Generally, masses of combustible metals do not represent unusual fire risks because they have the ability to conduct heat away from hot spots so efficiently that the heat of combustion cannot be maintained – this means that it will require a lot of heat to ignite a mass of combustible metal. Generally, metal fire risks exist when sawdust, machine shavings and other metal ‘fines’ are present. Generally, these fines can be ignited by the same types of ignition sources that would start other common fires.

Water and other common firefighting materials can excite metal fires and make them worse. The NFPA recommends that class D fires be fought with ‘dry powder’ extinguishing agents. Dry Powder agents work by smothering and heat absorption. The most common of these agents are sodium chloride granules and graphite powder. In recent years powdered copper has also come into use.

Some extinguishers use dry chemical extinguishing agents. This is easily confusable with dry powder. They are quite different, and using one of these extinguishers in error in place of dry powder can actually increase the size of a class D fire much like water.

Class-D fires represent a unique hazard because people are often not aware of the characteristics of these fires and are not properly prepared to fight them. Therefore, even a small class-D fire can spread class-A fires to the surrounding combustible materials. Most fire stations do not have class-D extinguishing agents available to them, making fighting these fires a logistical problem – however, in most places where these materials are found there is a hopper filled with the proper extinguishing agent.

In Europe and Australasia, a different classification system is used.

Class D: Fires that involve combustible metals, such as sodium, magnesium, and potassium.

http://www.nyc.gov/html/fdny/html/safety/extinguisher/classes.shtml

Class D

A fire extinguisher labeled with letter “D” is for use on Class D fires. Class D fires are fires that involve combustible metals such as magnesium, titanium and sodium.

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Class A

A fire extinguisher labeled with letter “A” is for use on Class A fires. Class A fires are fires that involve ordinary combustible materials such as cloth, wood, paper, rubber, and many plastics.

Class B

A fire extinguisher labeled with letter “B” is for use on Class B fires. Class B fires are fires that involve flammable and combustible liquids such as gasoline, alcohol, diesel oil, oil-based paints, lacquers, etc., and flammable gases.

Class C

A fire extinguisher labeled with letter “C” is for use on Class C fires. Class C fires are fires that involve energized electrical equipment.

Class D

A fire extinguisher labeled with letter “D” is for use on Class D fires. Class D fires are fires that involve combustible metals such as magnesium, titanium and sodium.

Class K

A fire extinguisher labeled with letter “K” is for use on Class K fires. Class K fires are fires that involve vegetable oils, animal oils, or fats in cooking appliances. This is for commercial kitchens, including those found in restaurants, cafeterias, and caterers.

http://www.nyc.gov/html/fdny/html/safety/extinguisher/classes.shtml