Spray Drying Equipment for Silica (White Carbon Black)

The Application of Spray Drying Technology in the Production of Silica

 1. Overview

Silica, also known as precipitated silica, has the chemical formula SiO₂·nH₂O and is an important reinforcing material in the rubber industry. Due to its microstructure and aggregate morphology, which are similar to those of carbon black, and its comparable reinforcing properties in rubber, it is referred to as white carbon black. White carbon black is soluble in caustic alkali and hydrofluoric acid, insoluble in water, heat-resistant, non-flammable, non-toxic, and odorless, and possesses excellent electrical insulation properties.

The primary methods for producing white silica are the gas-phase method and the precipitation method. The gas-phase method uses organosilicon compounds as raw materials; although the resulting product has high purity and excellent performance, the production process involves high energy consumption, high costs, and complex technology. Furthermore, the high price limits the production and use of such equipment. The precipitation method is currently the method used by most manufacturers. The production processes for precipitated silica include the dilute acid method and the concentrated acid method. Since the dilute acid method involves high water consumption and a heavy filtration load in subsequent processes, resulting in increased product loss, most newly built plants currently adopt the concentrated acid precipitation method. The chemical equations are as follows:

Na₂SiO₃ + H₂SO₄ → Na₂SO₄ + H₂SiO₃

(n-1)H₂O + H₂SiO₃ → SiO₂ + nH₂O

Precipitation-grade silica is primarily used in footwear, tires, and other light-colored rubber products. Other applications include serving as a carrier and flow agent in pesticides and animal feed, as an abrasive in toothpaste, and as a dispersant, anti-settling agent, and matting agent in the coatings industry.

After filtration through a filter press, silica appears as a filter cake with a solid content of 18–22%. Since it cannot be processed via spray drying, many manufacturers previously used rotary flash dryers for drying. However, the resulting products exhibited poor solubility and flowability, and production yields were low. With the introduction of advanced foreign technologies and the development of domestic techniques, the development and refinement of silica slurry mixers have led to the increasingly widespread application of spray drying in the silica industry. Single-unit production capacity has grown significantly, creating economies of scale. Centrifugal and pressure spray drying are now widely used in the silica industry, while air-jet spray drying has not gained widespread adoption due to high energy consumption and other factors.

pray drying requires the material to be in a liquid state and pumpable; therefore, the slurry preparation efficiency of the slurry mixer directly impacts the drying results and product quality. Consequently, when manufacturing silica drying equipment, we begin with the slurry mixer. The slurry tank must be equipped with a stirring device to prevent the silica slurry from settling.

In centrifugal spray drying, the slurry is pumped to the atomizer at the top of the tower. Under the centrifugal force generated by the atomizer, the slurry is dispersed into a uniform and fine mist. Cold air is heated to approximately 500°C in a coal-fired furnace and regulated to produce high-temperature, clean air. This air travels through high-temperature piping and a hot-air distributor to the upper section of the drying tower, where it comes into contact with the mist droplets and rapidly cools to approximately 100–140°C. The atomized liquid material rapidly exchanges energy with the hot drying air, transforming into a solid powder within a very short time. The dried powder is collected by a baghouse dust collector and fed into a silo, then discharged through a discharge valve into finished product bags for packaging. The exhaust gas is extracted by an induced draft fan and discharged outdoors.

Since the silica solution is acidic and the final product is a white powder, all parts of the production equipment that come into contact with the material must be made of stainless steel. This ensures the product’s color and low carbon content.

2. Process Layout

The size of the drying equipment and supporting facilities are designed according to the different production capacities and process requirements of various manufacturers. We offer several configurations of equipment:

(1) Basic Process Flow of the Rotary Atomizer Spray Drying System

The basic process of rotary atomizer spray drying includes a combustion furnace, drying section, feed system, dust collection system, and air supply system.

The basic process of rotary atomizer spray drying is generally similar across systems; the main difference lies in the pneumatic conveying system installed at the rear for product discharge. The figure illustrates the basic process of rotary atomizer spray drying, with a baghouse dust collector serving as the dust collection system. The hot air distributor is installed at the bottom of the atomizer. Hot air enters the tower from the bottom. When the hot air inlet temperature exceeds 500°C, it is best to use this type of hot air distributor to minimize heat loss.

Figure 2 shows the process flow with a pneumatic conveying system for the product installed at the rear. When the plant floor space is insufficient and on-site packaging is not feasible, a pneumatic conveying system is required to transport the product to another location for packaging.

(2) Basic Process Flow of Pressure Spray Drying

When larger particles are required, pressure spray drying may be considered. Hot air enters from the top, and the hot air distributor has the same structure as the rotary type. The pressure atomizer sprays from the bottom upward. The number of atomizers, nozzle angles, and distance from the nozzle to the top of the tower must all be reasonably designed. The process flow is shown in Figure 3. Figure 4 shows a system with pneumatic conveying.

(3) Hot Air Distributor

The hot air distributor is a critical component that establishes the flow pattern of air and droplets within the tower, preventing the dried product from adhering to the tower walls and the atomizing wheel. Proper air distribution plays a crucial role in ensuring a successful spray drying operation. Figures 5 and 6 show the structure of a hot air distributor with top air inlet.

Figure 7 shows the conventional hot air distributor structure with air intake at the bottom of the atomizer, which employs a single-stage hot air distribution. Figure 8 shows a vertical tube hot air distributor. When the gas contains solid particles, the structure shown in Figure 9—an improved two-stage hot air distribution system—can be used. This type of distributor provides better performance. When the hot air flow rate is extremely high, a structure with simultaneous air intake from both the top and bottom can be used, as shown in Figure 10. Figure 11 shows a schematic diagram of flue gas desulfurization with both top and bottom air inlets.

5. Future Plans

With years of experience in manufacturing silica drying equipment, Mingxing Drying has mastered the design methods for various drying systems used in silica production. Every system we have designed, manufactured, installed, and commissioned has been a success. As a manufacturer of drying equipment, we will focus our future efforts on the following areas: developing energy-efficient and resource-saving products; building upon our existing foundation to take concrete steps toward reducing production costs for our customers; and, for the silica industry, introducing comprehensive equipment solutions that prioritize energy conservation, resource efficiency, and emissions reduction.