General information on aluminum dust
A distinction is made between aluminum powder, not stabilized, and aluminum powder, phlegmatized. Non-stabilized aluminum powder is spontaneously flammable in the air, but is generally not used in the processing industry. The following information only relates to phlegmatized aluminum powder. A phlegmatizing effect is achieved by a light oxide layer that forms when aluminum powder comes into contact with atmospheric oxygen and in water or by other surface coating agents.
When aluminum is processed by grinding, brushing and polishing, combustible dusts are generated which, when mixed with air, can form an explosive atmosphere. If an ignition source effective for the dust / air mixture is present at the same time, there is a risk of explosion.
When it comes to evaluating dust explosions in the metal processing industry, the system group “dust extraction systems and separators” is the clear leader with a share of 44%.
The flammability of the dust / air mixture and the effect of the explosion depend, among other things, on the grain size, moisture and composition of the aluminum dust. Knowledge of the combustion and explosion parameters of the dust that occurs is an important basis for risk assessment and for determining protective measures, including the design of dust-processing plants.
There are different protective measures that are used in addition to dry separation. Separation with liquids using a reaction bath as well as an injection process (see Krahnen information) is the most common. The addition of an additive (mixed product of lime and volcanic rock flour) for so-called inerting according to VDI 2263 is rather rare. The proportion of aluminum dust in the polluted air (raw gas) is reduced to such an extent that greater security against fire and explosion is guaranteed. In powder form (particle size <500 μm), aluminum is very reactive due to its large surface, especially if it is not phlegmatized.
If necessary, the parameters of the aluminum can be determined by means of corresponding tests on a representative dust sample. Of course, Krahnen can carry out appropriate sieving in his dust laboratory to determine the particle size (see particle size distribution / safety parameters of combustible dusts).
Hydrogen formation possible
When aluminum dust comes into contact with water, e.g. during wet processing or dust separation in the wet separator, hydrogen gas can be produced. Under normal conditions it forms an explosive mixture with air from a volume fraction of approx. 4%. If an ignition source effective for the hydrogen gas / air mixture is present at the same time, there is a risk of explosion.
The required minimum ignition energy for hydrogen gas / air mixtures is comparatively low. It is on the order of 0.012 to 0.02 mJ.
Aluminum dust has the highest explosion pressure and the highest rate of pressure increase of problematic dusts that are traded in everyday operations. Years ago, Krahnen developed a special reaction liquid that prevents the formation of hydrogen in connection with aluminum dust.
Further information and protective measures can partly be taken from the trade association rule - DGUV rule 109-001 "Grinding, brushing and polishing of aluminum" (previously BGR 109).
An important basis for risk assessment and the definition of protective measures including the design of dust-processing plants.
With sufficient knowledge of the explosive composition of the substances, the appropriate measures can be selected on the basis of Chapter 4 of DGUV rule 109-001.
Certain chapters give clear instructions on how to proceed.
"18.104.22.168 In the dry process with dry separation of the dust, the dry dust that arises is extracted as completely as possible at the point of origin and fed to a dry separator. 22.214.171.124 By means of structural explosion protection measures, the possible effects of an explosion in the separator are limited to a harmless level.
126.96.36.199 The processing machines can only be operated if the suction and dry separation are effective. The suction can be monitored, for example, by means of a pressure monitor.
This is achieved, for example, by smooth surfaces, inclined surfaces with a sufficient angle of inclination and by avoiding horizontal surfaces; in pipelines through straight lines, avoidance of throttle and shut-off devices in horizontal sections of the suction line, bends with large radii and a flow speed in the pipelines of approx. 20 m / s.
Leaflet aluminum dust
Immediately take up spilled aluminum powder in a dry place avoiding the development of dust. A broom with natural hair bristles and conductive, non-sparking shovels with slow sweeping movements should be used. Avoid whirling up in the vicinity of potential ignition sources. The user should be fully equipped with conductive clothing, conductive rubber gloves and appropriate conductive footwear. Do not wash away with water or aqueous cleaners.
Do not use commercially available vacuum cleaners!
The use of explosion-proof vacuum cleaners is possible, provided that they comply with the applicable regulations and guidelines and are suitable for picking up flammable conductive dusts.
Attention, a marking e.g. Ex II 2 D T3 (suitable for use in Zone 21) does not say anything about the properties of the suction device with regard to the desired application.
We recommend industrial vacuum cleaners that have been certified or type-tested in their entirety by the notified body and explicitly permit use with flammable conductive dusts.
Because the design of the vacuum cleaner must be equipped with continuously electrically conductive components (including filter elements) on the raw gas and clean gas sides.
The vacuum cleaner must have appropriate safety devices (e.g. earthing clamp), which the device uses to maintain a fixed connection to the earth potential during use. Suction cups are recommended that do not pose a risk even in the event of a "two-fault case".
In the case of flammable conductive dusts, the manufacturer should always ask you about the type of application and at least about the minimum ignition energy of the suction material. Because in most cases, a case-by-case assessment must be made when the minimum ignition energy of the substance is <5 mJ.
Further information can be obtained from the Federal Institute for Occupational Safety and Health in the technical rules for operational safety, e.g. TRBS 60335-CC, BG RCI Merkblatt 109 or 2014/34 / EU.