Dosing and weighing

INSTALACIONES DE DOSIFICACIÓN Y PESAJE

In most industrial production processes involving the handling of bulk solids, it is necessary to prepare recipes or formulations for dosing the different components that make up the final product.

Depending on the industrial process, the raw materials used, the final product and the quantities to be dosed, different techniques are available for dosing and weighing the various components. These operations can be carried out either continuously or in batches, depending on the plant design and production requirements.

Although dosing is generally performed using gravimetric systems, volumetric dosing may also be used in certain applications when handling very low-density materials or when high dosing accuracy is not required. However, this method is less common in bulk solids dosing processes.

SOTECMA specializes in bulk solids dosing and weighing systems.

At SOTECMA, we have extensive experience in both gravimetric and volumetric dosing systems for continuous and batch (discontinuous) processes.

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Elements of a Concrete Dosing Plant and other dosing systems

In any dosing process, as well as in the production of concrete and other construction materials, at least three basic pieces of equipment are required:

The silo or storage hopper where the material is stored.
The material extraction equipment.
The continuous weighing scale.

A fourth component is virtually essential: the automation system. This system ensures precise dosing adjustments and guarantees consistent product characteristics throughout every production batch.

In continuous gravimetric dosing processes, an automatic control system is essential to ensure process accuracy, repeatability and stability.

1. Silos or storage tanks

The size and design of the silo depend on the characteristics of the material to be dosed and the quantity to be consumed over a given period of time.

Therefore, the storage capacity of the silo or hopper must be determined according to the product consumption rate and the selected supply method, whether by tanker truck, open-top truck, big bags or sacks. For example, if the consumption is approximately 5 t/h, the material would normally be delivered by tanker truck and stored in a silo. On the other hand, if the consumption is only 50 kg/h, the most suitable solution would be delivery in sacks and storage in 200 or 300-liter hoppers.

The physical properties of the material, such as particle size distribution, abrasiveness, cohesiveness, corrosiveness, moisture content, temperature and flammability, are also key factors in the design of silos and storage hoppers. These properties influence not only the construction materials and geometry of the equipment, but also the selection of the accessories required to ensure continuous, reliable and safe material discharge during the dosing process.

2. Dosing Equipment

This component plays a fundamental role in the dosing process, as it is responsible for extracting the material from the silo, conveying it and feeding it in a controlled manner to the weighing scale.

As with silos, the type of feeder and dosing equipment, as well as its design and dimensions, depend on the characteristics of the material to be dosed and the required conveying or feed rate.

The most common dosing systems include belt feeders, screw feeders, vibrating feeders and rotary valves. In certain applications where a high level of accuracy is not required, dosing can also be carried out using clamshell or butterfly valves. However, in these cases, accurate control of the material flow rate is more difficult to achieve.

At SOTECMA, we have an R&D department and a pilot plant where tests can be carried out on materials that present the greatest dosing challenges. We also have a highly experienced engineering team capable of determining the most suitable dosing system for each specific application.

3. Weightlifting equipment the measure

This component is responsible for measuring the weight or volume of the material being supplied by the dosing system, making its accuracy essential to ensure the precision of the process. As with the previous components, the design of the measuring system depends on the behavior of the material and the quantity to be dosed.

When handling liquids, the solution is generally simpler. In most cases, weighing is not required, as dosing is carried out using volumetric flow meters, since the density of the product usually remains constant. However, the physical and chemical properties of the liquid, such as corrosiveness, temperature, viscosity and required flow rate, must be considered when selecting the appropriate type and size of the measuring device.

For solids, except in very specific applications, dosing is normally carried out using gravimetric systems. This requires the weighing hopper to be designed according to the physical properties of the material, such as bulk density, internal angle of friction, wall friction angle, moisture content and cohesiveness, as well as its chemical characteristics.

It is also essential to determine the maximum weight to be measured in order to correctly size the load cells. This directly affects the scale resolution and, consequently, the accuracy and reliability of the dosing system.

4. Automation and control system

To complete a dosing system, it is now virtually essential to have an automation system capable of precisely controlling the operation of all the equipment involved in the process.

At SOTECMA, we have a specialized control and automation department where we develop control software and design the electrical control panels required for the fully automatic operation of our installations.

These systems ensure precise control of the material feed rate, optimize the dosing process and achieve a high level of weighing accuracy, guaranteeing process repeatability and consistent final product quality.

Types of solids and liquids dosing systems

Dosing systems can be classified in two different ways. According to the measurement method used, they may be gravimetric (weight-based) or volumetric. According to the operating mode, they may be continuous or batch (discontinuous) systems.

Continuous Gravimetric Dosing

Continuous gravimetric dosing is one of the most complex dosing processes and is generally used for bulk solids. Its operation requires a dynamic weighing system capable of continuously measuring the amount of material being fed into the subsequent process, which is typically a mixing system. For this reason, the design of the weighing equipment is one of the most critical aspects of this type of installation.

At SOTECMA, we have more than 30 years of experience developing continuous gravimetric dosing systems, using technologies such as dynamic belt weighing and loss-in-weight hopper dosing systems, a utility model patented by SOTECMA more than 20 years ago that provides a higher level of dosing accuracy.

Due to its complexity, this type of dosing system always requires an automatic control and automation system to ensure process stability, weighing accuracy and dosing repeatability.

This type of dosing system is used in our soil-cement and cement-treated aggregate plants for the continuous and accurate dosing of cement onto the aggregate, ensuring a homogeneous mixture and compliance with the specifications of each project.

Continuous Gravimetric Dosing

Batch Gravimetric Dosing

Batch gravimetric dosing is the most widely used system when a high level of accuracy is required, as the material is weighed statically, allowing precise adjustment of the feed rate before each weighing cycle is completed. In addition, it is easier to implement than continuous gravimetric dosing systems.

In any weighing system, it is essential to eliminate vibrations and any external influences that may affect scale accuracy, especially when dosing small quantities of material below 10 kg. For this reason, SOTECMA has developed and patented a high-precision weighing scale with a tilting discharge system, fully enclosed to prevent external interference and ensure maximum weighing reliability.

Although this type of dosing can be carried out manually by starting and stopping the dosing equipment, this approach is not recommended. Automating the process eliminates errors such as omitted ingredients, duplicate additions or overdosing caused by delayed shutdown of the feeder, thereby ensuring process repeatability and consistent final product quality.

This dosing system is used in our soil-cement and cement-treated aggregate plants for water dosing, ensuring the precise addition of this component so that the final mixture achieves the required moisture content and performance for each specific application.

Batch Volumetric Dosing

The only difference from the previous system is that the process is carried out in batches. However, this does not necessarily provide greater accuracy, as the measuring equipment used is generally the same for both continuous and batch processes.

Dosing and Weighing Equipment

Thanks to the experience gained over many years of project development, our range of equipment includes the following solutions:

Micro-dosing of additives by volume and by weight.
Continuous cement and water dosing systems for soil-cement and cement-treated aggregate plants.
Dynamic belt weighing systems.
High-precision tilting weighing system (patented).
Portable solid additive feeder for asphalt plants (patented).
Lime dosing silo for water treatment.
Complete chemical additive dosing systems for dry mortar and tile adhesive manufacturing plants.

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Plants and facilities with dosing systems: