Spray-dried polyaluminum chloride

Product Overview

Spray drying is the most widely used process in the liquid processing and drying industry. It is best suited for producing powdered or granular solid products from liquid raw materials such as solutions, emulsions, suspensions, and pastes. Therefore, spray drying is an ideal process when the particle size distribution, residual moisture content, bulk density, and particle shape of the final product must meet precise standards.

Working Principle

Filtered and heated air enters the air distributor at the top of the dryer, where the hot air is distributed uniformly in a spiral pattern into the drying chamber. The feed liquid is atomized into extremely fine droplets by a high-speed centrifugal atomizer at the top of the tower. These droplets come into contact with the hot air in a co-current flow and are dried into the final product within a very short time. The finished product is continuously discharged from the bottom of the drying tower and the cyclone separator, while the exhaust gas is vented to the atmosphere by a fan.

Product Features

The differences between spray-dried polyaluminum chloride and conventional drum-dried polyaluminum chloride are as follows: 1. The raw materials used in production differ. Spray-dried polyaluminum chloride uses aluminum hydroxide powder and high-quality hydrochloric acid, while drum-dried products generally use calcium aluminate powder and similar materials as raw materials. 2. The production processes differ. Spray-dried polyaluminum chloride undergoes the following steps: liquid raw material → plate-and-frame filtration → spray drying in a spray tower → finished product; drum-dried polyaluminum chloride undergoes the following steps: liquid raw material → precipitation → drum drying → finished product.

During the spray drying process, the nozzle evenly sprays the mother liquor into the drying tower, resulting in a uniform and stable Al₂O₃ content that is easily controlled within the specified range. This enhances the adsorption capacity of colloidal particles, achieving both flocculation and coagulation effects—something other drying methods cannot accomplish.

In water treatment, the level of basicity (also known as degree of polymerization) directly affects the water purification effect. The pressure-type granular spray drying method increases the product’s degree of polymerization while preserving the original activity of the mother liquor; it also allows for timely adjustment of the basicity level based on different water qualities. In contrast, drum drying easily disrupts the degree of polymerization, resulting in a narrow range of product basicity and limited adaptability to various water qualities.

Through the application of plate-and-frame filter press technology, excess impurities are filtered out, reducing water-insoluble solids in the solid phase to below 0.3% and in the liquid phase to below 0.1%. This ensures smooth flow through users’ pipelines while also improving the utilization rate of the chemical—a level of performance unattainable by natural sedimentation methods.

Spray-dried polyaluminum chloride offers excellent stability, broad water adaptability, rapid hydrolysis, strong adsorption capacity, large floc formation, dense and rapid sedimentation, low effluent turbidity, and superior dewatering performance. Under identical water quality conditions, spray-dried products require lower dosages. In poor water quality conditions, the dosage of spray-dried products can be reduced by half compared to drum-dried products. This not only reduces the labor intensity for workers but, more importantly, lowers the user’s water treatment costs. In addition, the use of spray-dried products helps prevent water-related incidents, providing greater assurance for the safety of residents’ drinking water.

Changzhou Mingxing Drying Equipment Co., Ltd. is a leading enterprise in China’s general machinery drying industry. As a renowned manufacturer specializing in the production of equipment for pharmaceutical processing, drying, granulation, mixing, grinding, and heat sources—with capabilities in custom design—the company is a designated manufacturer for both the China Pharmaceutical Equipment Industry Association and the China Chemical Equipment Industry Association. It is recognized as a “Contract-Honoring and Creditworthy” enterprise and holds an AAA credit rating. Leveraging the long-standing production and management experience of the Sino-Japanese joint venture, Changzhou Mingxing Drying Equipment Factory, and embracing a globalized business philosophy, the company has established a unique position in the drying equipment sector.

The company’s new factory is located south of the Grand Canal and north of the Changzhou Railway Freight Station, offering convenient access to both water and land transportation. The company possesses robust equipment and a sound management foundation. Key production personnel have extensive experience in drying equipment manufacturing, and we have engaged more than ten renowned experts in the drying industry as technical advisors. Our talent pool, advanced equipment, ISO 9002 international quality certification, and comprehensive management system ensure that our product quality leads the industry. The company manufactures 14 major series and over 80 varieties of drying equipment. With a newly constructed 20,000-square-meter facility in the Sanhekou Development Zone, the company is capable of meeting the drying needs of various sectors and industries.

The company possesses substantial financial and technical capabilities. In terms of management, we have established a local area network (LAN) and invested heavily in developing custom management and design software tailored to our specific needs, providing an effective foundation for the company’s rapid growth. On the technical front, we have consistently engaged renowned domestic experts as senior consultants, effectively ensuring the high quality of our products. We also place a strong emphasis on technological innovation, having collaborated with major research institutes and universities in Shanghai, Wuhan, Shenyang, and other cities to develop numerous new products. These include the LPG series of high-speed centrifugal spray dryers, the DW series of high-efficiency, energy-saving jet belt dryers, the SXG series of rotary flash dryers, the YZG and FZG series of low-temperature high-vacuum dryers, and the PGL series of spray granulation dryers. Furthermore, the company independently manufactures the KRH series of heat exchanger (direct-fired) coal-fired (oil/gas-fired) high-temperature hot air furnaces to complement these systems. Adhering to the principle that “science and technology are the primary productive forces,” the company launches more than two new products annually, which are highly favored by users. In the drying equipment industry, our company is one of the few enterprises capable of independently manufacturing both heat source equipment and dryers.

Mingxing Drying Equipment Co., Ltd. prioritizes technological advancement, actively incorporates global best practices and expertise, and continuously develops new products to meet the diverse drying needs of various customers, while also offering custom design solutions. In 2002, the company secured consecutive contracts for large-scale projects in the food, sugar, and non-mineral sectors in Beihai, Quanzhou, Luzhou, and Huaibei.

After completing two sets of LPG3000-type (evaporation capacity 3000 kg/h) spray drying equipment in Fujian in 1997, our factory subsequently completed three large-scale LPG2000–3500-type spray drying units in Yangshan, Guangdong; Haihua, Shandong; and Tonghua, Jilin in 1999 and 2000. Among these, three units were equipped with KRH320 high-temperature heat-exchange hot air furnaces, while the remainder utilized oil-fired hot air furnaces. In 2002, our factory won the bid for the Beihai project, undertaking one set each of the LPG2000, LPG3000, and LPG4500 models, as well as one set each of the Huaibei LPG2000, Maoming LPG2000, Beijing LPG1500, Sanming LPG6000, Luzhou LPG1800, Jiangyin YPG1200, and the Yunnan 30,000-ton-per-year LPG4000-type drying tower (see the performance table for details). All of these were equipped with heat-exchange type coal-fired hot air furnaces. The equipment has been put into operation, and the customers are satisfied. As our factory has drawn on the best practices from various sources, the first LPG3000-type spray drying unit was able to operate normally within a very short period of time, and subsequent units were successfully commissioned on the first attempt. We therefore possess practical experience relevant to undertaking this project.

Design Proposal

This design has been developed in accordance with the client’s requirements, drawing on advanced domestic and international experience as well as our factory’s many years of practical expertise. The design strictly adheres to all specifications provided by the client and guarantees the achievement of all performance targets. The guiding principles of the design are to ensure advanced technology, reliability, sufficient design margins, and cost-effectiveness.

(I) Process Description (Refer to the Flow Chart)

A polyaluminum chloride slurry with a solids content of approximately 40% is pumped from the feed tank to the high-speed atomizer at the top of the tower via a screw pump. Under the centrifugal force generated by the atomizer, the slurry is broken down and dispersed into a dense cluster of fine droplets.

The hot air used for drying is heated by a coal-fired furnace and passes through a hot air distributor to come into direct contact with the droplet clusters, rapidly drying the slurry into powder.

