LU88319A1 - METHOD FOR PRODUCING FINE-GRAIN RUBBER POWDER FROM OLD RUBBER VULCANISATES - Google Patents

Description

DESCRIPTION

Process for the production of very fine-grained rubber powder from waste rubber vulcanizates

The invention relates to a method for producing very fine-grained rubber powder from used rubber vulcanizates, which are primarily in the form of used tires.

The economic recycling of old and waste materials by returning them to the material cycle is a priority of our society. Recycling prevents waste of raw materials, saves landfill space and avoids thermal disposal with the known disadvantages. Waste rubber vulcanizates, and primarily the millions of used tires per year, are a waste product that is produced in large quantities.

Rubber products are synthetic or natural rubbers cross-linked with sulfur by vulcanization, the thread molecules of the rubber being cross-linked by sulfur blocks. Depending on the degree of crosslinking or the sulfur content, vulcanized soft rubber or hard rubber is formed. For example, rubber tires contain about 0.2 to 10% sulfur and thereby retain the properties of thermosets, i.e. they are no longer plastically deformable due to an increase in temperature.

The sensible disposal and recycling of vulcanized waste rubber products, such as used tires, has been a problem that has so far been unsatisfactorily solved. A number of methods have recently been proposed for treating or recovering the waste rubber for reuse.

Processes are known in which these waste rubber products are ground and pressed into shaped articles with the aid of binders. The disadvantage is that the regrind is still relatively coarse and therefore only low quality products can be produced.

Methods for the regeneration of vulcanizates by treatment with chemicals at elevated temperatures and under pressure are also known; these procedures are very complex. With these regeneration processes, such as the alkali oil pann HD method, it is not possible to specifically crack the sulfur or the sulfur bridges in the rubber. The vulcanizate breaks both at C-C bonds and at C-S bridges, and granules with active valences are formed which, when added to rubber, are only partially cross-linkable with sulfur during the new vulcanization. The binding activities are relatively low and the regenerated particles are still quite large. Due to the low binding forces, weak points form in new rubber, which negatively affect the physical properties.

All in all, the processing methods known today for recycling products of the type mentioned at the outset have disadvantages with regard to the quality of the end products, the extensive limitation in the choice of processing and application options and in the profitability of manufacture. In addition, these approaches cannot reuse the entire amount of rubber waste.

The invention is therefore based on the object of developing a process for producing very fine-grained rubber powder from waste rubber vulcanizates, primarily waste tires, which, on an industrial scale, makes a decisive contribution to the removal and sensible reuse of rubber waste and which produces new, high-quality end products.

This object is achieved according to the invention in that the waste rubber vulcanizates are broken open, the textile-steel composites are read out and the rubber parts are broken down into granules which are subjected to a deep-freeze treatment and are subjected to very fine grinding in the deep-frozen state, and in that the very fine ground material is cleaned and separated into sieve fractions whose finest grains are subjected to bacterial desulfurization.

According to the invention it is provided that the granules have a grain size of 0.5 to 3.0 mm.

For the finest grinding of the rubber granulate, it proves to be expedient that the deep-freeze treatment is carried out at - 170 ° to -180 ° C.

On the basis of the method according to the invention, it is possible for the very fine grinding to take place on grains of less than 200 μm.

An essential feature of the invention is given by the fact that very fine ground grain with a grain size of less than 80 pm is used for bacterial desulfurization.

It is provided according to the invention that very fine ground material with a grain size of 80 to 200 pm is processed in conjunction with recycling thermoplastics.

On the other hand, according to the invention there is also the possibility that very fine ground material with a grain size of 80 to 200 μm is subjected to treatment with chain breakdown and regeneration agents.

The advantages achieved with the invention consist in particular in the fact that very fine ground material can be produced, that on the one hand in combination with other polymers results in composite recyclates with excellent properties and economic usability, and on the other hand provides a high-quality rubber regenerate after bacterial desulfurization.

An exemplary embodiment of the invention is explained in more detail below with reference to the process flow diagram shown in the figure.

