Cost Analysis and Control of Asphalt Recycling Mixing Equipment

Abstract: This article introduces the structural forms (intermittent and continuous) and recycling performance of plant-mixed hot recycling mixing equipment, analyzes the construction cost and production operation cost of asphalt recycling mixing equipment, and proposes control measures to reduce operation costs.

Key words: Cost of asphalt recycling mixing equipment

With the increase in the mileage of highways in our country and the development of high-grade highways, the asphalt pavement expressways that were built and opened to traffic in the early days have entered the major repair and maintenance period, thus a large amount of old asphalt mixture is milled out. The reuse of mixtures has attracted widespread attention. Countries that started the development of expressways earlier have long adopted asphalt recycling technology for the regeneration and maintenance of road surfaces, and their recycling equipment and construction techniques are relatively mature. However, China has only just begun to introduce and adopt this technology, and there are many issues worth discussing. The author will analyze the construction cost and production operation cost of the plant-mixed hot recycling mixing equipment. It is of certain help for the selection and use of recycled mixing equipment.

While the mechanized construction of high-grade highways benefits mankind, it also brings about health hazards to humans and environmental pollution, such as waste gas, toxic gas, dust, waste oil, noise, etc. Especially the large amount of asphalt mixture in the major repair period of high-grade asphalt pavement causes even greater pollution to the environment. Therefore, scientific and technological personnel have tried every possible way to recycle and reuse the used asphalt mixture. There are also many methods, including on-site hot regeneration, on-site cold regeneration, field-mixed hot regeneration and field-mixed cold regeneration. When hot regeneration is used to heat and recycle old asphalt mixtures, a large amount of asphalt fumes and toxic gases will be produced. At relatively contradictory times, people designed special drying cylinders and burners for intermittent mixing equipment. Secondary air intake devices were installed around the burner opening to ensure that the flame of the burner combustion is slender and does not spread to the periphery, thus preventing the asphalt in the old recycled materials from aging. As a result, the utilization rate of thermal energy generated by the burner is extremely low, and a large amount of thermal energy is discharged by the induced draft fan, causing a huge waste of energy. With international fuel prices remaining high, how to save energy has once again become the primary issue to be addressed. Based on the above contradictions, it is necessary to conduct an economic analysis of the current old asphalt recycling equipment.

Asphalt plant-mixed hot recycling equipment can be divided into two structural forms: intermittent type and continuous type

The technology of heating recycled materials in the second drying drum is generally recognized for intermittent asphalt recycling equipment. This type of equipment needs to be equipped with a dedicated system on the basis of the original mixing equipment. This system includes the crushing and screening system for the old asphalt mixture, the supply system, the lifting system, the drying system, the combustion system, the storage and weighing system, and the treatment system for harmful gases. The asphalt mixing equipment produced by foreign manufacturers such as Beninghoff, Marini, Rigong, and Oman all have this function. However, the same problem exists: the drying system must not only ensure that the old recycled materials reach a certain temperature but also guarantee that the asphalt adsorbed on the surface does not age. In terms of the treatment of harmful gases, it is required that the harmful gases released during the heating and storage of old mixtures be absorbed and treated without causing pollution to the environment. To ensure the uniform mixing of new and old materials and the full and even coating of new and old asphalt, the stirring time needs to be increased, so the output of the equipment has decreased. Theoretically, the recycling rate of old asphalt mixture can reach 100%, but in actual production, it can only be added to less than 50%. Although the recycled materials have undergone screening treatment, their gradation cannot meet the requirements of asphalt mixtures. Even the temperature of the old recycled materials cannot meet the requirements of the asphalt mixture (usually, when the old recycled materials are heated above 130℃, they will produce a large amount of smoke and harmful gases that cannot be fully absorbed). New aggregates must be added to increase the temperature of the new aggregates and use part of the heat from the new aggregates to heat the old recycled materials.

The most widely used and representative continuous asphalt recycling equipment is the double-drum recycling mixing equipment produced by the American ASTEC Company. After the aggregates are sieved and graded and dried, they enter the mixing cylinder on the outer wall of the drying cylinder at one end of the burner and mix with the old recycled materials that have just entered. The old recycled materials are heated by the heat radiation from the hot aggregates and the outer wall of the drying cylinder, which completely isolates the old asphalt mixture from the flame and prevents the asphalt from aging. The mixture is forcibly stirred by the agitator installed on the outer wall of the drying cylinder (similar to a single horizontal shaft agitator). As the drying cylinder is relatively long, up to seven meters, the stirring time of the new and old materials in the double drums can reach 60 to 90 seconds, ensuring uniform mixing of the mixture. During the mixing process of new and old materials, the harmful gases produced return to the combustion chamber for combustion, significantly reducing the amount of asphalt gas and harmful gases entering the dust collector. It has reduced secondary pollution to the atmosphere. This equipment is continuous from the measurement, transportation, heating, stirring of aggregates to the transportation of finished materials. Its structure is relatively simple, with a high output and a low failure rate. It can add 50% of the old recycled materials.

Analysis of the Purchase Cost of Asphalt Recycling equipment

At present, the most representative intermittent asphalt recycling equipment in China is the H series of environmentally friendly products jointly developed by Xizhu Factory and Benning Hoffer. The basic model is the H4000 environmentally friendly and energy-saving asphalt mixing equipment, equipped with the MGAR150 type site hot recycling equipment. Both sets of equipment have independent control systems, but they can also be matched with each other to achieve automatic system control.

