(70r) A Granulation Process for Producing Spherical Particles of a Rubber Antioxidant in a Water Cooling Tower

Rubber antioxidant is one of the most important rubber assistants, which is mainly used to prevent rubber from becoming soft, hard or cracked. Rubber antioxidant needs to be produced into particles for easy transportation and using. The popular granulation technology in industry is a water-cooling-belt molding granulation. In this granulation process, melted rubber antioxidant is dropped onto a moving belt from a drum, and cooling water is sprayed onto the back surface of the stainless steel belt. Subsequently, solidified rubber antioxidant flakes are scraped off from the belt by a scraper. There are several disadvantages in this granulation technology. Firstly, because the heat conductivity of rubber antioxidant is not very high and heat transfer from rubber antioxidant to the water is indirect via the belt, so the cooling efficiency is very low, leading to a long belt usually 6-8 meters in length. Secondly, only hemisphere flakes or even worse shape particles can be produced. Thirdly, harmful dust forms when the flakes are scraped off from the belt and rubber antioxidant is toxic and irritating, leading to environment pollution and damage to workers. Besides, the equipment has a very complicated structure and a relatively high failure rate in the granulation process.

A new technology for producing spherical rubber antioxidants particles is developed in this research, which can overcome the deficiencies of present molding techniques. It makes use of the properties of the insolubility between rubber antioxidant and water in granulation. The rubber antioxidant drops are cooled down directly in a water tower with much higher efficiency of heat transfer. The granulation materials are 4020 (6PPD, C18H29N2) and 4010NA (IPPD, C15H18N2), which are two kinds of most frequently used rubber antioxidants in tires and shoes. A pilot-plant of 3000 t/y for 4020 or 4010NA granulation has been built and runs well, spherical particles about 4 mm are produced. The granulation principle and process are introduced as followed, and some key factors in the granulation process are analyzed.

The design of granulation principle is based on the insolubility of the granulation material in a cooling medium and the interface tension in air and water. Because of their insolubility, the melted rubber antioxidant and water will not contaminate each other. The surface tension enables the rubber antioxidant drop to form a spherical shape in air and water. When the melted rubber antioxidant drops into the cooling water, the heat transfer between drop and water is at high efficiency, leading to the drop solidification and spherical particle formation, which can greatly reduce the granulator size. After dewatering with dry air, the satisfied spherical particles are obtained.

The main apparatus in the granulation flow include pre-crystallizer, granulator (it includes granulator head, a water tower and heat exchanger for circulating water), dewatering screen and drier. The granulation materials are demanded to have a certain nucleus concentration in the melted liquid via a pre-crystallizer and then are fed to the granulator. Drops are produced from the granulator head, and are dropped in the water tower in which they are cooled and solidified. The cooled particles settle down to the bottom of the water tower and then are discharged to the dewatering screen. The dewatered particles are dried in a fluidized bed with dry air, after a few minutes drying, qualified spherical particles are produced. The followings are the detail descriptions.

The rubber antioxidants 4020 and 4010NA in melted phase are typical super-cooling materials, which do not solidify even at temperatures quite below their freezing-points if there is no crystal nucleus in the melted liquids. A pre-crystallizer is necessary in the granulation of rubber antioxidant 4020 and 4010NA, in which the enough nuclei form and they can induce the melted rubber antioxidant to crystallize and solidify.

The melted material for granulation from a tank firstly goes into a pre-crystallizer in which the nuclei form through the scraping effect between special designed blades and the wall of the pre-crystallizer, and the scraping results from the rotation of these blades. A circulating cool water takes away the heat released from the nucleation. As the nucleus concentration increases, the liquid viscosity increases, leading to an increment of electric current in motor that rotates the blades. The suitable nucleus concentration for granulation can be estimated from the electric current value of the running motor. The feed from the pre-crystallizer should contain enough nuclei for the drop crystallization and solidification in water tower. In this new granulation process, the pre-crystallizer in other granulation techniques is used.

Then the feed flow goes into the granulator head. The structure of granulator, which consists of a chamber and distributors, determines the particle shape and granulation stability. The melted rubber antioxidant is fed into a chamber at a fixed flow rate. A combined distributor includes a pre-distributor and a distributor. The distributor is 800*800 mm in size, on which 2500 nozzles (Di=2 mm, Do=3 mm) are installed. Drops formed at the nozzles due to its surface tension. The pressure in the granulator head is well controlled, if the pressure is too small, there may be no drop formation at some nozzles and if the pressure is too high, some nozzles may form a continuous flow. In a stable granulation, discrete drops form at nozzles and fall down in the water tower. The feed rate was controlled about 600 kg/h. The temperature of feed in the chamber and distributor is also well controlled to keep the nuclei in feed from melting. The temperature of the feed in the chamber is about 49°C, and the temperature of the distributor should be about 51°C. So the nucleus concentration can be maintained constantly for a stable granulation. The maintenance of the constant temperature is achieved by thermostated water.

A certain height from nozzle to the water surface is necessary for the drop to enter water in the tower properly. Experiments have shown that 200-300 mm height is suitable. The water tower is 1500 mm high and just under the granulator head. The rubber antioxidant is hydrophobic and the spreading coefficient (the difference of surface tension of water and feed in air minus the interface tension of feed liquid in water) is positive when it contacts the water, so the drops might float on the water surface instead of entering. This is harmful for the particle formation because the floating drops can't keep the spherical shape and worsen the formation and cooling of the subsequent drops. Therefore, some surfactants need to be added into the water to reduce the surface tension of water and increase the drop wettability, then drops can fall into the water easily. The surfactant concentration is at ppm level which do no harm to rubber antioxidant but can reduce the surface tension of water obviously. A kind of surfactant AEO7 is selected, which is also used in rubber industry and friendly to rubber. AEO7 can reduce the surface tension of water notably and form little foam when using. The concentration in water tower is controlled at about 10*E-5~10*E-4mol/L.

At the bottom of the tower, cooled particles are discharged out into dewatering screen. The water was cooled and pumped back into the tower. The backed water is divided into two parts, one part is used for improving the particles discharge with the flow and the other part for loosing the particle stack at the bottom. Thus the particles in the tower can be discharged out properly and the particles in the tower are not disturbed by circulating flow. The dewatering screen is operated at a low amplitude but high frequency vibration in order to avoid the friction and collision among particles.

The particles from the dewatering screen then enter the drier. The water quantity on the particle surface is about 2-3%, which is not difficult to dry with a fluidized bed. By adjusting the air temperature, air flow rate and air humidity, the water on particles can be dried up to trace quantity. The suitable operation is that the resident time of particles in the fluidized bed is kept about 4 min, and the apparent gas velocity is about 2 m/s.

Experiments show that this novel technology can produce spherical particles with the simple granulation flow sheet. It has many special advantages. This granulation method has solved the main problems of present molding techniques and show many special advantages. The technology can also be used in many similar material granulations for producing many kinds of spherical particles.