Mixer Design for Explosives and Composite Propellants
The mix composition of explosives and composite propellants includes substances such as inorganic salts, metal powders, organic crystals, special solid additives and a minor quantity of liquids.
Explosives are classified depending on the type of hazard that they posses in conditions of storage, confinement, process of manufacture and during processing at elevated temperature and pressure, besides other special considerations. The type of hazards include
- Mass explosion hazard
- Serious projectile hazard
- Fire hazard, minor blast or projectile hazard
- Fire or slight explosion hazard
Due to the explosive characteristics and their high viscosity, these materials offer several mixing challenges. Mixing is to be carried out in a controlled environment and with utmost care. The equipment used for mixing has therefore to be designed so as to provide the right conditions for mixing and to prevent any type of explosion hazard during the mixing process.
The horizontal sigma blade kneader and the vertical planetary mixer are the most popular equipment used for mixing of explosives and propellants. The following are the guidelines, considerations for design and construction of mixers for explosives and composite propellants:
Container profile – The geometry of the mixing container and its construction, manufacturing to the required tolerances is important. There should be no voids or gaps in the mixing vessel. Gaps can be a potential zone for material deposition of explosive material and would be difficult to clean.
Mixing blade profile – The blade profile and geometry of the sigma blade and the planetary mixer, for this application is different from the conventional blade designs used in these mixers.
Mixing blade speed - The selection of blade (mixing element) speeds and the speed ratios are critical. The mixer should be equipped with variable speed operation.
Clearance – The clearance between the blade (mixing element) and container (mixing vessel) should be carefully selected. Low clearance can lead to higher friction and heat generation, which is undesirable. Higher clearance may lead to non-uniform mixing.
Drive System – Motors should be explosion proof and should be specified with respect to the group classification. The selection of service factors for mechanical components needs special considerations.
Mixer Seals - The sealing arrangement of the mixing vessel is extremely critical. The sealing between the mixer shaft and the area from where it exits the container assembly is critical. Depending on mixer operating parameters, stuffing box design or mechanical seals may be deployed.
Heat transfer - The mixer container should be provided with an external jacket for circulation of cooling or heating media, so as to maintain the desired temperature of the explosive mass.
Vacuum Operation – It is desirable to design the mixer design for operation under vacuum.
Instrumentation – There should be a provision for temperature measurement of the process material. The container assembly should be provided with pressure sensors and pressure safety devices. Safety interlocks should be provided in case the temperature, pressure rises beyond the acceptable limits.
Control System - Mixers for explosives are generally operated using PC based control system with SCADA for remote operation of the mixer. Appropriate selection, specification of electrical components such as motor and electrical switchgears, electronic components is essential.
Overload Protection – The mixer should be equipped with torque limiting devices for mechanical safety and mechanical overload protection. The mixer control system should be equipped with electrical overload protection.
Inspection and Testing – It is extremely important that a thorough inspection and testing procedure be followed before putting in the mixer into production. Mixing trials should be conducted with similar non-explosive compositions. Mixer performance, operating parameters should be established prior to usage.
Unique Mixers & Furnaces Pvt Ltd
Contact Person: Mr Jayesh R Tekchandaney
Phone – +91-22-25801214/25822385
Website - www.uniquemixer.com