Which features increase operational safety when handling dusty, potentially explosive powders in fertiliser blends?
When handling dusty, potentially explosible powders – for instance in NPK blends, fertilisers containing ammonium nitrate or formulations containing micronutrients – an integrated, multi-stage safety concept is required. It combines preventive measures, ignition source control and constructional explosion protection and is based on an ATEX-compliant, dust-tight plant design.
An ATEX-compliant design encompasses the zone classification for dust atmospheres (Zone 20/21/22) and the selection of suitable equipment with matching equipment categories. Electrical components are of dust-tight design, with suitable protection classes and enclosures, in order to avoid dust ingress, short circuits and ignition on hot surfaces. Vessels, filters and mixers are of explosion-pressure-shock-resistant construction and designed for a reduced explosion pressure.
Constructional explosion protection includes pressure relief devices such as bursting discs or explosion vents on silos, filters and mixers, in order to discharge overpressure in a controlled manner into safe areas. Explosion suppression systems with rapid detection and injection of extinguishing agents can suppress the flame front within milliseconds. Explosion decoupling is achieved by quick-acting slide valves, non-return valves, rotary valves with flame arresting capability or extinguishing barriers, in order to prevent flame and pressure propagation into adjacent parts of the plant.
Avoiding ignition sources is a central level of protection. Continuous equipotential bonding and consistent earthing of all conductive and dissipative parts of the plant – including big bag stations – prevents electrostatic spark discharges. Mechanical and thermal ignition sources are limited by non-sparking materials, suitable bearing technology and temperature monitoring on rotating components. Surface temperatures are kept below the smouldering temperature of the dust concerned. Sensors such as spark detection in extraction ducts, temperature monitoring of bearings and housings as well as vibration sensors for the early detection of overheating complete the protection strategy.
A low-dust, closed process design reduces the formation of explosible dust-air mixtures. Vacuum or closed material transfer systems, for instance vacuum-based powder charging systems or closed pneumatic conveying, minimise dust release. Charging and transfer points are secured with defined extraction, and filter units with online cleaning and differential pressure monitoring ensure reliable dust capture. Critical areas can be inerted: mixers, silos or pipework are blanketed with inert gas and the oxygen content is lowered below the limiting oxygen concentration, so that flame formation and propagation are prevented.
Measurement and safety instrumentation provides the ongoing evidence of process safety. In inerted systems the oxygen content is monitored continuously, and CO sensors detect smouldering fires in silos at an early stage. Fill level, temperature and further safety-relevant variables are recorded in appropriately rated loops. Emergency stop and shutdown concepts with a suitable safety controller enable a rapid, defined response to critical deviations.
Cleaning, maintenance and hygiene are likewise part of the safety concept. A design with few dead spaces avoids dust deposits in areas that are difficult to access and reduces the risk of local ignition sources or smouldering fires. Smooth, readily cleanable surfaces in product contact facilitate the removal of dust, optionally in a CIP-capable design. Readily accessible inspection openings with dust-tight and gas-tight closures support inspection and documentation. Defined and documented cleaning intervals, for instance with specifications for maximum permissible dust layer thicknesses, limit the quantity of combustible dust.
A complete safety concept also includes documentation and dust characterisation. An explosion protection document systematically records hazards, zone classification, protective measures and responsibilities. Dust characteristics such as KSt, Pmax, minimum ignition energy and lower explosion limit are determined and form the basis for the constructional and process engineering design. Risk assessments, for instance in the form of HAZOP or PAAG analyses, consider the overall process, identify critical scenarios and define suitable preventive and mitigating measures.
The combination of these elements produces a staged safety concept: primary explosion protection by avoiding explosible mixtures, secondary protection by excluding ignition sources and tertiary protection by limiting the effects. In this way operational safety when handling dusty, potentially explosible powders in fertiliser mixing plants is sustainably increased.
How amixon® implements explosion protection for dust-explosible powders
Dust avoidance in the mixing process
When combustible dusts are processed, an explosible atmosphere can form under certain conditions; the prerequisite is the simultaneous presence of combustible dust, oxygen and an effective ignition source. Preventive explosion protection therefore aims to exclude reliably, or effectively limit, at least one of these prerequisites. At amixon® the basis is the dust-tight design of the apparatus: it reduces the escape of product dust into the surroundings and thereby helps to avoid hazardous dust deposits and explosible dust atmospheres outside the mixer. The SinConcave®/SinConvex® helical ribbons generate the forced restratification required for this without raising dust from the product unnecessarily.
