What ATEX requirements apply to powder mixers in detergent production, and how do I implement inerted processes?
Powder mixers in detergent production can process combustible organic dusts, oxidising constituents, fragrance carriers, surfactants, polymers, enzyme granulates and other functional components. Whether this creates an explosive atmosphere cannot be inferred from the product group "detergent" but must be assessed for the specific recipe, its fine-dust fraction and possible vapour-dust hybrid mixtures. The basis is a risk assessment, safety-relevant material data and an explosion protection document.
The ATEX Product Directive 2014/34/EU governs the requirements for equipment and protective systems used in potentially explosive atmospheres. The ATEX Workplace Directive 1999/92/EC concerns operator obligations, including zone classification, selection of suitable equipment, organisational measures and documentation. In Germany, these requirements are given concrete form, among other things, through the Ordinance on Industrial Safety and Health, the Hazardous Substances Ordinance and the Technical Rules for Hazardous Substances.
Zone classification and equipment selection
The interior of a mixer, silo, filter or dust-conveying line is frequently Zone 20 if an explosive dust atmosphere can be present there continuously, for long periods or frequently. Zone 21 can exist at filling, discharge, sampling or filter points if dust is occasionally released during normal operation. Zone 22 designates areas in which a hazardous dust atmosphere occurs only rarely and briefly. The specific classification depends on actual release sources, dust properties, tightness, extraction, cleaning measures and the geometry of the installation room. It must not be derived blanket-fashion from the plant function.
Zone 20 generally requires Category 1D equipment. In Zone 21, equipment of Category 1D or 2D can be used; in Zone 22, equipment of Categories 1D, 2D or 3D is permissible. This assignment applies not only to motors, sensors and switch cabinets but equally to mixer shafts, bearings, seals, mixing tools, flaps, rotary valves, conveying equipment and filters. All components must be assessed with regard to mechanical, thermal, electrical and electrostatic ignition sources.
The relevant material characterisation comprises at least KSt, maximum explosion pressure pmax, minimum ignition energy, minimum ignition temperature of the dust cloud, minimum ignition temperature of a dust layer, particle-size distribution, moisture and electrical conductivity. With recipes containing volatile liquids, solvents or fragrances, possible hybrid mixtures must also be taken into account. Where inerting is used, the oxygen limit concentration, internationally usually referred to as the limiting oxygen concentration, is additionally required for the specific product-inert-gas system.
Ignition sources and protection concept
Avoiding explosive atmospheres and effective ignition sources takes priority. Inside the mixer, in particular bearing and seal temperatures, tool-to-wall contact, foreign bodies, jamming, friction, hot surfaces, electrostatic discharges and electrical equipment must be considered. Suitable measures can include temperature and torque monitoring, foreign-body separators, limited tool speeds, safe sealing concepts, grounding and equipotential bonding, conductive or dissipative components, and preventive maintenance. TRGS 723 states, as a general rule, a safety margin of 75°C between the minimum ignition temperature of a dust layer and the surface temperature for dust layers, unless the risk assessment justifies a different value.
Sodium percarbonate is an oxidising substance and not a combustible dust in the usual sense. It can, however, intensify fires, is moisture-sensitive and can release oxygen upon decomposition. Safety data sheets note that it should be kept away from heat sources and can react critically with combustible materials. For detergent recipes containing percarbonate, temperature control, moisture ingress, material compatibility, contamination with organic substances and possible oxygen release must therefore be taken into account in particular. Inerting with nitrogen prevents neither an exothermic decomposition nor the fire-intensifying effect of an oxidiser.
If an explosion cannot be prevented with sufficient certainty, constructive measures come into consideration: explosion-pressure-resistant or explosion-pressure-shock-resistant design, explosion pressure relief, flameless pressure relief, explosion suppression and explosion-technical decoupling. The design must cover the complete process train. Filters, silos, conveying lines and discharge points can be affected by pressure and flame in the event of an incident. Rotary valves, non-return flaps, quick-closing slide valves or extinguishing barriers are only effective if they are suitable for the respective decoupling function, correctly sized and regularly tested.
