Restricted Space? Costly Explosion Protection?
Kek Gardner has the answer : Closed-Loop Fine Grinding Mill
The initial decisions that are made when defining a mill system design are determined by the actual milling application. Primarily, the characteristics of the material to be milled i.e. product hardness and the required milled particle size determines the specification of the grinding media as well as the relative need for explosion protection. The throughput rate required affects the size of the mill that will most suitable. Having established the need to mill and determined the general mill type that is required, cost and space considerations are the next stage in defining a fine grinding mill’s specification, especially when aiming to boost production with limited space available. This often conflicts with the obligation to protect the mill system against the consequences of a powder explosion, particularly in the pharmaceutical industry where products of higher toxicity require the highest standards of containment. In a traditional, normally aspirated mill system, the provision of explosion protection can significantly add to the cost, size, clean-ability and complexity of the system.
The Problem
Traditional milling system designs have a filtered air intake at the material feed point. Air acts as the 'carrier' for the product through the mill and as a result has to be filtered out of the system at the milled product collection point. Given that, for example, a standard KEK Universal Mill is capable of generating an airflow of 530m3 /hr, it is easy to appreciate the size of the typical filter unit required to separate milled product from the air. This can be very costly and cause significant problems where space is tight. The explosion risk associated with finely divided powders entrained in an air stream is an additional complication. It is not always acceptable for product resulting from an explosion to be
vented to atmosphere.
This paper discusses the four recognised methods of dealing with the explosion risk in a milling system and their inherent problems and possible solutions:
Venting:
Use of a rupture disc/panel to relieve explosion pressure. Venting is a cheap, reliable and low tech. operation and requires very little maintenance. Typically, vented systems are used in conjunction with large filter installations for the safe venting of pressure in the event of an explosion. This may be problematic dependant upon site layout and obviously, as this method vents product to atmosphere, it is not an option when dealing with potent, toxic or otherwise hazardous products.
Suppression:
Detection & suppression of an explosion before it can cause damage. This method of protection allows the system to be sited anywhere and does not compromise containment in the event of an explosion. However, a suppressed system does require periodic checking & maintenance and may require extensive system cleaning after operation which can be costly as well has holding up production. Probably the most significant point is that it does not actually prevent an explosion from taking place and may not be suited for use with sensitive, low M.I.E materials.
Inerting:
Use of an inert gas such as nitrogen or argon to reduce the oxygen level in the mill to below the limit that will support combustion. This method allows the system to be sited anywhere and remains the only practical solution to eliminate the possibility of an explosion. As such, it is particularly suitable for the milling of low M.I.E materials. However, this system does have high initial cost and the ongoing expense of supplying the inerting agent. Adequate protection against hazards of asphyxiation & “cryo burns” must also be implemented as well as provision for venting of added gas.
Containment:
Designed to contain the maximum pressure rise during an explosion. The milling system is built to withstand a 10 bar over-pressure situation without any venting to atmosphere. However, this setup requires a large and costly up rated filter and the question of where to site it which may be a problem if available space is limited. Contained systems involve a higher initial cost of manufacture and of course, they do not prevent an explosion from taking place. This may not be suitable for use with sensitive, low M.I.E materials.
Conventional Containment System with filters etc
Solution
To solve the problems of cost, complexity and sheer size associated with the options detailed above, Kek-Gardner has developed a highly innovative Closed-Loop design for its range of KEK Fine Grinding Mills.
Comparison between Vented, Containment with filtration and Closed Loop mill systems, shown in the same scale
Closed loop systems can be based upon the containment principle. However, they are much more compact and less expensive than traditional containment designs.The mill process gas is re-circulated around the system, which eliminates the need for filters, slam shut valves, etc. These systems offer easier cleaning with less chance of cross contamination.
