Dry sintering of nepheline – a new, more energy efficient technology

A new technology for dry sintering of nepheline and limestone to produce alumina has been developed in collaboration between Pikalevo Soda, FLSmidth and the Russian National Mineral Resources University. The new technology is a result of laboratory research activities and semi-industrial tests conducted by FLSmidth in Denmark and the USA.

The technology was tested in a semi-industrial setting, which consisted of a three-stage preheater with calciner and rotary kiln. This type of equipment has already been in operation in the cement industry for more than 40 years.

The tests proved a success due to the ability to control feed chemistry and particle size distribution, as well as temperature profile and retention time in kiln system. Control of the above parameters is the basis for achieving high-quality sintering and required alumina extraction.

Up until today, sintering of nepheline and limestone has been done by means of wet technology. In spite of the fact that nepheline contains substantially less alumina than bauxite, its chemical components can still produce significant alumina percentages. In regards to energy efficiency, the wet technology is unable to compete with newer technology. The conversion to a dry process offers the potential to reduce the specific fuel consumption by nearly 50%.

Laboratory research and semi-industrial testing have been key in the effort to develop a dry technology for nepheline sintering. The result of this testwork shows that dry technology is now a valid replacement for wet technology. The processing of nepheline to alumina contains many sub-processes. The sintering of nepheline and limestone is the first part of the production of alumina.

Sintering process (existing wet process)
The raw mix preparations are made up of two steps. First, limestone is ground separately with white slurry in an open-circuit system followed by co-grinding with nepheline in the last step, also in an open-circuit system.

White slurry is a waste product from the desilication step, but as it contains residual alumina it is re-introduced to the process to minimize waste products and achieve as close to 100% utilization of the chemical components as possible.

Preceding the burning zone is a preheating and calcining zone. In the preheating zone the material is heated up and the moisture in the raw mix is removed. In the calcining zone the most important reaction is the decomposition of calcium carbonate.

This reaction is also central in cement production and is one of the reasons for the good synergies that are present when comparing the two processes. It is to a great extent the same equipment applied, and thereby processes from cement production can be used in the nepheline sintering process.
Final sintering takes place in a rotary kiln at a burning zone temperature of 1,250–1,325°C. There is a very narrow temperature range where the final sintering takes place, and attention to the amount of liquid phase is crucial as this is responsible for the final reactions and structure of the sinter.

The solid material retention time in the kiln is 2½–3 hours, resulting in an alumina extraction of 87–89% in the sinter. The heat consumption is approximately 1,290 kcal/kg sinter. The result of the burning tests with the raw mix sample shows that the best conditions were a burning temperature of 1,325°C with a duration of 30 minutes.

Testing
Extensive laboratory and pilot testing was performed over a span of several years. Initial pilot operations in the dry process system yielded unsuccessful results in terms of process stability, efficiency and alumina extraction. Adjustments were made to the method of feed preparation, and the pilot system was redesigned to improve cyclone performance. The end results were significant improvements to feed chemistry control, higher cyclone efficiencies, more stable flash calciner and kiln operation, and better control of the sinter liquid phase in order to support target alumina conversion levels. The clients were present to observe these efforts and see firsthand the ability of the dry system to satisfy their expectations.

Conclusions
Dry sintering of nepheline and limestone is a new approach to the sintering process and results in a much more energy efficient technology. The process has been proven in a semi-industrial test, and the alumina extraction results are in line with what is achieved today when a wet technology is applied.

Additionally, when comparing the two technologies the following process knowledge and experience has shown:

  • Separate grinding resulting in similar particle size distribution of the nepheline and limestone
  • Enhanced focus on raw mix homogenization and chemical control to ensure optimal conditions for sintering
  • Control of calcination degree and temperature profile
  • Special care to the temperature profile in the rotary kiln as the burning zone is narrow
  • Retention time is needed to allow for the sintering to reach completion

Further optimization is possible, and industrial operated systems should be custom-built to enable the dry process to achieve excellent performance.

This new, more energy efficient technology for sintering of nepheline and limestone has been developed and now proven semi-industrially, with more than 50% reduction in heat consumption. Furthermore the dry technology offers the possibility of having bigger production units.

Based on this study, Pikalevo Sodium and FLSmidth have signed an agreement on technological and project cooperation aimed at upgrading the facilities of the Pikalevo manufacturing complex. The 3,700-tpd dry process sinter system designed by FLSmidth will support a significant reduction in plant fuel consumption and enable an increase in plant alumina production levels. A basic engineering contract was signed with the plant in February 2014, and this engineering work will continue for several months in order to yield a final firm solution for the plant.

CONTACT: Sine Bogh Skaarup