At present, the most representative continuous asphalt recycling equipment is the double-drum asphalt recycling equipment produced by the American company Asterco. Its main systems include the double-drum (drying - mixing) system, control system, recycled material batching and metering system, heat transfer oil combustion system, asphalt (including regenerant) metering pump, cold aggregate batching and metering system, and filler metering system. The original imported Estek products are adopted, while the rest of the component systems are provided by domestic factories, but they must meet the technical requirements of Estek Company (see Table 1).

Economic Analysis and Control Measures for the Operation of Asphalt Recycling Equipment

Economic analysis

From the cost chart of asphalt recycling mixing equipment, it can be seen that in the mixtures mixed by the two types of equipment, raw materials account for 76% to 84%, while consumed materials (fuel, electricity) and equipment costs account for 15% to 23%. Here, factors such as management costs, waste materials, heat transfer oil heating, equipment malfunctions, and the differences in raw material prices in different regions have not been taken into account. However, on the whole, it relatively intuitively reflects the difference in the cost of mixing and blending materials between the two types of recycling equipment: the cost of intermittent asphalt recycling equipment is 8% higher than that of continuous asphalt recycling equipment in terms of material consumption and equipment expenses. Upon further detailed analysis, it is not difficult to identify the reasons: In the intermittent recycling equipment, the drying cylinder and burner specially designed to prevent the aging of old asphalt in the heating of recycled materials are not conducive to the full utilization of heat, and most of the heat energy is discharged through the induced draft fan. When the continuous regenerative mixing of the mixture is carried out, the heat radiation on the outer wall of the drying cylinder and the heat exchange of the new aggregates are fully utilized. Intermittent equipment has led to a decline in productivity and an increase in equipment costs due to the increased measurement of old recycled materials and the extended mixing time during mixing to ensure thorough and uniform mixing of new and old materials as well as uniform coating of new and old asphalt.

(2) It can also be seen from the cost charts of various mixed materials that by adopting asphalt recycling technology, the cost of each ton of finished material has decreased by 15% to 20%, avoiding the pollution to the environment caused by the disposal of old recycled materials and reducing the waste of resources.

Control measures

Whether it is intermittent or continuous recycling equipment, in addition to the structural differences analyzed above that cause different costs of the mixture, it is also necessary to analyze and control in sequence according to the size of the mixture cost. The sequence is: asphalt → aggregate (recycled material) → mineral powder → reliability of the equipment → management link.

(1) Control of asphalt measurement accuracy

The accuracy of the amount of asphalt not only directly affects the quality of the mixture but also influences its cost. Firstly, it is necessary to frequently measure and analyze the asphalt content in the old recycled materials. When there is a significant fluctuation in the asphalt content, the amount of new asphalt should be adjusted in a timely manner. The metering system of the new asphalt should also be frequently calibrated, and it is best to ensure that its metering accuracy is within ±0.1%. It is conducive to reducing costs and improving economic benefits.

(2) Control of aggregates (including recycled materials)

The moisture content of new aggregates and recycled materials has a significant impact on cost control. For every 1% increase in moisture content, production capacity drops by approximately 15%, and an additional 15% of fuel is burned. When processing crushed stones and stone chips, it is best to wash them with water to remove dust and mud particles smaller than 0.075mm. This is because when heating aggregates, these fine-grained materials all enter the dust removal system. Often, it has a high mud content and cannot be used, causing unnecessary waste and environmental pollution to the surrounding areas. The dust content is usually around 5% to 8%.

(3) Reliability of the equipment

The reliability of equipment encompasses the accuracy and stability of various measurements, high-quality components, the interchangeability of parts, and the overall stability of operation. Ensuring the normal operation of equipment, reducing the frequency and duration of downtime due to faults, improving the utilization efficiency of equipment, and being conducive to reducing cost expenditures.

(4) Control of fuel

To ensure that the fuel burns fully and emits the maximum heat energy, a reliable and high-quality burner is required. Whether the energy can be effectively absorbed by the aggregate also requires the drying cylinder to have good heat preservation and heat exchange capabilities. The burner is a key component in asphalt mixing equipment. Fuel consumption accounts for 12% to 18% of the operating costs. Therefore, it is required that the burner be safe and reliable with excellent quality assurance to save fuel. When reducing operating costs, burning heavy oil is also a means. Although heavy oil has a slightly lower calorific value, the price difference is significant.

3 Conclusion

A cost analysis of intermittent and continuous asphalt recycling equipment reveals that the intermittent equipment wastes a large amount of heat energy due to the secondary air intake device designed for the burner of the second drying cylinder. In contrast, the continuous type utilizes the heat radiation from the outer wall of the drying cylinder to be absorbed by the old recycled materials, thus saving fuel.

(2) The variability of old recycled materials is relatively high. The principle of intermittent and continuous gradation control is the same. The addition amount of recycled materials should be within 30%, and the quality of the mixed materials can all meet the specification requirements for high-grade pavement construction.

(3) Continuous asphalt recycling equipment has certain limitations. As the technical specifications for asphalt pavement construction have not been revised yet, it cannot be used for the new pavement construction of expressways at present. However, it has an advantage in asphalt recycling.