Recycling of Waste and Valuable Materials in the Rotary Kiln

Requirements for Process Engineering and a Comparison of Grate Firing and Fluidized Bed

Our economy is tied to the availability of raw materials, which are limited, however, especially in the case of geological deposits. In addition, mining is faced with ever greater challenges when easily accessible resources are depleted – whether due to technical complexity or environmental and social concerns. Against this background, the recovery of raw materials, recycling and the treatment of wastes is increasingly becoming the focus of society and the economy.
These trends place new demands on process technology, as increasingly complex processes become economically relevant.

New Recycling Markets Open

New framework conditions will bring processes that previously appeared as a marginal phenomenon or research project into the focus of companies.

One example is the processing and recycling of building materials, such as residues from the demolition of streets and buildings; they can replace the extraction of dwindling resources. Gypsum is of particular interest here. The phase-out of coal energy means that FGD gypsum will no longer be a by-product of flue gas desulphurization – being the primary source of the important building material today. Otherwise, the serious gap in supply could only be closed only with the massive exploitation of natural resources. The recovery and reuse of gypsum from the waste of demolished buildings offers an attractive solution.

Another aspect is the reduction of CO2 emissions that arise during the production of concrete – the biggest climate impact of a single industry worldwide. A higher proportion of concrete made from recycled raw materials can offer massive savings potential here.

Other examples are the recycling of phosphorus from sewage sludge; the expansion of the recovery of rare earths or the regeneration of used catalyst materials from the chemical industry.

Recycling Challenges in Rotary Kilns

The treatment of waste places particularly high demands on process engineering, especially in the case of thermal treatment, because strong fluctuations in the quality of the material must be covered. Sewage sludge, for example, is often produced as a homogeneous liquid-pasty mass with a high water content, while the residues from the demolition of roads or buildings are lumpy, highly inhomogeneous and have a wide particle size distribution. The chemical composition also shows high fluctuations.

The result is a wide range of calorific values that fluctuate strongly. Therefore, the choice of treatment technology plays a decisive role in the implementation of recycling projects with thermal treatments.

The Rotary Kiln Compared to Alternatives

For the recycling processes under consideration, rotary kilns, grate firing or the fluidized bed are particularly suitable. Differences in the procedures are reflected in the suitability for different input materials.

The rotary kiln scores particularly well when it comes to the large number of different types of waste and residual materials. Thermal treatment of lumpy waste from automobile recycling is just as possible as the burnout of pasty sewage sludge. The incineration of such a broad spectrum of waste cannot be achieved with either grate or fluidized bed incineration. On the other hand, a slightly reduced energy efficiency is to be expected with the rotary kiln.

With the appropriate engineering design, the rotary kiln can cover a wide range of recycling processes, and fluctuations in the feed quantity are also unproblematic. A grate furnace offers similar advantages in processing different quantities, the fluidized bed however cannot be used as variably.

Due to the wide range of possible treatment conditions (temperature and atmosphere), a large number of processes can be implemented in the rotary kiln, which opens up a broader range of applications compared to competing technologies, although energetic optimization for a certain process can be more complex. The direct incineration of hazardous waste for thermal recycling in rotary kilns is a proven method. At the other end of the spectrum, old tires can for example, be pyrolyzed in the absence of oxygen in order to produce environmentally friendly carbon black.

Water-containing sewage sludge can also be treated: For drying or for thermal conversion into inertized ash. With appropriately optimized process parameters, bound organic pollutants are completely destroyed, even inorganic pollutants such as mercury or arsenic can be expelled. As a recycling product, you get an ash rich in bioavailable phosphorus which can be used directly as a mineral fertilizer.



Grate Firing

Fluidized Bed

Rotary Kiln



Stair grates:

  • Coarse and ash-rich fuels such as wood, processed garbage or (today more rarely) brown coal


Roller grates:

  • Waste incineration with more problematic, non-prepared, inhomogeneous fuel mixtures, with a tendency to stick or slag.
  • Only limited slagging operation possible, high slag quality, if necessary with slag after-treatment

Applicable for:

  • Sludges
  • Lumpy, granular waste with high ash content
  • Conditionally suitable for gaseous waste
  • Conditionally suitable for liquids


Wide range of application:

  • Solid fuels: Coarse, rich or low in ash
  • Dusty substances
  • Liquids
  • Pasty fabrics
  • High and low caloric substances
  • Inhomogeneous fuels, also with different phases


Driving style:

  • Incinerating
  • Slagging
  • ("slag lake" – high slag quality achievable)



  • Many mechanical components with high maintenance
  • Extensive corrosion protection with high corrosion potential (water cooling, special steels)


  • Strongly influenced by changes in operating parameters
  • Feed material may need to be pre-sorted
  • Not suitable for metals and rocks
  • Not suitable for bulky and coarse waste
  • Robust and proven mechanics
  • “Simple”
  • corrosion protection via brickwork and the use of “normal”  boiler plate
  • Higher heat losses than rust
  • Maintenance: Drive, brick lining


With our many years of extensive experience in planning and operating rotary kilns for a wide variety of purposes, we are happy to support you with our service in the implementation of your recycling projects. Just give us a call or write an email to discuss your project!


Christian Kühnert

Technical Sales

Telephone: +49 - 3643 8649 - 89
E-Mail:        kuehnert(at)

German, English


Dr. Nico Zobel

Director Sales Services & Products

Telephone: +49 - 3643 8649 - 38
E-Mail:        zobel(at)

German, English

Robert Süße

Sales, International

Telephone: +49 - 3643 8649 - 730
E-Mail:        suesse(at)

German, English


Dr. Stefan Schwarz

Head of Sales Cathode Material

Telephone: +49 - 3643 8649 - 386
E-Mail:        schwarz(at)

German, English

Steffen Vogel

Sales, Engineering

Telephone: +49 - 3643 8649 - 40
E-Mail:        vogel(at)

German, English

Elahe Isazadeh

Technical Sales

Telephone: +49 - 3643 8649 - 234
E-Mail:        isazadeh(at)

German, English, Persian