New heat treating method for the curing of rubber profiles. (Process Machinery).

In recent years, ecological requirements have brought about developments that open completely new perspectives for salt bath technology. One ideal application is the curing of rubber profiles.

A new process developed by Houghton Durferrit, called the Durferrit Reclaiming System (DRS), is a salt bath system that is effluent-free (composed of EPDM, natural rubber or full silicone) and completely contained, providing manufacturers with a safer way to cure all kinds of extruded rubber compounds and profiles. The DRS system operates without effluents occurring, meaning that the treated parts are rinsed thoroughly, using a minimum of fresh water in a cascade washing system, and the relatively small amount of rinsing water can be processed very economically. The DRS system allows water to be completely reclaimed, eliminating the waste, as well as air-borne contaminants. The recycled salts can be used again or disposed of in accordance with the local regulations.

Using salt bath technology, extruded profiles can be vulcanized more efficiently with less energy loss than competing technologies such as hot air, microwaves and fluidizing bed. Due to the intrinsic salt properties and density of the salt bath, heat is exchanged quickly and homogeneously to the rubber profile.

Using a salt bath for curing rubber profiles

In a separate electrically-heated or gas-fired salt bath furnace, the salt melt is warmed up to curing temperature, which is between 200 [degrees] C and 280 [degrees] C. By means of an electromotive salt bath pump especially developed for this purpose, the salt is pumped into the curing bath through a pipeline or sprinkler unit.

At the end of the curing salt bath pot, the molten salt runs back into the supply salt bath via an outlet. In this central supply salt bath there is a desludging area with a strainer screen through which the salt flows back into the pot. At the same time, floating particles, sludge and carbonates are separated from the curing salt bath, and can be removed easily.

The curing salt carded out with the rubber profiles is concentrated in the rinsing cascade, solid particles are filtered out, and the reaction foam (i.e., stearites) is automatically removed.

By means of an automatically controlled pump, the concentrated salt solution is pumped out of the wastewater collecting tank and sprayed onto the surface of the separate supply and regeneration salt baths. The rinsing water evaporates, and the reclaimed salt is again in the supply cycle.

There are numerous advantages of this technology compared with conventional curing salt baths. First of all, almost all the salt dragged out of the bath is regained and reused. This continuous filtering and cleaning of the curing salt bath ensures high quality results. In addition, due to the continuous inlet and outlet of molten salt, the level of the curing salt bath remains constant, so that the pressure on the rubber profiles is always the same.

In addition, feeding the curing system with molten salt ensures a uniform temperature. Fluctuations in temperature due to filling the curing salt bath with solid fresh salt do not occur. Continuous desludging ensures there is no fire risk due to rubber particles floating in the curing system. Plus, the supply and regeneration salt baths can be gas-fired, and are therefore particularly economical to operate.

Salt bath system design

The classic curing system is made up of single units, in which profile running speeds of 80 m/min, are possible. Several profiles can run through the system at the same time.

The following system design can be regarded as standard:

* Profile inlet band between extruder and salt bath features an infinitely variable drive.

* Spraying system for precuring the profiles ensures a pressureless coating of molten salt around the profiles over a wide area. Blocking up of the spraying system does not occur due to the continuous filtering of the molten salt. The spraying units are provided with an electric motor that enables them to be moved vertically, so that new profiles can be easily inserted.

* In two or three further units, powered rollers or steel bands guide the rubber profiles under the surface of the salt bath. These units can also be moved vertically to enable new profiles to be inserted.

* Behind the unit is the centerpiece of the curing system, the regeneration salt bath for the filtering and reclaiming of the carried out salt, and the transfer of the molten salt. The rubber profiles run over this salt bath in a closed tunnel. By means of a special nozzle system, most of the adhering salt is blown from the surface of the profiles into the salt bath below.

* Subsequently, the profiles are run through a three or four tank cleaning and cooling cascade and exit the system absolutely salt free, dry and at the required temperature. They can then be guided on to a winding device or a caterpillar drawing device.

Usually the salt baths are between 12 and 30 meters long, depending on the parameters requested by the customer. A further 8-10 meters are required for the cascade.

Modern control systems with connections to the extruder, and documentation, are in accordance with the DIN ISO 9000 ff and can be integrated within the system.

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Cleaning cascade

Economical use of this newly developed vulcanizing technique requires a high salt concentration in the rinsing water. To accommodate this, a cascade technology has been developed, which consists of three or four lower tanks designed as a washing cascade. The rinsing water within the cascade flows in the opposite direction of the rubber profile to be cleaned.

The upper sections are located on top of the washing cascades. The doors of these upper sections can be opened pneumatically or manually so that when the system is closed, the running profile is cleaned by high pressure spraying in the tunnel.

The independent cascade areas are separated by means of a compressed air curtain; the adhering salt-bearing rinsing water is pushed back into the previous cascade unit. The drying process is a special function of the last air curtain.