ATEX Zone 20 as a consistent design basis
At amixon®, the mixing chamber is designed for ATEX Zone 20 across all series. This includes the single-shaft mixers EM, the vertical and cone mixers VM, HM, AM, KS, SH and GM, the container mixers COM, the continuous mixers AMK as well as the mixer-dryer reactors VMT and AMT. ATEX Zone 20 designates areas in which an explosible dust atmosphere is present permanently or for long periods. This design forms the consistent basis for Zone 20/21 concepts in powder processing. The regulatory framework is provided by the ATEX product directive 2014/34/EU and the operating directive 1999/92/EG. The zone classification of the plant is carried out by the operator; amixon® supplies qualified apparatus for this together with complete documentation.
Avoidance of ignition sources through low-speed operation
The avoidance of effective ignition sources is a central element of preventive explosion protection. In many applications amixon® mixers work with comparatively low rotational frequencies; the circumferential speed of the mixing tools can, if the process concept requires it, be limited to less than 1 m/s. The low tool speed reduces mechanical stresses and can lower the risk of ignition-effective friction, impact or spark events. Because the mixing geometry generates an intensive three-dimensional product movement, a low circumferential speed does not mean a lower mixing performance. The specific assessment of possible ignition sources must nevertheless always be made with reference to the application – among other things on the basis of the properties of the input materials, possible foreign bodies, electrostatic charging, the design of bearings and seals, temperature monitoring and the electrical and mechanical equipment of the apparatus.
Inerting by means of vacuum technology
A further approach to preventive explosion protection is the inerting of the process chamber. amixon® mixers can be built vacuum-tight. The air present in the mixing chamber can thereby first be evacuated and then replaced by an inert gas; depending on product requirements, process and safety concept, nitrogen, carbon dioxide or noble gases, for example, are possible inert gases. By lowering the oxygen content below the limiting oxygen concentration relevant for the particular dust, the formation of an explosible atmosphere inside the mixer can be prevented. Decisive here are the dust-specific characteristic data, the tightness of the overall system, the intended mode of operation and suitable measurement, control and monitoring technology.
Pressure-resistant and pressure-shock-resistant design
If an explosible atmosphere cannot be reliably avoided, design protection measures may be necessary. amixon® mixers can be realised either in a pressure-resistant or in a pressure-shock-resistant design. The design is based on the characteristic values determined for the particular dust, in particular on the maximum explosion overpressure and the rate of pressure rise. A pressure-resistant construction withstands the maximum explosion pressure to be expected; a pressure-shock-resistant design takes account of the reduced explosion pressure that results from a matched pressure-relief concept. Pressure relief is not an isolated component feature but part of a coordinated protection concept: alongside the sizing of the relief device, the safe discharge of flames and pressure, possible dust emissions, the endangerment of personnel and the interaction with connected components have to be taken into account.
An individual explosion protection concept
Which protection concept is suitable cannot be laid down in general terms. Decisive factors include:
- combustibility and explosion characteristics of the powder,
- minimum ignition energy and ignition temperatures,
- limiting oxygen concentration,
- particle size, moisture and product temperature,
- operating pressure and process management,
- integration of feeding, discharge, filtration and periphery,
- ATEX zone classification at the installation site,
- requirements of the risk assessment and of the explosion protection document.
amixon® therefore designs the mixing apparatus specifically for the application – from dust-tight process management through vacuum-assisted inerting to a pressure-resistant or pressure-shock-resistant design with matching pressure relief.
Verification in the amixon® pilot plant
Every investment is preceded by evidence from the pilot plant. At the headquarters in Paderborn, 35 test units of various sizes are available for this. These are complemented by pilot plants in Japan, India, Thailand, China, South Korea and the USA. The trials are carried out with original products, at real fill levels and batch sizes and within the intended temperature and pressure range. Mixing quality, product protection, energy input, cleanability and reproducibility in later series operation are assessed. The results are evaluated and documented together with amixon® experts. This produces a robust basis for decision-making that markedly reduces technical and economic risks before purchase.