Powdered detergents frequently contain combustible and in part thermolabile constituents such as surfactants, zeolites, percarbonates or enzyme granulates. In combination with fine dust and atmospheric oxygen, explosible dust-air mixtures can form during the mixing process. Mixers whose interior is classified as Zone 20 and whose surroundings are classified as Zone 21 must therefore meet the requirements of ATEX Directive 2014/34/EU and the relevant standards (including EN 1127-1 and EN 80079-37) in terms of construction, electrical design and process engineering.

Zone classification and equipment categorisation

• Interior of the mixer: in industrial detergent production, the mixing chamber is typically classified as Zone 20 (equipment category 1D, EPL Da) because of the dust cloud frequently or permanently present there during operation.
• Surroundings of the mixer: charging and discharge points, maintenance flanges and immediately adjacent areas are by contrast usually classified as Zone 21 (equipment category 2D, EPL Db), since an explosible dust atmosphere can occur occasionally there in normal operation.
• Marking of the mixer: typically Group II, dust Ex range, e.g. II 1D/2D Ex h/Ex tb IIIC T… Da/Db. The temperature specification (T …) is derived from the lowest relevant ignition or smouldering temperature of the product.

Requirements for maximum surface temperatures

To avoid ignition sources from hot surfaces, the following principles apply under EN 1127-1:

• With dust clouds, the maximum surface temperature must not exceed two thirds of the minimum ignition temperature (MIT) of the dust cloud in °C.
• With dust layers, a safety margin of at least 75 K to the smouldering temperature of the dust layer (tested at a layer thickness of 5 mm) must be maintained.

Because detergents frequently contain thermolabile constituents such as percarbonates and ignition-sensitive fine dusts, the following points are particularly critical:

• temperature monitoring at bearings and shaft seals (frictional heat),
• control of housing and jacket temperatures,
• compliance with suitable temperature classes (e.g. T125 °C) derived from the product data (minimum ignition and smouldering temperature), and
• avoidance of electrostatic discharges.

Detergent dusts can become strongly charged on plastic surfaces and poorly earthed metal parts. To avoid electrostatic ignition sources:

• continuous earthing and equipotential bonding of all conductive plant components, including internals, flaps and sight glasses,
• use of dissipative or conductive materials for product-contact plastics, seals and hoses,
• avoidance of insulated conductive parts such as unbonded screws, plates or sensor housings,
• limitation of filling velocities and free fall heights during charging in order to reduce spray charging,
• no large-area, non-dissipative plastic linings in the mixing chamber,
• regular testing of the dissipation resistances as part of the recurring inspections.

Constructional and mechanical requirements

The mixer itself must be built so that mechanically generated ignition sources are excluded or effectively limited.

• Defined, monitored clearances between the mixing tool and the wall, so that no contact and no friction occur.
• Material pairings without a risk of sparking; light metals with a high aluminium, magnesium or titanium content are to be avoided in the dust area.
• Rolling bearings and shaft seals outside the product space or with temperature monitoring and a defined maintenance interval.
• Speed and torque monitoring to detect blockages, since a blocked tool heats up quickly.
• Dust-tight design of housing, doors and passages, in order to avoid dust deposits and thus secondary explosions.
• Where required, explosion-pressure-shock-resistant construction or pressure relief, in each case with explosion decoupling from upstream and downstream machines.

Process engineering measures

In addition to construction and electrical design, measures on the process side reduce the probability of an effective ignition source.

• Closed product guidance from charging to discharge, supplemented by extracted transfer points.
• Inerting, provided the oxygen concentration can be kept permanently and verifiably below the limiting oxygen concentration.
• Limitation of mixing intensity and circumferential speed to the level required by the process.
• Control of foreign bodies by magnetic separators or screens upstream of the mixer, since metallic foreign bodies can generate impact sparks.
• Cleaning intervals that reliably keep dust layers on surfaces and in the surroundings below critical layer thicknesses.

Documentation and evidence

For Zone 20/21, the evidence is as much a part of conformity as the technology itself. Required are an ignition hazard assessment to EN 80079-36/-37, the operating instructions with the intended operating conditions, the marking to 2014/34/EU and details of the dust characteristics on which the design is based. If the operator changes the product, the formulation or the mode of operation, it must be checked whether the minimum ignition and smouldering temperatures originally assumed still apply.