Formation mechanisms of electric furnace dust and filter cake

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The formation of filter cake in the waste acid treatment plant The scale of the hot rolled or annealed stainless steel strip predominantly contains FexO (x=0.67, 0.75 and 1), Cr2O3, NiO, SiO2, NiCr2O4 and FeCr2O4 [66]. The formation mechanisms of the filter cake vary according to the treatment process. For the traditional three step treatment process, the filter cake contains amorphous phases which consist of metal ions such as Cr, Fe, Ca and Ni, crystalline phases such as calcium fluoride (CaF2) [60,61], calcium sulphate (CaSO4) [61], Cr2O3 and spinel phase (FeCr2O4) [62].
Based on Chapter 2 The characteristics, formation mechanisms and treatment processes of Cr (VI)- containing pyrometallurgical wastes: a review an analysis of the production process, the amorphous phases are mixtures of the metal hydroxides that precipitated from the alkaline solution. CaF2 and CaSO4 however might precipitate due to their low solubilities (solubility limits of 0.016g/l and 2.09g/l respectively [67]). Cr2O3 and the spinel phases (e.g. FeCr2O4) are associated with stainless steel scale that remained after pickling.

Treatment processes of electric furnace dust and filter cake

A number of problems currently hamper the treatment of Cr(VI)-containing electric furnace dust and filter cake: (a) The variation in composition of the dusts (as shown in Table 2.3) due to changes in heats, raw materials, grade and operational parameters which require that the technology can treat all kinds of dusts; (b) Volatile substances in the dusts can impact on the normal operation of the furnace if it is simply charged back to the furnace; (c) Leachates from the electric furnace dusts are highly basic, and therefore require large volumes of acid in order to adjust the pH [68]; (d) the high sulphur content in the filter cake makes it difficult to recycle back to the steelmaking plant [63]. (e) Cr (VI) is highly soluble, and has very low regulation limits. As an alternative to land filling or stockpiling, different methods have been developed to deal with these wastes : 1) Minimisation of the wastes at source by optimising the operational parameters; 2) Direct recycling of the dust back to the electric furnace by injection with air [69-73]; 3) Recovery processes, which include hydrometallurgical methods [68,74-89] and pyrometallurgical methods [90-112]; 4) Solidification/stabilisation methods, for instance, cementation and vitrification or glassification processes [3,5,113-123]; and 5) Use as a raw material for fertiliser, glass and pigment plants [124,125].

Contents :