The exhaust gas containing the dried powder enters the dust collector, and the exhaust gas filtered by the dust collector is extracted and discharged into the atmosphere by an induced draft fan.

Due to the highly corrosive nature of the material, all components in contact with the material are made of 316L stainless steel to ensure the purity of the product and prevent contamination.

To ensure the safety of the dust collector and air heater, interlocks are installed in the system.

A. When the exhaust gas temperature is too high, the screw pump will spray a large amount of water into the tower to prevent the filter bags from burning out, while simultaneously stopping the blower and cutting off the power to the heater. Under normal operating conditions, the feed rate is controlled by adjusting the screw pump’s speed via a variable frequency drive (VFD), using the exhaust gas at the drying tower outlet as a reference, to maintain a constant product moisture content.

B. For the installation, disassembly, and operation of the high-speed atomizer, please refer to the instruction manual (the supplier is responsible for training the buyer’s maintenance and operating personnel).

(II) Design Features:

1. This design utilizes a clean heat source generated by a high-temperature coal-fired hot-air furnace for heat exchange as the drying medium.

2. Highly effective and reliable measures are in place to protect the filter bags. A clean water tank is installed at the top of the tower, equipped with an automatic device to maintain the water level, connected to a centrifugal pump and a variable-flow electric valve. When the outlet temperature exceeds 120°C, the centrifugal pump at the bottom of the water tank automatically activates, and the computer-controlled variable-displacement electric valve automatically supplies an appropriate amount of water to the atomizer, thereby reducing the outlet temperature.

3. This solution features a high degree of automation, excellent safety performance, and low energy consumption. The design utilizes a centralized computer control system via a simulation panel, supporting automatic, semi-automatic, and manual operation modes. Equipment operating parameters are displayed on the screen, with adjustable control settings, audible and visual alarms, and automatic safety shutdown functions.

4. The atomizer is independently developed and manufactured by our factory. The technology is mature and reliable, ensuring seamless compatibility with the spray system. A 4,000 kg/h atomizer is paired with a drying tower of 8.5 m diameter. Through extensive practical application, the atomizing disc operates at 12,000 rpm without material adhesion to the tower walls.

5. Regarding hot air distribution technology, our company has successfully manufactured an air intake and distribution system capable of handling 240,000 m³/h of air volume. The technology utilizes components from the Danish company Niro, which operates ideally in conjunction with the drying tower.

6. The hot air furnace system is designed, developed, and manufactured by our factory. The design of this system, along with the heating system, drying tower, and gas-solid separation system, incorporates a 5–10% margin of safety to ensure product output and quality.

System Design Features

Due to the high corrosiveness of polyaluminum chloride, particularly its chlorine ion content, the equipment must feature a specialized corrosion-resistant design. Consequently, the atomizer—a core component of the drying system—is made of titanium alloy, while the tower body and cyclone sections are constructed from imported 316L stainless steel. The exhaust gas components are made of non-metallic materials such as fiberglass-reinforced plastic (FRP). The product has poor flowability; therefore, the feed chute taper should be minimized, and the discharge opening should be enlarged. Additionally, an air vibrator is incorporated to ensure continuous discharge.

1.0 Description of Major Process Systems:

1.1 Feeding System: Consists of a feed pump, associated valve assemblies, and piping.

Material feeding is accomplished using GF series electromagnetic stepless speed-regulated screw pumps. The screw pump is an internally meshed, sealed rotary positive displacement pump. It features stable, disturbance-free delivery, stable pump pressure, good suction performance, excellent flow regulation capabilities, and high efficiency. It is capable of conveying viscous slurries and concentrated slurries containing small amounts of solid particles. Suitable for conveying various slurries with a medium temperature range of (-10 to 80) °C and a viscosity range of 20,000 to 200,000 MPa·s. It is widely used for the conveyance of viscous fluids such as crude oil, oily waste, chemical products, adhesives, coatings, and concentrated wastewater. The equipment is constructed from 316L stainless steel.