Inerted processes
Inerting lowers the oxygen fraction in the process space by adding a suitable inert gas, generally nitrogen, to a value at which an explosion can no longer propagate. Carbon dioxide or other gases can be suitable in individual cases but must be matched to product chemistry, corrosion, pressure level, temperature and occupational safety. The oxygen limit concentration is not a general constant for "detergent dust". It is determined experimentally for a specific fuel-inert-gas system under defined conditions.
A maximum permissible oxygen value for operation is derived from the oxygen limit concentration. This value must include a safety margin, for example for fluctuations in product, temperature, pressure, moisture, measurement uncertainty, air leakage and oxygen release. The inerting guideline therefore distinguishes between the experimentally determined oxygen limit concentration, the maximum permissible oxygen concentration and the trigger value for a safety shutdown. Blanket setpoints such as 8 to 12 volume percent oxygen or a blanket margin of 2 to 5 percentage points are not reliable without substance-specific evidence.
In batch processes, inerting can be achieved by repeatedly evacuating and flooding a vacuum-resistant mixer. Alternatively, the air is displaced before the process begins by a defined inert-gas purge. During operation, a controlled inert-gas top-up may be required to compensate for air leakage, gas displacement through charging and possible oxygen ingress. A slight overpressure can reduce air ingress but may only be set within the permissible pressure stage, tightness, safety devices and the overall process.
Safe inerting requires a sufficiently gas-tight plant, defined purge sequences, reliable oxygen measurement, suitable measuring points, alarm limits, safety interlocks and an emergency strategy. Mixer motor, product feed, liquid addition or heating may only be enabled in accordance with the defined safety logic. If the safe oxygen value is exceeded, the plant must be brought into an assessed safe state. The functional safety required for the measurement and interlock chain is determined on a risk basis; a blanket SIL 2 requirement cannot be derived for every plant.
Inerting does not protect against every hazard. It prevents neither self-ignition, exothermic decomposition, thermal decomposition of sodium percarbonate, nor the release of oxygen from oxidising components. CEN/TR 15281 explicitly notes that inerting sufficient to avoid an explosion does not automatically protect against fires, self-ignition, exothermic reactions or deflagrations in dust deposits.
Operation and testing
Safe operation includes the regular testing of the inert-gas supply, tightness, oxygen analysers, alarm and interlock functions, and all measures for explosion decoupling and pressure relief. The test intervals result from the risk assessment, manufacturer specifications, the protective systems used and the applicable regulations. Changes to recipe, raw-material quality, moisture, fragrance addition, throughput or plant geometry must be assessed through documented change management.
Operating personnel require briefings on ATEX zones, avoidance of ignition sources, safe cleaning, behaviour in the event of alarms and the particular risks of inerting. Nitrogen and carbon dioxide can lead to oxygen deficiency in surrounding areas. Gas-warning concepts, adequate room ventilation, labelling, access restrictions and rescue measures therefore form part of the occupational safety concept.
How amixon® supports explosion protection with detergent powders
ATEX-compliant design of the product space
In detergent production, fine solids, organic constituents, fragrance carriers and recipe fine dusts can form an explosive dust atmosphere. The interior of a mixer must therefore frequently be assessed as Zone 20. Zone 20 designates an area in which an explosive dust atmosphere is present continuously, for long periods or frequently. The final zone classification of the overall plant, however, is part of the risk assessment and is the operator's responsibility. It covers not only the mixer but also dosing, charging and discharge, filters, silos, conveying lines, filling and the respective installation area.
amixon® can design various mixers, granulators, vacuum mixing dryers and mixing reactors for a product space classified as Zone 20. This includes, depending on series and project configuration, single-shaft mixers, vertical and cone mixers, container mixers, continuous mixers, and the vacuum mixing dryers and reactors VMT and AMT. The specific equipment category, the maximum permissible surface temperature, electrical and non-electrical equipment, and the ATEX documentation must be established for each plant on the basis of the actual recipe and process conditions.
The ATEX Product Directive 2014/34/EU governs the requirements for equipment and protective systems for potentially explosive atmospheres. The ATEX Workplace Directive 1999/92/EC obliges operators to carry out a risk assessment, classify zones, prepare and update the explosion protection document, and select and test suitable equipment. For the mixing chamber in Zone 20, a very high level of protection is generally required, which regularly corresponds to a Category 1D design.