The first step in the processing of used rubber vulcanizates, such as car and truck tires, is the separation of the rubber from the steel wire and textile cord composites and the shredding of the rubber parts to a granulate size in the range of about 0.5 to 3.0 mm.

The steel-textile rubber banding is broken up and shredded in granulators that have been specially designed for this purpose. The metal parts are separated in a metal separator, the textile cord is further opened in a large cutting mill and removed in a downstream cord reader. The rubber granulate is further reduced to the desired grain size of 0.5 to 3.0 mm.

For the subsequent fine grinding of the rubber granules to grain sizes below 200 pm, the granulated polymers are deep-frozen to temperatures below their glass point. For this purpose, a freezer double trough screw was adapted to the required conditions. The rubber granulate entering through a metering trough and cellular wheel sluice is cooled by spraying with a coolant and then passes through a cooling bath. Liquid nitrogen with a temperature of -195.8 ° C. is advantageously used as the coolant and cooling bath.

By varying the conveying speed, the dwell time of the rubber granulate in the cooler of the freezer double trough screw and thus the throughput quantity and the temperature of the rubber granulate can be controlled. As a result, the rubber granulate reaches a temperature of -170 to - 180 ° C, which is far below the glass point and causes strong embrittlement. This is important for the achievable fineness and surface design of the ground material.

The fine granulation of the rubber granulate takes place in a micro vortex mill. The grinding principle is based on the fact that the incoming, deep-cold rubber granulate is introduced into an air stream which is placed in a large number of vortices in grinding chambers and accelerates the rubber particles to almost the speed of sound. In extremely short time intervals, the rubber particles experience both changes in direction and speed, which lead to collisions of the rubber particles with one another and with the walls. As a result, the brittle particles literally burst apart and collisions of the fragments lead to a further crushing of the ground material.

At a given temperature and thus brittleness, the smaller the fragments, the more violent the collisions. This brittle fracture comminution takes place so rapidly that heating by energy conversion is too slow to cause the rubber particles to heat above their brittle point. In addition, the temperature in the grinding chambers can be controlled by spraying in refrigerants.

It is desirable to give the finest ground material as large a specific surface as possible in order to create areas of attack for chemicals or bacterial strains during the subsequent treatments. Under certain gap conditions, such as impact speed, granule temperature, air flow eddy and residual moisture of the rubber particles, microcracks of different penetration depths form in the surface of the rubber particles, which enlarge the specific surface considerably.

The finest ground material or the air-dust mixture is cleaned with a filter cyclone after the grinding process and then separated into different sieve fractions. In order to ensure minimal residual moisture for the subsequent treatments, the outlet temperature of the finely ground material from the micro-vortex mill is very important. It has been shown that an outlet temperature of 2 to 8 ° C is very favorable, which can be easily adjusted by appropriately controlling the heating generated during the grinding process.

According to the invention it is provided that the finely ground material separated into sieve fractions is further treated in accordance with its grain size (see process flow diagram). An essential feature of the invention is that the finely ground fraction with grain sizes less than 80 pm is subjected to a selective bacterial sulfur breakdown. Selected bacterial strains, such as' Thiobacillus oxidans, Thiobacillus ferrooxidans and the like, act alone or in mixtures in a bioreactor on the fine-grained rubber powder. The finest ground material is mixed with water in a reactor tank by means of a turbo agitator and the adequate amount of bacteria is added to the nutrient solution. The microorganisms use the sulfur in rubber as an energy source for their metabolism and convert the sulfur to sulfuric acid according to the following simplified reaction equations: S + 1,5 '02 + H20 bacteria ”H2S04 or

R-Sx-R + X ’1.5’ 02 + X * H20 bacteria "X * H2S04 + R-R, where R is the organic component of the sulfur.

The pulp mass is kept in motion by a stirring mechanism in the tanks and is also circulated as a total mass by a pump system. The circulation system allows control of the reaction and, if necessary, interventions in the reaction process.

