The Application of Drying Equipment in the Production of Instant Sodium Silicate

The Application of Drying Equipment in the Production of Instant Sodium Silicate (Part 1)

Sodium silicate has a wide range of applications, and most sodium silicate products currently on the market are either solid or liquid. Liquid sodium silicate presents certain challenges in packaging, transportation, and storage; the dissolution of solid sodium silicate is a highly complex and time-consuming process. Solid granules must be placed in a drum mixer and gradually dissolved using steam. The entire process involves complex physicochemical reactions such as hydration, preliminary dissolution, hydrolysis, and gelation. Furthermore, dissolving solid sodium silicate granules takes 4–5 hours, and the resulting aqueous solution must undergo precipitation or filtration to remove impurities such as unmelted quartz sand before the final product can be obtained. These factors cause significant inconvenience in the use of sodium silicate products and limit their industrial applications; therefore, research into new types of sodium silicate products is urgently needed.

  1. Production Process Flow

  Powdered sodium silicate, also known as instant-dissolving sodium silicate due to its rapid dissolution rate, is already produced by several domestic manufacturers. The production process primarily involves filtering liquid sodium silicate, adjusting its modulus, and then obtaining the final product through spray drying; however, product quality varies due to differences in the spray drying methods used by various manufacturers. Based on years of experience in manufacturing spray drying equipment and through process trials, Qinghai 3419 Drying Equipment Co., Ltd. has established a production process for powdered sodium silicate, as shown in Figure 1.

Spray drying is classified into several types based on the form of the atomizer, including pressure-type, centrifugal-type, and air-jet-type drying.

In centrifugal atomization, the centrifugal force generated by high-speed rotation flings the feed liquid horizontally. Under the combined effects of airflow and gravity, the feed liquid follows an approximately parabolic trajectory and is dried as it falls, resulting in spherical particles of uniform size with pores.

Pressure atomization uses the pressure generated by a feed pump as the primary atomization force; this pressure energy is converted into kinetic energy. The dried product takes the form of microparticles, with an average particle size generally below 100 mesh, and exhibits good flowability.

Air-jet atomization uses the kinetic energy generated by high-speed airflow as the primary atomization force; the feed liquid is dispersed by the velocity difference between the airflow and the feed liquid at the atomizer outlet, resulting in a fine powder product.

The author applied these three types of spray drying equipment to dry liquid sodium silicate. Since the primary technical indicator for instant sodium silicate is dissolution rate, dissolution tests were conducted on the dried powdered sodium silicate products. The test results are shown in Table 1.

Market research indicates that many industries require powdered sodium silicate with a particle size of less than 200 mesh; therefore, the author selected centrifugal spray drying as the drying method. However, liquid sodium silicate has high viscosity and is highly prone to sticking to the walls during the drying process, which significantly impedes product quality and yield.

2. Solution

Due to the characteristics of liquid sodium silicate, wall adhesion occurred during drying with spray drying equipment, resulting in some material exceeding moisture content limits, non-conforming products, or the presence of lumpy material in the product, which required further grinding and screening.

To address the issues encountered during production, the author made several adjustments and modifications to the conventional drying equipment.

2.1 Spray Drying Tower Diameter

When designing the diameter of a centrifugal spray drying tower, the spray distance of the centrifugal atomizer must be taken into account. It can be calculated using the following empirical formula:

   (R99)⁰.⁹ = 3.46D⁰.³G⁰.²⁵N⁻⁰.¹⁶

  Where: D is the disc diameter, m; G is the feed rate, kg/h; N is the disc rotational speed, r/min.

The actual spray distance does not match the theoretical value. The primary reason is that the density of the droplets decreases due to rapid water evaporation after ejection, thereby reducing their flight distance; the diameter of the dried droplets shrinks or decreases due to fragmentation; and the flow of hot air inside the dryer affects the droplets’ flight path. It is generally accepted that the actual spray distance is approximately 15% shorter than the theoretically calculated value; therefore, when determining the dryer diameter, an approximation of 2R99 is sufficient to meet the requirements.

Application of Drying Equipment in the Production of Instant Sodium Silicate (II)

2.2 Centrifugal Atomizer

The operating principle of a centrifugal atomizer is as follows: when liquid is injected onto a high-speed rotating dispersion disc, it is subjected to centrifugal force and gravity. Under the combined action of these two forces, the liquid is accelerated, fragmented, and atomized. Simultaneously, friction at the interface between the liquid and the surrounding air also promotes the formation of droplets.

The atomizer is the core component of spray drying, and its performance plays a decisive role in the spray drying process. In the early stages of the experiment, a mechanical gear atomizer was selected. After 10 days of continuous operation, severe wall adhesion occurred, making it impossible to continue production. Subsequently, a high-speed electric atomizer was used, which alleviated the situation. However, after a period of production, wall adhesion reappeared on one side of the drying tower’s inner wall, while the other side remained unaffected. After analysis and investigation, the author discovered that the structure of the atomizer’s feed liquid distribution plate was inadequate. The feed liquid entered the atomizer from one side and, before being evenly distributed, was flung out of the atomizer by high-speed centrifugal force. This uneven distribution of the feed liquid caused material to adhere to one side of the drying tower. The author resolved this issue by making minor modifications to the distribution plate structure; the modified structure is shown in Figure 2.

2.3 Hot Air Distributor

The primary function of the hot air distributor is to introduce hot air and other drying media into the drying tower, where they mix thoroughly and uniformly with the droplets being dried to achieve high heat and mass transfer efficiency; and to minimize or prevent the material from sticking to the walls or ceiling or from coking. Since a centrifugal spray dryer has only one atomizer, installed at the center of the drying chamber ceiling, the hot air always enters the drying chamber from the top center. This ensures a more uniform airflow distribution and reduces wall adhesion. See Figure 3.

Its shape resembles a snail shell, with a circular inner edge. The centrifugal atomizer is installed in the center, and numerous air deflectors are arranged on both the inner and outer sides of the cone. These deflectors serve to control the direction of the hot air, ensuring that the mixture of mist droplets and hot air meets design specifications, preventing material from adhering to the walls, and guaranteeing that the drying tower operates in optimal condition.

3 Final Moisture Content of the Material

The dissolution rate of instant sodium silicate is a matter of great concern to users. During the research and development process, it was found that the dissolution rate of instant sodium silicate is not only related to the material’s modulus but also closely related to the final moisture content of the dried product. During production, the drying temperature must be strictly controlled to achieve an acceptable final moisture content. The specific specifications for instant sodium silicate are shown in Table 2.

Drying is one of the most critical stages in the production of instant sodium silicate. The quality and properties of sodium silicate products are closely related to the drying process. Centrifugal drying is the preferred method for this product. Based on actual production experience, the author has summarized the following points:

① The diameter of the dryer should be set to 2R99;

② The centrifugal atomizer should be properly designed, and the spray holes on the spray disk should be evenly spaced to prevent uneven distribution of the feed solution;

③ The hot air distributor should be installed at the center of the top of the drying chamber to ensure uniform distribution of the hot air flow.