Acknowledgements
Abstract
Contents
List of Figures
List of Tables
Chapter 1 Introduction
- 1.1 Background
- 1.2 Objectives of this project
- 1.3 Structure of the thesis
Chapter 2 The characteristics, formation mechanisms and treatment processes of Cr (VI)-containing pyrometallurgical wastes: a review
- 2.1 Introduction
- 2.2 The characterisation of electric furnace dust and filter cake
  - 2.2.1 Electric furnace dust
  - 2.2.2 Filter cake from the waste pickling acid treatment plant
- 2.3 Formation mechanisms of electric furnace dust and filter cake
  - 2.3.1 Dust formation in the stainless steel plant
  - 2.3.2 Dust formation in the ferrochrome plant
  - 2.3.3 The formation of filter cake in the waste acid treatment plant
- 2.4 Treatment processes of electric furnace dust and filter cake
  - 2.4.1 Recycling
  - 2.4.2 Recovery
- 2.4.3Solidification/stabilisation
- 2.5 Conclusions
Chapter 3 The characteristics of the Cr (VI)-containing electric furnace dust and filter cake from a stainless steel waste treatment plant
- 3.1 Introduction
- 3.2 Experimental
  - 3.2.1 Waste materials and sample preparation
  - 3.2.2 Analytical procedures
- 3.3 Results
  - 3.3.1 Particle Size Distribution, Bulk Density, Moisture Content and pH
  - 3.3.2 Chemical Composition and Phase Composition of the EF Dusts and FC
  - 3.3.3 TG/DTA Analysis
- 3.4 Discussion
- 3.5 Conclusions
Chapter 4 The formation mechanisms of Cr(VI)-containing electric furnace dust and filter cake from a stainless steel waste treatment plant
- 4.1 Experimental
  - 4.1.1 Sample Preparation
  - 4.1.2 Analytical Methods
- 4.2 Results
- 4.3 Discussion
  - 4.3.1 The formation mechanisms of stainless steel plant dust
  - 4.3.2 The formation mechanisms of the ferrochrome dust
  - 4.3.3 The formation mechanisms of filter cake
  - 4.3.4 Measures to reduce waste generation
- 4.4 Conclusions
Chapter 5 The leachability of the Cr (VI)-containing electric furnace dust and filter cake from a stainless steel waste treatment plant
- 5.1 Introduction
- 5.2 Experimental
  - 5.2.1 Sample preparation
  - 5.2.2 Experiment methods
- 5.3 Results and discussion
  - 5.3.1 TCLP and ASTM D 3987-85 tests
  - 5.3.2 Static distilled water and nitric acid leaching tests
  - 5.3.3 Effect of stirring speed on the leachability of Cr (VI) from stainless steel dust
- 5.3.4 Effect of temperature on the leachability of Cr (VI) from stainless steel dust
- 5.3.5 Effect of pH on the leachability of Cr (VI) from stainless steel dust
- 5.3.6 The leachability of Cr (VI) from ferrochrome dusts and filter cake
- 5.4 Conclusions
Chapter 6 The aging behaviour of Cr (VI)-containing Electric furnace dust and filter cake from a stainless steel waste treatment plant
- 6.1 Introduction
- 6.2 Experimental
  - 6.2.1 Sample preparation
  - 6.2.2 Leaching experiment and analysis methods
- 6.3 Results
  - 6.3.1 Effects of temperature and time on the aging behaviour of Cr (VI)
  - 6.3.2 Effect of particle size of the wastes on the aging behaviour of Cr (VI)
  - 6.3.3 Effect of atmosphere on the aging behaviour of Cr (VI)
- 6.4 Discussion
- 6.5 Conclusions
Chapter 7 Stabilisation of Cr (VI) through sintering using silica-rich clay, Part I: Synthetic samples
- 7.1 Introduction
- 7.2 Experimental
  - 7.2.1 Materials
  - 7.2.2 Analytical techniques
- 7.3 Results and discussion
  - 7.3.1 Characteristics of the clays
  - 7.3.2 Leaching behaviour of Cr(VI) from the sintered brick
  - 7.3.3 Chromium emissions during sintering process
- 7.4 Conclusions
Chapter 8 Stabilisation of Cr (VI) through sintering using silica-rich clay, Part II: Electric furnace dust and filter cake
- 8.1 Introduction
- 8.2 Experimental
  - 8.2.1 Sample preparation
  - 8.2.2 Experimental methods
  - 8.2.3 Analytical methods
- 8.3 Results and discussion
  - 8.3.1 Effect of clay type on the leachability of Cr (VI)
  - 8.3.2 Effect of leach time on the leachability of Cr (VI) in the modified TCLP test
  - 8.3.3 Influence of sinter temperature on the leachability of Cr (VI)
  - 8.3.4 Influence of the SPD content of the brick on the leachability of Cr (VI)
  - 8.3.5 Influence of sinter time on the leachability of Cr (VI)
  - 8.3.6 The leachability of other toxic substances from the stabilised wastes
  - 8.3.7Crystalline phases present in and microstructure of the sintered brick
  - 8.3.8 Cr (VI) stabilisation in the sintered brick
  - 8.3.9 Stabilisation of Cr (VI) in ferrochrome fine dust and filter cake by sintering
  - 8.3.10 Chromium emission during the sinter process
- 8.4 Conclusions
Chapter 9 Stabilisation of Cr (VI) through sintering using silica-rich clay, Part III: Leaching behaviour of chromium from the stabilised wastes
- 9.1 Introduction
- 9.2 Background
  - 9.2.1 Leaching model based on initial wash-off or interface reaction kinetics
  - 9.2.2 Leaching model based on matrix diffusion
  - 9.2.3 Leaching model based on dissolution or corrosion
  - 9.3 Experimental
  - 9.3.1 Sample preparation
  - 9.3.2 Leaching test
- 9.4 Results and discussion
  - 9.4.1 Leaching mechanisms of chromium from the stabilised product
  - 9.4.2 Leaching mechanisms of Cr(VI) from the stabilised wastes
- 9.5 Conclusions
Chapter 10 Summary and Conclusions
- 10.1 Summary
  - 10.1.1 Stainless steel plant dust (SPD)
  - 10.1.2 Ferrochrome plant dust (FCD1, FCD2 and FCD3)
  - 10.1.3 Filter cake (FC)
- 10.2 Conclusions
- 10.3 Recommendations for future work
  - 10.3.1 Modelling of the formation mechanisms of Cr (VI)
  - 10.3.2 Simultaneously treatment of stainless steel plant dust and pickling acid
  - 10.3.3 Properties of the bricks
- References
- Appendix I Cr (VI) and total chromium determination using spectrophotometer
- Appendix II Thermal characteristics of clays
- Appendix III Mass balance of the sintered brick
- Appendix IV Calculations on the acceptable Cr (VI) concentration in the leachate
- Appendix V The production process of synthetic calcium chromate
- Appendix VI Details of experiments on leaching behaviour of chromium from the
- stabilised wastes