire Extinguisher page at ILPI, from which the pictures are copied. All rights reserved.