In order to maintain optimal conditions for the bacteria, a temperature range must be maintained which varies somewhat from strain to strain, but is usually between 35 and 45 ° C. In addition, constant replenishment of oxygen must be ensured. This is done by blowing in air and constantly stirring and moving the reaction mass.

The process is monitored by pH probes, which ensure that the most favorable pH range from 6.5 to 7.5 is maintained. Deviations on the acid side, i.e. pH <6.5, are neutralized by adding carbonate solutions; Deviations after the basic side, i.e. pH> 7.5, can hardly be expected.

The sulfuric acid resulting from the bacterial metabolism is immediately neutralized by a carbonate (alkali carbonate NaHCOg or Na2C03) according to the following reaction equations:

HgSO ^ + 2 NaHCOg -> Na2S04 + 2 C02 + 2 HgO Or H2S04 + Na2C03 -> Na2S04 + C02 + HgO, the resulting C02 ~ gas being used to monitor the reaction.

The beginning, course and end of desulfurization can be determined and tracked using the COg concentration in the exhaust air. After the end of the reaction, i.e. if it is determined that no more C02 is produced, the bi.oreactor tanks are emptied. The reaction mass is placed in a basin in which the bacteria are killed by a high acid concentration, for example acetic acid. Horizontal stirring arms keep the reaction mixture in motion and in intimate contact with the acid, which also serves as a prewash liquid.

The acidic void solids are separated on an automatic Röckspöl filter, the acid is returned and the separated rubber is separated after neutralization and further washing out of the biomass formed by the bioreaction and dewatered in a continuous strand filter press. After checking the filter cake for acidity and foreign matter content,

Release dried in a countercurrent drum. A regenerated rubber is formed, the residual sulfur content of which depends on the grain size of the rubber powder used and the micro-cracking of the particles. With a penetration depth of the cracks of, for example, 20 pm into the grain and a grain distribution of 50 pm, desulfurization to a residual sulfur of less than 20% takes place; with a grain size distribution of, for example, 100 pm, 50% of the total sulfur is still broken down in the rubber.

These high-quality regenerates can be regarded as a rubber batch that no longer requires any other additives when vulcanized again and has properties similar to fresh rubber.

According to the process flow diagram shown, either a composite with recycling thermoplastics of the same grinding fineness or a breakdown of the molecular chains is provided for the finest ground material with the grain sizes 80 to 200 pm. In combination with thermoplastics such as PVC or polyethylene, new raw materials with elastic soft to hard properties and very good economic usability result.

By adding chain breakdown agents, e.g. 4 - 6% Renacit 6, and regenerating agents, e.g. 10 - 12% Dutrex, the very fine ground material becomes plastic, and a batch regenerate is created which also vulcanizes in new rubber mixes, giving vulcanizates with better physical properties than those produced from conventional regenerates Vulcanizates.

Claims (7)

1. A process for the production of very fine-grained rubber powder from waste rubber vulcanizates, which are primarily in the form of waste tires, characterized in that the waste rubber vulcanisates are broken up, the textile-steel composites are read out and the rubber portions are broken down into granules which are subjected to a deep-freeze treatment and in deep-frozen state undergoes fine grinding, and that the very fine ground material is cleaned and separated into sieve fractions, the finest grains of which are subjected to bacterial desulfurization. 2. The method according to claim 1, characterized in that the granules have a grain size of 0.5 to 3.0 mm. 3. The method according to claim 1 and 2, characterized in that the freezing treatment is carried out at -170 ° to -180 ° C. 4. The method according to claim 1, 2 and 3, characterized in that the fine grinding takes place on grain sizes less than 200 pm. 5. The method according to one or more of the preceding claims, characterized in that very fine ground grain of less than 80pm is used for the bacterial desulfurization. Flexokem 1892 11 6. The method according to one or more of the preceding claims, characterized in that very fine ground grain of 80 to 200 pm is processed in conjunction with recycling thermoplastics. 7. The method according to one or more of the preceding claims, characterized in that very fine ground material of the grits 80 to 200 pm is subjected to a treatment with chain breakdown and regeneration agents.