Keeping solids on the move
18 Mar 2010
Options to keep powders and other solid materials flowing
On most process plants it is assumed that expertise is required in the selection and use of the type of vibrator, air pad, pneumatic hammer or other discharge aid needed to obtain reliable discharge of material. However, the discharge pattern that develops inside the storage tank, and the potential for operational problems, is really defined by the bulk flow properties of the particulates and the original design of the vessel.
This issue recently led Imerys Minerals to turn to the Wolfson Centre to help with erratic and unreliable discharge from its china clay silos, which made accurate tanker loading difficult. By characterising the materials, the Wolfson team was able to determine the critical hopper geometry and most suitable feeder selection to achieve smooth mass flow. The tanker loading is now accurate, and turnaround times have been minimised.
A similar problem was experienced by Wardle Storeys in Blackburn, a manufacturer of flexible polymeric noise insulation and vibration damping products. These are, typically, manufactured from polymer granules, pellets and powders, which, after mixing, are stored in a large hopper feeding a mixer.
Wardle Storeys had experienced an increase in bridging problems at the hopper outlet, leading to an inconsistent flow delivery. Ajax Equipment was called in to investigate, and conducted powder flow characterisation testing of the hopper contents.
The materials were found to be poor at flowing due to the polymer’s ’stickiness’ and tendency to bind together; this resulted in mix segregation within the hopper and bridging at the outlet. Asa result, Ajax Equipment replaced Wardle Storeys’ pyramid-shaped hopper with a wedge-shaped unit. This pushes the stored materials in two directions to create a single planar flow, preventing segregation and encouraging mass flow.
The Wolfson Centre, itself, faced problems when a pneumatic conveying test rig used to monitor the practical problems of discharging and conveying powders, was experiencing blockage problems when handling even mildly cohesive materials because a butterfly valve in the eight-inch line presented only a three-inch free space when open. The problem was solved by using a 10-inch roller gate valve from Vortex Valves incorporating custom-built, square-to-round transitions to fit smoothly into the existing pipework.
Many mineral processing plants need to detect any jams or blockages in chutes or transfer lines before major plant damage or downtime results. In these typically hostile and dusty plant conditions, one of the few detection systems that works reliably uses gamma radiation to monitor reduced transmission through the increased mass of the blockage.
Blockage detection
Hycontrol has introduced the Microsense as a simpler, safer alternative for detecting blockages. It monitors the increased 24GHz microwave attenuation across the shute when it is blocked. The system is, typically, being adopted by mining and quarrying companies that are uneasy with the difficult and costly ownership aspects associated with nucleonic technology.
Microsense systems have been installed as far apart as in the Hanson Aggregates Batts Combe Quarry in Cheddar, where 4,000 tonnes of limestone are crushed every day, to the Bajo de la Alumbrera copper-gold mine, operated by Minera Alumbrera and located in the Catamarca province of Argentina.
In any powder mixture, it is important to maintain the correct proportions between the different ingredients, despite particle size variations. This is a significant problem in pharmaceutical tablet production where the challenge is to formulate active pharmaceutical ingredients, which are often very fine crystalline powders, with generally much coarser excipients to give a blend that tablets successfully.
A common approach is to use wet granulation, converting the fine powders into granules with a much larger particle size - this also gives improved flow properties and a reduced dust hazard. Granules can then be milled and dried to give a powder with a uniform composition, and which can then be sized to suit the further processing needed.
A common approach is to use wet granulation, converting the fine powders into granules with a much larger particle size - this also gives improved flow properties and a reduced dust hazard
Wet granulation is usually carried out by blending the dry constituents in a high shear mixer and adding water. Controlling the end-point of the water additions to achieve the desirable granulate is achieved by sampling the mixture and using a powder rheometer, such as the FT4 from Freeman Technology, to measure the basic flow energy (BFE); this is the energy required to induce a specific flow pattern in a conditioned powder bed.
At low water contents BFE remains approximately constant, but at a water concentration of 40-45% it begins to rise sharply, providing an easy decision for the end-point measurement. As a result, this dynamic powder property is proving to be a valuable parameter for wet granulation, since it sensitively detects the transition from wet mass to granulate.
Characterising the endpoint in terms of BFE provides a target for processing that is independent of scale. It also helps to accelerate process scale-up since control variables at each stage are simply manipulated to reach the same BFE rating.
The FT4 instrument is equipped with special hopper design software. This shows operators how to measure the powder parameters that are needed to specify the optimum hopper design parameters for achieving the desirable conditions that would produce mass flow for any powder discharges.
Cadbury’s Gum Base facility in Tallaght, Dublin had until recently suffered problems with the valves fitted to its chewing gum production process. The valve units were becoming increasingly jammed due to the ingress of vapour, as well as the sticky nature of the ingredients passing through them.
In search of a solution, the Irish operation contacted Vortex Valves, which recommended a HDP design that lends itself very well to highly abrasive and sticky applications, as well as being able to handle a 5-bar pressure differential.
A DN150 valve was supplied on a ’try-before-you-buy’ basis by Vortex, to enable Cadbury to evaluate its suitability without having to commit to any cost. The valve performed well, but still succumbed to the sticky chewing gum ingredients after a period of time.
Vortex engineers serviced the valve to keep the Cadbury plant going and, in the meantime, identified what was needed to take things forward and resolve the underlying design problems.
This work led to the development of a ’Cadbury spec’ HDP, with customised internal modification and some Teflon coating of selected components, providing a valve capable of handling the particularly tough conditions in this application.
Other plants within the Cadbury group are now showing an interest and have taken up the option of a free trial for “quite different, if equally taxing, applications,” reports Vortex.
Containerised systems
Dosing in a box
Spiroflow and ejector company Transvac Systems have jointly met a requirement for Containerised Dosing Systems in which the solids additive handling system, the dosing ejector system and all controls, including safety and alarm systems, are housed in an ISO container.
According to Spiroflow, the approach massively reduces the cost of water or waste treatment in situations where a building would otherwise have to be erected. And, of course, containerised systems are mobile and can be available for transfer to any location at short notice to deal with a seasonal or emergency situation.
Spiroflow applied its expertise in the conveying of dry bulk solids and bulk bag discharging to design and manufacture the bulk bag unloader and the flexible screw conveyors that transfer additives, such as hydrated lime or powdered activated carbon.
The solids are transferred to the Transvac ejector, which introduces them into the water flow. The big challenge, said Spiroflow, was to accommodate it all, and to make it work efficiently, in the restricted height of an ISO container.