Controlling ignition sources
Mixing intensity and circumferential speed influence the mechanical stress on the product and are therefore also relevant for the ignition-source assessment. Many amixon® vertical mixers can be operated at low circumferential speeds. For the VM and HM series, amixon® states an adjustable range of approximately 0.8 to 3.5 m/s. A low tool speed can limit friction, impact and abrasion energy and be gentle on the particle structure. It is not, however, sole proof that no effective ignition source can occur.
The ignition-source assessment must include bearings, seals, shafts, tools, possible tool-to-wall contact, foreign bodies, jamming, surface temperatures, electrostatic charging and electrical components. Sensible measures can be temperature and torque monitoring, limitation of tool rotational speed, foreign-body separation, suitable sealing and bearing concepts, grounding, equipotential bonding and preventive maintenance. In Zone 20, the overall concept must also take account of rare operational faults.
Tightness and process operation
Mixing chambers with few joints, ground weld seams and suitable sealing systems support dust-tight process operation. According to amixon®, Clever-Cut® inspection doors with OmgaSeal® seal are designed for low-dead-space sealing of the product space. The actual tightness, however, additionally depends on pressure differentials, seal material, wear, maintenance and the real operating conditions.
Closed product transfer points can reduce dust release and thus deposits in the installation area. This reduces the risk of dust dispersal and secondary explosions but does not replace a cleaning and maintenance concept. Dosing points, filters, sampling, discharge, cleaning openings and maintenance work must be included in the zone concept.
In detergent processes, liquids such as surfactants, oils or fragrance formulations can be introduced into the moving powder via suitable lances and nozzles. Droplet size, spray pattern, introduction position, liquid quantity, temperature and viscosity must be matched so that local over-wetting, caking and lump formation are limited. Completely deposit-free processing cannot be promised in general. With sensitive components such as enzyme granulates or sodium percarbonate, moisture, temperature, mechanical stress and contact times must be controlled particularly carefully.
For high throughputs, the continuous mixer AMK can be used. The AMK operates with a controlled fill level and a residence time that, according to amixon®, can be influenced via fill level and throughput. Liquid addition, the dosing concept and the mixing intensity must be matched to the recipe. With volatile fragrances, highly viscous surfactants, enzymes or oxidisers, trials with the original product are necessary in order to demonstrate product stability, homogeneity, moisture, discharge and cleanability.
Inerting and pressure processes
Vacuum mixing dryers and mixing reactors of the VMT and AMT series can be built gas-, vacuum- and pressure-tight. They are therefore suitable for processes in which mixing, reacting, drying or conditioning is to take place under vacuum, overpressure or an inert-gas atmosphere. The achievable vacuum level, the permissible operating pressure and the tightness requirements must be established project-specifically. Pressure or vacuum resistance is not automatically equivalent to an explosion-pressure-resistant design in accordance with DIN EN 14460.
For batch processes, the process space can be inerted by repeated evacuation and subsequent flooding with a suitable inert gas. This lowers the oxygen content step by step. The permissible oxygen concentration must lie with a sufficient safety margin below the oxygen limit concentration determined for the specific product-inert-gas system. A sufficiently tight plant, reliable oxygen measurement, defined alarm and shutdown values, safety-related interlocks and a suitable inert-gas supply are required.
Inerting, however, prevents neither the thermal decomposition nor the oxygen release of oxidising detergent constituents. Sodium percarbonate, for example, is an oxidising substance; it can release oxygen upon decomposition and intensify fires. With such recipes, temperature, moisture, contamination and material compatibility must additionally be controlled.
Manufacture and testing
The technical design follows the operator's User Requirement Specification as well as the safety-relevant material data. Particularly relevant are KSt, pmax, minimum ignition energy, minimum ignition temperatures of dust cloud and dust layer, electrical conductivity, moisture, particle-size distribution and possible hybrid mixtures. From these result the zone concept, equipment category, temperature limits, tightness requirements, grounding, inerting and, where applicable, pressure relief, suppression or decoupling.
amixon® can investigate process-engineering suitability with the original product in the pilot plant. This allows mixing quality, temperature development, liquid distribution, possible caking, particle integrity, discharge and cleanability, among other things, to be assessed. These trials support the selection of the mixing system and process parameters but do not replace the necessary safety assessment of the overall plant by qualified explosion-protection specialists.