Rather than inducing airflow into the milling chamber, this new design incorporates a recycle loop around the mill system. Once closed up, and ready to run, no additional air is introduced into the loop and therefore there is no requirement to remove it after milling. The air carrying the product is continually re-circulated back
to the mill inlet. This results in a much smaller mill system which is much cleaner and negates the need for dust filtration.
Due to the compact nature of the closed loop system, it is particularly suited to a 10 bar explosion containment approach offering a compact, quiet running mill which can be sited
anywhere. During relatively short batch runs, it is possible to provide a closed loop system which operates without any temperature rise problems. Kek-Gardner point out that, where longer runs are required, a chiller can be added to the system to maintain an ambient milling temperature.
Closed loop Mill systems have many advantages but they may still not be suitable if the product has a low MIE and is prone to dust explosions. To cater for this class of materials has led to the development of the Inert/Closed Loop Hybrid System. This offers many of the advantages of the Closed Loop method, (compactness, clean-ability, etc.) with the additional advantages that the inert processing atmosphere enables its use with even the most sensitive materials.
They do require the use of filtration, but because it only requires the venting of ‘top up’ volumes of gas, these can be small “throw away” units. Also they do not require construction to withstand pressure shock containment.
GloveBox Design
For the ultimate in containment of toxic materials, milling systems can be enclosed in "gloveboxes". Closed Loop & Inerted Closed Loop systems are ideally suited for use within Gloveboxes. Their compact design requires a smaller enclosure with fewer penetrations through the walls. Where required, the gas flow through the enclosure can be chilled to limit the temperature rise of the mill.
Summary
Explosion protection and / or filtration are often a pre-requisite during the specification of a fine grinding mill system. The Closed Loop design offers a more compact and less expensive alternative to traditional methods. Benefits to the Closed Loop Milling process are typically:
• Minimal moisture induced into the mill, leaving a drier product
• Much simpler explosion hazard protection through inertion or containment
• Compact design so ideal if space is a consideration
• Easy clean with less risk of contamination
• Less complex therefore less expensive design
Kek-Gardner Ltd offers a range of Powder Processing Equipment including PPS Air Classifier Mills, KEK Universal Mills, Cone Mills and Kibbler Pre-breakers and Gardner Mixers.
For information on a demonstration of the Closed Loop System at our Test Centre:
Call: +44 (0) 1625 412000
Email: sales@kek-gardner.com
Visit: www.kekgardner.com
Containment:
Designed to contain the maximum pressure rise during an explosion. The milling system is built to withstand a 10 bar over-pressure situation without any venting to atmosphere. However, this setup requires a large and costly up rated filter and the question of where to site it which may be a problem if available space is limited. Contained systems involve a higher initial cost of manufacture and of course, they do not prevent an explosion from taking place. This may not be suitable for use with sensitive, low M.I.E materials.
For the ultimate in containment of toxic materials, milling systems can be enclosed in "gloveboxes". Closed Loop & Inerted Closed Loop systems are ideally suited for use within Gloveboxes. Their compact design requires a smaller enclosure with fewer penetrations through the walls. Where required, the gas flow through the enclosure can be chilled to limit the temperature rise of the mill.
Summary
Explosion protection and / or filtration are often a pre-requisite during the specification of a fine grinding mill system. The Closed Loop design offers a more compact and less expensive alternative to traditional methods. Benefits to the Closed Loop Milling process are typically:
• Minimal moisture induced into the mill, leaving a drier product
• Much simpler explosion hazard protection through inertion or containment
• Compact design so ideal if space is a consideration
• Easy clean with less risk of contamination
• Less complex therefore less expensive design
Kek-Gardner Ltd offers a range of Powder Processing Equipment including PPS Air Classifier Mills, KEK Universal Mills, Cone Mills and Kibbler Pre-breakers and Gardner Mixers.
For information on a demonstration of the Closed Loop System at our Test Centre:
Call: +44 (0) 1625 412000
Email: sales@kek-gardner.com
Visit: www.kekgardner.com
