Лако-красочные материалы — производство

The furnace black process was developed in the United States in the 1920s, and since then, it has been greatly refined. It is a continuous process, carried out in closed reactors, so that all reactants can be carefully controlled [4.8]. Today most semi-reinforcing rubber blacks (carcass or soft blacks) with specific surface areas of 20-60 m2 g-1 and the active reinforcing blacks (tread or hard blacks) with specific surface areas of 65-150 m2 g-1 are manufactured by this process, as well as to an increasing extent, pigment-grade carbon blacks with much greater specific surface areas and smaller particle sizes. In addition to the specific surface area, other quality specifications such as structure, measured as DBP absorption, and application prop­erties of rubber such as abrasion resistance, modulus, and tear strength or jetness and tinting strength for color blacks can also be systematically varied in the furnace black process by adjusting the operating parameters. This flexibility is necessary to meet the very narrow specifications required by customers.

The heart of a furnace black production plant is the furnace in which the carbon black is formed. The feedstock is injected, usually as an atomized spray, into a high-temperature and high-energy density zone, which is achieved by burning a fuel (natural gas or oil) with air. The oxygen, which is in excess with respect to the fuel, is not sufficient for the complete combustion of the feedstock, which therefore is, for the most part, pyrolyzed to form carbon black at temperatures of 1200-1900 °C. After the reaction mixture is quenched with water and further cooled in heat exchangers, the carbon black is collected from the tail gas by using a filter system.

Figure 4.4 shows a schematic drawing of a furnace black plant. The feedstock, preferably petrochemical or carbochemical heavy oils, which usually begins to crys­tallize near the ambient temperature, is stored in heated tanks equipped with circu­lation pumps to maintain a homogeneous mixture. Oil is conducted to the reactor by means of rotary pumps via heated pipes and a heat exchanger, where it is heated to 150-250 °C to obtain a viscosity appropriate for atomization. Various types of spray­ing devices are used to introduce the feedstock into the reaction zone. An axial oil injector with a spraying nozzle at its tip, producing a hollow-cone spray pattern, is a frequently used device. One — and two-component atomizing nozzles [4.9] are in use, air and steam being the preferred atomizing agents in the latter case. However, the feedstock is injected into other reactors as a plurality of coherent or atomized streams into the accelerated combustion gases perpendicular to the direction of stream [4.10].

As the carbon black structure may be reduced by the presence of alkali metal ions in the reaction zone [4.11], alkali metal salts, preferably aqueous solutions of potassium hydroxide or potassium chloride, are often added to the oil in the oil injector. Alternatively, the additives may be sprayed separately into the combustion chamber. In special cases, other additives, e. g., alkaline-earth metal compounds, which increase the specific surface area, are introduced in a similar manner.

The high temperature necessary for pyrolysis is obtained by burning fuel in excess air in a combustion chamber. Natural gas is still the fuel of choice, but other gases, e. g., coke oven gases or vaporized liquid gas, are occasionally used. Various oils including the feedstock are used as fuel for economic reasons. Special burners, depending on the type of fuel, are used to obtain fast and complete combustion (Figure 4.5).

The air required for combustion is compressed by rotating piston compressors or turbo blowers. It is preheated in heat exchangers by hot gases containing carbon black leaving the reactor. This conserves energy and thus improves the carbon black yield. Preheated air temperatures of 500-700 °C are common.

Important progress has been made on the reactor throughput. A production plant with a capacity of 20,0001 a-1 (2.5 t h-1) was previously run with 12 furnaces, which in the last decades have been replaced by only one high-performance reactor for the same capacity. Modern plants are one-stream units with only one aggregate for each process step (reactor, collecting system, beading device, dryer). From a technical point of view, even larger units could be built. However, due to the great variety of

carbon black types required, the capacity of one unit is economically limited by the frequency of switching over to other types and the amount of off-grade carbon black produced during this procedure.

The reactors of modern furnace plants vary considerably in internal geometry, flow characteristics, and the manner in which fuel and feedstock are introduced. Nevertheless, they all have the same basic process steps in common, producing hot combustion gases in a combustion chamber, injecting the feedstock and rapidly mixing it with the combustion gases, vaporizing the oil, pyrolyzing it in the reaction zone, and rapidly cooling the reaction mixture in the quenching zone to temperatures of500-800 °C.

Most furnace black reactors are arranged horizontally. They can be up to 18 m long with an outer diameter of up to 2 m. Some vertical reactors are used especially for the manufacture of certain semi-reinforcing blacks (Figure 4.5) [4.12]. Further reactors are described in Ref. [4.13].

The properties ofcarbon blacks are dependent on the ratios offuel, feedstock, and air, which therefore must be controlled carefully [4.14]. The particle size of the carbon black formed decreases, in most cases, with increasing amounts of excess air relative to the amount needed for the complete combustion of the fuel. Since the excess air reacts with the feedstock, a greater amount of air leads to higher oil combustion rates, resulting in rising temperatures in the reaction zone. As a consequence, the nucleation velocity and the number of particles formed increases, butthe mass of each particle and the total yield decreases. This allows semi-reinforcing carbon blacks to be manufactured with better yields than active reinforcing carbon blacks. The yields, which depend on the carbon black type and the type of feedstock, range between 50 and 65% for semi-reinforcing blacks and 40 and 60% for reinforcing blacks. Pigment blacks with large surface area and markedly smaller particle sizes than rubber blacks gives lower yields.

Other parameters influencing the carbon black quality are the manner in which the oil is injected, atomized, and mixed with the combustion gases, the type and amount of additives, the preheating temperature of the air, and the quench position. As long as the carbon black is in contact with the surrounding gases at the high reaction temperature, several reactions on the carbon surface occur (e. g., Boudouard reaction, water gas reaction), so that the chemical nature of the carbon black surface is modified with increasing residence time. When quenched to temperatures <900 °C, these reactions are stopped and a certain state of surface activity is frozen. Carbon black surface properties can also be adjusted by varying the pelletizing and drying conditions.

The mixture ofgas and carbon black leaving the reactor is cooled to temperatures of 250-350 °C in heat exchangers by counter flowing combustion air and then conducted into the collecting system.

Generally, the collecting system consists of only one high-performance bag filter with several chambers, which are periodically purged by counter-flowing filtered gas or by pulsejets. Occasionally, an agglomeration cyclone is installed between the heat exchanger and the filter [4.15]. Depending on the capacity of the production unit, the

filter may contain several hundred bags with a total filter area of several thousand square meters.

The fluffy carbon black coming out of the filter is pneumatically conveyed into a first storage tank. Small amounts of impurities (“grit”, e. g., iron, rust, or coke particles) are either removed by magnets and classifiers or milled to an appropriate consistency.

Freshly collected carbon black has an extremely low bulk density of 20-60 g L-1. To facilitate handling and further processing by the customer, it must be compacted. Densification by “put gassing”, a process by which the carbon black is conducted over porous, evacuated drums, is the weakest form of compacting which allows the carbon black to retain its powdery state [4.16]. This form of compacting is used for certain pigment blacks for the paint, ink and plastic applications in which good dispersibility must remain.

The carbon black leaving the beading machine contains ca. 50 wt.% water. It is dried in dryer drums, indirectly heated by burning tail gas. The dried carbon black is transported via conveyor belts and elevators to the storage tank or packing station. Bulk densities of wet-pelletized carbon blacks are between 250 and 500 g L-1.

4.4.2