Sealed hopper

The overall Vemeuil apparatus, shown in 6.9.2. on the next page, consists of a sealed hopper to contain the powder, the TORCH itself, a refractory pedestal to hold the growing crystal, and an after-furnace to anneal the crystal. 

The solid reactant is fed from a self-sealing hopper at the top of the reactor and the flow is restricted by means of a suitable outlet hopper, star feeder, or valve, at the base of the column. A distribution plate with holes... 

A bank of four horizontal stirred beds is used, one above another, of the screw conveyor type. The top one is for calcination in air to remove traces of nitric acid, water and carbonaceous products, the middle two for hydrofluorination and the lower one mainly for partial sintering and den-sification of the product in hydrogen fluoride. Each stirred bed reactor is 20 ft long and 16 in. internal diameter, the lower ones made of inconel and the upper one of stainless-steel. The powder flows from a hopper at the top, via a seal hopper between the calcination and hydrofluorination sections, to another hopper and feeding system at the bottom, as shown in Fig. 5.8. Air and anhydrous hydrogen fluoride pass in counter-current flow to the solids in the appropriate reactors, the efBuent gases being conducted to suitable filters and dust collectors. 

In the first commercial process for melt-spinning nylon, polymer chips were stored under nitrogen pressure in a sealed hopper from which they flowed by gravity to a pancake coil heated by a central Dowtherm system (62). The nitrogen pressure... 

Furnace feed consists primarily of sinter of the appropriate composition to give the required slag formulation, and coke preheated to around 700°C. Other feed materials such as briquettes made from zinc oxides, and metallic scrap can also be added to the furnace feed. Feed is charged to the furnace using weighed buckets, which are dumped into a double bell seal hopper arrangement located in the furnace roof. The bell seals are water-cooled steel construction and efficient sealing is very important. 

It may be necessary to contain dust by enclosing a weigh hopper and using dust seals or flexible connections to seal openings. Figure 19 shows an arrangement where the top of the hopper is fixed to the stmcture, and the hopper must have an effective vent which minimizes even transient pressure... 

Rotary V lve Feeders. Devices known as rotary valve feeders are commonly used for circular or square configured outlets. These are particularly useful when discharging materials to a pneumatic conveying system where a seal is required to prevent air flow through the hopper outlet. The discharge rate is set by the speed of rotation of the vanes or pockets of the valve. 

Top-feed drum filters are used for the treatment of very fast settling solids that cannot easily be kept in suspension. A drum filter may be preferable, rather than a horizontal pan or similar filter, because of its low cost, simplicity and reliability, and because of the relative ease with which it can be fitted with appropriate accessory features. A top feed drum filter incorporates multi-compartments, drainlines and a rotary valve identical to those in a conventional drum filter. The feed slurry is introduced at, or just before, top dead centre, and the cake is discharged from 90° to 180° from the feed point. The feed slurry can be distributed across the drum by sprays or a weir box, or it may be contained within a 3-sided head-box which is sealed against the moving drum at each end and across the back on the ascending filter surface. For materials which do not cake together sufficiently to be self-supporting on the drum surface, a hopper-type top feed filter may... 

Semiclosed systems for handling solids often involve the use of big bags or tote bins. In these systems the solid is shipped in alarge (>1 t) container which is then lifted into place over a closed hopper or feed mechanism and a sealed connection is made. It is sometimes necessary to rap or agitate the big container to keep the solid moving and this action can result in deterioration or damage to the container or seals with consequent leaks. Also, there is some release of solids when containers are connected or disconnected as well as opportunities for dust generation. 

The second rig involved the release of approximately 2 kg of hot alumina from an electrically heated hopper in a sealed box. The alumina was dropped onto a plate at the bottom of the box and the dust liberated during the fall and subsequent impact was collected in petri dishes and weighed to determine the specific dust generation rate G = (dust liber-ated/mass of parent material). The motivation for this work came from the need to understand the fundamentals of dust generation during hot solids handling processes, such as those involved in cement and coke production. 

When an empty hopper is not the cause of machine output failure, plastic might have stopped flowing through the feed throat because of screw bridging. An overheated feed throat, or startup followed with a long plasticator operating delay, could build up sticky plastics and stop flow in the hopper throat. Plastics can also stick to the screw at the feed throat or just forward from it. When this happens, plastic just turns around with the screw, effectively sealing off the screw channel from... 

Once weigh-out has been completed, the formula is usually transported to a mixer where the ingredients must be mixed to uniformity. The ingredients are usually manually dumped into the mixer, after which the lid is sealed and the mixer is operated until the formula reaches the desired uniformity. After mixing, the formula is usually manually removed from the mixer and placed into a hopper or container with a controlled outlet and is ready for the next operation. 

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