1.2 Atomization System: Consists of a high-speed centrifugal atomizer, specialized components, a cooler, and a circulating oil pump.

The atomizer is driven by a vertical two-stage motor via two sets of speed-increasing gears, achieving a linear velocity of the atomizing disc exceeding 120 m/s. The atomizing disc features vertical blades and a rectangular feed channel. The motor drives the drive shaft via a coupling; the drive shaft drives the intermediate shaft via gears; and the intermediate shaft drives the main shaft via gears. Since the atomizing disc is mounted at the lower end of the main shaft, it rapidly and continuously atomizes the slurry into a fine mist. The increased specific surface area of the particles facilitates efficient heat exchange with clean hot air, causing the moisture to evaporate instantly and be extracted and vented by the induced draft fan.

The atomizer’s main bearings are imported SKF bearings. The bearings for the drive shaft and intermediate shaft are high-grade radial-thrust ball bearings from Harbin. The gear oil pump is manufactured by Shanghai Changjiang Hydraulic Equipment Factory. The main shaft, intermediate shaft, and drive shaft are made of high-quality alloy steel, forged, heat-treated, and ground. The gears are also made of high-quality alloy steel, forged and heat-treated, and finally ground on a gear grinding machine.

1.3 Hot Air Supply System (to be provided by the user): Consists of a blower, air volume control valve, hot air furnace, and hot air ductwork.

The hot air furnace is a fuel-fired type, characterized by high thermal efficiency, ease of operation, and low cost.

1.4 Drying System: Consists of a hot air distributor, main drying tower, insulation layer, cleaning and inspection doors, and an internal lighting system.

Hot air is fed into the tower through the hot air distributor at the top of the tower, where it comes into contact with fine droplets atomized by the atomizer. Under the action of the high-temperature airflow, moisture evaporates rapidly, and the product within the droplets dries into a solid, resulting in a qualified product.

The hot air distributor allows adjustment of the hot air entry angle and rotation speed based on material characteristics to control the residence time of the material within the tower; the adjustment range is 15–45°. The tower is equipped with two opposing cleaning doors, a lighting fixture at the top, and full insulation. To facilitate discharge, an air vibrator is installed on the lower cone to agitate the tower walls. The inner walls of the tower are made of 316L stainless steel, while the outer shell is constructed of color-coated steel plates.

1.5 Product Collection System: Consists of a cyclone separator and discharge device.

A portion of the dried solid powder is discharged directly from the tower bottom, while another portion is mixed with exhaust gas and recovered via the cyclone separator for packaging. Air vibrators are installed in the conical sections to facilitate discharge.

The drying tower and cyclone dust collector are constructed of 316L stainless steel.

1.6 Exhaust Gas Treatment and Emission System:

A small amount of fine powder cannot be recovered by the cyclone; however, direct discharge into the atmosphere would pollute the air and increase production costs. Therefore, a wet scrubber is used to dissolve this powder in water, and it is periodically recovered. This approach meets environmental requirements while improving product recovery rates.

The equipment is constructed from non-metallic corrosion-resistant materials.

1.7 Control System:

1.7.1 Voltage and current display; automatic overvoltage protection

1.7.2 Inlet air temperature detection, display, and alarm for the drying tower

1.7.3 Internal mixing temperature detection and display for the drying tower

1.7.4 Exhaust air temperature detection, display, and alarm for the drying tower

1.7.5 Feed pump: Variable frequency speed control, interlocked with exhaust air temperature

1.7.6 Automatic pulsed vibration of the air vibrator

1.7.7 Voltage-reduced start-up for high-power motors

1.7.8 Interlock between atomizer and feed pump: The atomizer rotates first, followed by the start-up of the feed pump; once the atomizer stops rotating, the feed pump immediately stops feeding.

1.7.9 Cooling water linked to exhaust air temperature: When the exhaust air temperature exceeds the operating limit of the wet dust collector, the cooling water valve automatically opens to spray water onto the FRP dust collector tower, preventing damage from high temperatures.