Briquetting Succsss

Roll-press briquetters can improve the handling and use of powdery, granular or shredded materials.

There are a number of reasons scrap processors should consider compacting powdery, granular or shredded materials into briquets. Briquetting increases bulk density for easier handling, shipping or disposal; prevents caking or bridging in silos, hoppers and metering equipment; reduces dusting and segregation of fines in handling; improves remelt yields; makes scrap reclamation and recycling easier; and, in the case of fuels, improves burning characteristics. Such benefits can improve plant productivity, cut costs, and make products more competitive.

Roll-type presses are efficient tools for this task, due to their simplicity and low cost-per-ton of final product. These machines can compact a variety of materials into many different briquet shapes and sizes, ranging from 2 grams (0.07 ounces) to 10 pounds in weight. They can accommodate throughput capacities ranging from as low as 20 pounds-per-hour (for bench scale laboratory briquets) up to 50 tons-per- hour (for large scale production machines).

The basic components of a roll-type briquetter are two rotating rolls, usually having widths of no more than one-half their diameter, with half-briquet cavities (pockets) cut into the circumference of each. The rolls are indexed so the cavities on opposing rolls match up as the rolls turn, forming whole cavities as both halves pass through the roll centerline.

Loose material is fed between the rolls at their nip region -- where the rolls come together -- filling the cavities just prior to closure. Pressure applied to the material by the closing of the cavities as they pass through centerline compresses the materials into a solid mass. As each cavity passes beyond roll centerline, its two halves naturally move apart, allowing the formed briquet to slide out and discharge from the machine.

In modern roll presses, one roll is mounted in fixed bearing blocks and the other is mounted in moveable blocks. The force required to hold the rolls together is provided by hydraulic cylinders pressing against the bearing blocks that support the moveable roll. By adjusting the pressure in these hydraulic cylinders, the force between the rolls can be adjusted. Since this equals the force required to move the rolls apart, it is usually referred to as "roll separating force."

Hydraulic systems have virtually eliminated the earlier spring systems, which did not afford much control over spring pressure adjustment. In the hydraulic method, pressure is maintained by a nitrogen-filled accumulator which accepts the hydraulic fluid displaced from the cylinders as the rolls are forced apart. This allows the rolls to move apart at virtually constant pressure. Once charged, the hydraulic system simply acts like a spring – absorbing and giving back energy – and requires no pump or motor. Here, that stiffness of the spring-like response is controlled easily by adjusting the gas pressure in the accumulator.

SCREW FEEDERS

Another critical factor in producing good quality briquets is ensuring that the proper amount of material is fed into each cavity. While earlier briquetting machines used gravity feed, in which the material simply dropped into the roll cavities (and that method still has some applications today), most modern machines use a screw (auger) feeder to force material into the rolls.

One advantage to the screw feed method is that its variable-speed drive permits the feed rate to be adjusted to match volumetric requirements of the roll cavities. This adjustment can be automated by controls that monitor the motor current load on the roll drive and vary the feeder motor speed to maintain a constant roll load. Better feed control usually allows faster machine speeds for higher production.

Especially important for dry and finely divided materials, screw feeders provide the benefit of precompacting and deaerating the incoming material for higher infeed densities. They also can crush infeed particles to achieve a more favorable size consistency before compacting, and there is evidence to suggest that particle mobility caused by screw rotation may help the particles align their crystal axes more favorably for briquetting. In some cases, heat generated by the screw feeding process also has shown to be beneficial.

In addition, the invention of screw feeders has allowed the alternative of having vertically-paired rolls fed from the side, rather than horizontally paired rolls fed from above. This has expanded the potential applications for roll presses in that horizontal material flow to the rolls avoids the effects of gravity and can be more tightly controlled for greater uniformity. Horizontal feeders also are often more effective in handling fine, aerated, dry materials such as dust reclaimed from baghouse or cyclone accumulators.

BINDERS HELP

The briquetting process involves many mechanisms of particle-to-particle bonding. Among them are mechanical interlocking, molecular forces, Vanderwaals adhesion forces, capillary forces, electrostatic forces, magnetic forces and free chemical forces. In most cases, the strength of the particle-to-particle bond depends on the contact pressure and surface area of contact between particles. Therefore, machine pressure and particle size distribution play major roles in determining optimal briquetting conditions.

With most materials, optimum briquetting pressure depends upon the properties of the compacted materials, and upon the combination of stresses that the forming rolls exert on those materials. In general, in order to form whole briquets, the shear strength of the material under compression must be greater than the shear induced by the briquetter, otherwise the briquet tends to fracture in the process.

Many materials require use of a binder or a lubricant to produce briquets. Binders are additives that increase the strength of the agglomerate, whereas lubricants decrease the coefficient of friction between individual particles, or between the surfaces of the agglomerates and the rolls that form them. Some materials function both as binder and lubricant.

There are three types of binders: matrix binders, which help the briquet achieve structural strength by embedding the particles in a continuous pattern; film binders, which act as a glue to bond the surfaces of particles; and chemical binders, which depend upon chemical reactions occurring between the components of the binder.

When briquetting with binders, mixing adds another important variable to briquet quality. Proper mixing also can minimize costs by allowing use of less binder. Overmixing, however, can make the material too wet or gummy.

Binder mixing typically relies on pug mills or mullers. Pug mills basically comprise a trough with one or two rotating shafts fitted with paddles. Pug mill mixing is usually less intense, but offers high capacity for the capital cost and is adequate for many applications. Mullers consist of rotating wheels that roll over the material with a sliding action, generally using plows to push material into the path of the wheels. Mullers are very effective in coating particles with liquids, but typically have the disadvantage of batch operation rather than continuous.

BRIQUETTING ROLLS

Briquet machine rolls are classified according to their construction, as integral, solid or segmented. Integral means the roll and shaft are formed in one piece. Because they have no joints, integral rolls are frequently specified for food or pharmaceutical uses where cleanabilty is of primary concern. Integral rolls also can be heated or cooled to aid briquetting effectiveness.

Solid rolls – sometimes called "tires" – are the most commonly used type, and consist of replaceable rings keyed or shrink-fitted onto their shafts. This allows rings and shafts each to be made from the most suitable materials, whereas integral rolls often require some compromise in material properties to satisfy both roll and shaft specs.

Segmented rolls are made in a series of working-face sections that are mechanically clamped onto the shafts to build the roll’s circumference. Segmented rolls further simplify frequent machine changeovers, and are especially well suited for briquetting hot or highly abrasive materials that need specialized roll surfaces or frequent replacement due to wear.

PRESS DESIGNS