Contaminated soil must be crushed before thermal treatment when pieces or lumps exceed a particle size of approximately 60 to 80 mm. Larger fragments prevent uniform heat transfer in the rotary kiln and cause contaminants inside the material to not be fully desorbed. The following sections answer the most important questions about the mechanical pre-treatment of contaminated mineral waste.
Which soil properties make crushing necessary?
Crushing is necessary when contaminated soil contains pieces that are too large, too compact, or too inhomogeneous to be uniformly heated throughout in the rotary kiln. The decisive factors are particle size, moisture content, the proportion of cohesive components, and the presence of construction debris, stones, or concrete chunks.
Cohesive soils such as clays and silts tend to cake into compact lumps during transport and storage. These lumps cannot be broken up simply by placing them on a conveyor belt. The same applies to soils with high moisture content that form a hard crust when drying. Construction debris and concrete residues, which frequently arise during the remediation of industrial sites, also introduce very hard, large-sized fractions that must be mechanically broken up.
The distribution of contaminants within the material also plays a role. When contaminants such as PAHs or TPHs are bound deep inside larger lumps, external heat exposure alone is not sufficient to drive them out completely. Crushing creates new surfaces and significantly shortens the diffusion pathways for volatile contaminants.
How does particle size affect desorption in the rotary kiln?
Particle size is one of the most important parameters for effective thermal desorption in the rotary kiln. The larger a piece of material, the longer it takes for heat to reach its core. With excessively large particles, the material exits the kiln before the internal temperature is sufficient to fully mobilize contaminants.
In the rotary kiln, the material is continuously moved through the rotating drum and heated evenly in the process. Fine-grained and medium-grained materials up to approximately 60 to 80 mm benefit optimally from this mixing action. They absorb heat quickly, and contaminants can be efficiently transferred into the gas phase. Larger pieces, on the other hand, roll through the kiln without being heated all the way through.
Another aspect is the homogeneity of the feed material. Widely varying particle sizes lead to uneven heat distribution and make process control more difficult. Targeted mechanical pre-treatment ensures a defined feed particle size and thus stable, reproducible treatment results. This is particularly important when the treated material is to be reused or recycled afterwards.
Does every contaminated soil have to be crushed before thermal desorption?
No, not every contaminated soil needs to be crushed before thermal desorption. Sandy or gravelly soils with a naturally favorable particle size distribution can often be fed in directly. Whether a crushing stage is required depends on the soil type, the processing condition, and the excavation state of the material.
Loosely deposited, non-cohesive soils with a maximum particle diameter below the plant threshold can generally be treated directly. Soils that have already been mechanically loosened during excavation and show no clumping can also be introduced into the plant without an additional crushing stage.
The situation is different for soils that have been stored for a long time, are heavily compacted, or contain a high proportion of cohesive fractions. In these cases, mechanical pre-treatment is generally indispensable to ensure treatment quality and to avoid impairing plant operations. The decision is made on the basis of a material analysis and a visual assessment of the delivered waste.
Which crushing methods are used for contaminated mineral waste?
For contaminated mineral waste, jaw crushers, impact crushers, and roller crushers are primarily used. The choice of method depends on the material composition, the initial size, and the desired target particle size. For highly cohesive or moist materials, screens and size-reduction units are frequently combined as well.
Jaw crushers
Jaw crushers are well suited for hard, lumpy materials such as concrete chunks or construction debris. They reduce the material by compressive stress between a fixed and a movable jaw plate. Wear is high with abrasive minerals, but the machines are robust and designed for use under difficult conditions.
Impact crushers and roller crushers
Impact crushers operate with high impact energy and are suitable for medium-hard material. Roller crushers, on the other hand, are particularly suitable for moist, sticky soils that tend to clog. They tear and squeeze the material without overheating it, which is important with contaminated waste to prevent uncontrolled release of contaminants. All units used must be suitable for handling contaminated materials and must be appropriately sealed or enclosed to prevent emissions.
What happens if contaminated soil is not adequately pre-crushed?
If contaminated soil is not adequately pre-crushed, contaminants can remain in the core of larger lumps and exit the treatment process without having been fully desorbed. The result is an output material that may not meet the required limit values and cannot be recycled.
Additional risks arise for plant operations. Oversized pieces can mechanically stress the rotary kiln, block conveyor systems, or cause uneven material throughput. This not only impairs treatment quality but can also lead to unplanned downtime.
From a legal perspective, incomplete treatment is problematic. The treated material is only considered harmless once contaminant levels fall below the permissible limit values. If the output material is produced without adequate pre-treatment and thus without complete desorption, rejection of the material or costly post-treatment is at risk. Thorough mechanical-physical pre-treatment is therefore not an optional step but an integral part of proper disposal.
Who decides whether a crushing stage is required?
The decision as to whether a crushing stage is required is made by the plant operator on the basis of a material assessment prior to acceptance. The basis for this is the analytical data of the waste, a visual assessment of the material structure, and knowledge of the plant requirements. In practice, this assessment takes place as part of the acceptance procedure.
In material flow management, the delivered material is first characterized. Information from the pre-inquiry, analytical certificates, and the origin of the soil is taken into account. Soil from the excavation of a former gasworks behaves differently from soil material from an agricultural environment. This contextual information helps to plan the necessary pre-treatment steps before delivery.
Customers who are unsure whether their material requires pre-crushing can contact the plant operator at an early stage. Close coordination before delivery saves time, avoids rejections, and ensures a smooth treatment process. Further information on material flow management helps to understand the entire disposal pathway from acceptance to recycling.
How ARE Deutzen GmbH helps with the pre-treatment and thermal treatment of contaminated soils
ARE Deutzen GmbH supports civil engineering companies and industrial clients from the initial material assessment through to the completed treatment of contaminated soils and mineral waste. With over 30 years of experience in thermal desorption and a clear focus on technical quality, ARE Deutzen GmbH offers concrete solutions for demanding remediation projects.
The range of services includes, among other things:
- Material assessment and acceptance inspection to determine pre-treatment requirements
- Mechanical-physical pre-treatment including crushing, screening, and homogenization
- Thermal desorption in the rotary kiln at temperatures of up to approximately 600 °C, depending on the material and contaminant load
- Treatment of a broad spectrum of contaminants: TPHs, BTEX, PAHs, PCBs, PFAS, PCDD/PCDF, Hg, cyanides, and others
- Multi-stage exhaust gas purification to ensure reliable compliance with statutory emission limit values
- Material flow management and interim storage for a seamless disposal chain
If you are planning to dispose of contaminated soil material and are unsure whether and how it needs to be pre-treated, contact ARE Deutzen GmbH at an early stage. Get in touch and describe your project so that the appropriate treatment pathway can be determined together.