This cleaning line is completed by a storage and filter tank, as well as a control cabinet. The entire cleaning cascade is made of stainless steel.

The cleaning cascade fulfills several tasks. A concentrated salt solution is obtained in the first cascade so that the evaporation which follows in the separate supply salt bath can be done economically and without harmful substances. The profile is washed thoroughly so that almost 100% of the adhering heat transfer salt is removed and can be regained. Compared with other cleaning procedures, water requirements are cut down to a minimum. In each individual cascade, the rinsing water is filtered so that when the salt is later reused, no microparticles are brought into the curing bath. The profiles are cooled so that they are warm to the touch, even though the amount of water used in cooling has been greatly reduced.

Effects on the environment

Waste air

A small waste air cleaning unit consisting of a fume scrubber used with a cyclone cleans the salt-bearing waste air, which contains salt and organic residues originating from the curing system. At regular intervals, the waste water, which occurs in the fume scrubber, is fed back to the vulcanizing cycle via the waste water collecting tank. The quality of the waste air either meets or exceeds the official standards.

Waste water

At intervals, the highly concentrated salt water from the washing cascade is fed into the regeneration salt bath from the waste water collecting tank. The salt returns in molten condition to the curing cycle, and the water vapor is discharged above the roof.

Waste salt

Small amounts of carbonates and rubber particles accumulate in the filter basket of the reclaim furnace, which can be disposed of in accordance with the official regulations for the waste management of solid salt residues.

Modification of existing salt bath equipment to the DRS

The compact, environmentally-friendly combination of cleaning cascade and salt reclaim furnace can be integrated in existing salt bath curing equipment, which continuously cleans and cools rubber profiles during treatment and returns the carried out salt to the curing cycle. The technological development of the process, in meeting market requirements, has led to the compact and space-saving design of this system. In fact, this environmentally-friendly technology has become popular for use in existing salt bath curing equipment.

Special equipment

A 75 m long salt bath has been built for the continuous curing of hoses up to a diameter of 50 mm.

Two hoses of different dimensions and running speeds can pass through the curing system parallel to each other. During the process, the hoses move across the surface and are covered with molten salt by a special spraying system. The system allows a maximum speed of 30 m/min. In the subsequent cooling area of approximately 20 m length, the hoses are cooled to room temperature.

Testing system for treating new profiles

A mobile testing system for the optimization of new profiles and rubber combinations is available. This system, with a total length of approximately 5 m, consists of a salt bath and water rinse basin. Inside the salt bath is a steel band system for speeds up to 20 m/min., which moves vertically. At the end of the salt bath there is an infinitely variable drive to assist the movement of the profiles.

This testing system ensures that the existing production equipment is not used for the introduction of new mixtures and profiles.

Cooling equipment for the UHF technology

In the past, long water cooling units were necessary to cool both air circulation furnaces and rubber profiles cured in microwave plants down to room temperature.

Subsequent to the system developed for the cleaning and cooling of the profiles, Houghton Durferrit has developed a cooling system that gives the required cooling performance in extremely short units.

Based on a total system length of 2 m, a reduction of 25 kW thermal energy/hour can be obtained. Combining several cooling blocks permits the individual matching of the rubber profiles to the running speed in relation to the cross-section.

Cooling of flocked profiles

A special modification of the cooling technology previously described permits cooling and thorough drying of flocked profiles.

Air cooling equipment

A highly effective air cooling unit has been designed for rubber profiles whose running speeds and cross-sections permit a relatively mild cooling. In the cooling channel of the unit there is a fiberglass band, the speed of which is infinitely controllable and can be adjusted to the running speed of the profile. The pneumatically operated lid permits easy insertion of the profiles. After closing the lid, the high performance cooling blower starts automatically, and an all-around intensive cooling takes place. The compact unit of ergonomic design is supplied ready for installation and can be integrated into existing UHF equipment without difficulty.

Technical data of the standard design

Total length is 9,300 mm; width is 600 mm; height of the profile inlet is 1,120 mm; and cross-section of the profile inlet is 70 x 40 mm.

Closing

The salt bath system for the curing of rubber profiles provides an effluent-free solution and strong recycling capabilities, as it regains and reuses nearly all salts. As for safety elements, desludging eliminates any risk of fire and clean water is used to rinse the rubber parts. The use of water is both safe and economical, as is the operation of the supply and regeneration salt baths, which can be gas-fired.

The efficiency of salt bath curing results from the continuous filtration and cleaning

Combined, the cleaning cascade and salt reclaim furnace continuously clean and cool rubber profiles during treatment and return the carried out salt to the curing cycle. This process prevents blockage, maintains consistent pressure on profiles, maintains uniform temperature and produces top quality results. The system is compact, environmentally-friendly, and can be integrated in existing salt bath curing equipment.