South Africa holds more than three-quarters of the world's viable chromium ore (chromite) reserves. Such an investigation is of particular importance within the local context, given the large amounts of waste produced by South Africa's Cr-related industry.
MOTIVATION AND OBJECTIVES
Project Motivation
- Water availability in South Africa
- Ozonation for water purification
- Ozonation for the recovery of Cr from waste materials
Ozonation is widely used in Europe and North America for water purification (Geldenhuys et al., 2000). This is mainly due to the limitations of the extraction techniques applied (Erdem et al., 2005; Riekkola–Vanhanen, 1999).
Project objectives
Thus, if Cr(III) in Cr-containing wastes can be converted to Cr(VI), it may facilitate the recovery of Cr waste units. Therefore, in addition to investigating the possible formation of Cr(VI) during water purification by ozonation (Par 1.1.2), the potential for the application of ozonation in the recovery of valuable Cr units from waste materials was investigated.
LITERATURE SURVEY
Introduction on chromium
- Historical perspective
- Natural occurrence
- General chemistry of chromium
- Consumption and uses
The first successful attempt was made in 1821 by Pierre Berthier, who produced metallic chromium by reducing the metal's oxides with carbon. Chromium is a member of the d-block transition metal in Group VIB (or Group 6) of the Periodic Table.
Chromium in South Africa
- Chromite ore reserves and production
- Ferrochrome and stainless steel production
- Cr(VI) chemicals production
According to 2009 International Chromium Development Association production statistics (Figure 2.3), South Africa produced approximately 39% of global chromium consumption (ICDA, 2010). South African stainless steel production began in the 1960s with RMB alloys and SX stainless steel with a capacity of 20 kt·y-1.
Core process
- Chromite mining and beneficiation
- Ferrochromium production
- Chromium chemicals production
Most of the South African semi-closed furnaces are operated on an acid slag, with a basicity factor (BF) smaller than 1. However, there are also pelletizing and sintering sections built at plants where the pellets feed through conventional semi-closed ovens.
Chromium containing wastes in South Africa
There are currently three furnaces in regular commercial operation in South Africa for the production of ferrochrome and they usually use a basic slag regime. During the production of the chemical Cr(VI), chromite ore is intentionally oxidized to Cr(VI) by an alkaline roasting process (Antony et al., 2001). This is in contrast to the ferrochrome industry, where the production of low Cr(VI) waste materials is an unintended by-product.
Certain facilities associated with Cr(VI) chemical production have been forced to shut down in South Africa due to occupational health and safety concerns. Thus, all coal-burning industries in South Africa (e.g. coal-fired power plants, coal for liquid fuel production, boilers) also produce fly ash and clinker containing chromium (Nel et al., 2011;.
Carcinogenicity and toxicity of chromium
Thus, given the difference in toxicity of Cr(III) and Cr(VI), determining the oxidation state of chromium is important if one wishes to assess its potential health effects. The importance of processes responsible for the conversion of Cr(III) to Cr(VI) and vice versa is also becoming clear.
Aqueous chemistry of chromium
- Speciation
- Precipitation/Dissolution
- Adsorption/Desorption
- Cr(VI) compounds
- Cr(VI) reduction
- Cr(VI) reduction by inorganic compounds
- Cr(VI) reduction by organic compounds
- Cr(III) oxidation
- Possible aqueous oxidation of Cr(III) by dissolved oxygen
- Oxidation by manganese oxides
- Cr(III) oxidation by the advanced oxidation process
The solubility of Cr(OH)3 also kept the chromium concentration below the drinking water limit of 50 ppb for total chromium (WHO, 2008; DWA, 1996) between pH 6 and 11. An excess of phosphate in the equilibrium solution completely prevents adsorption of Cr(VI) through soils (Bartlett & Kimble, 1976). Naturally occurring organic reducing agents of Cr(VI), such as citric acid, increased the ability of Cr(VI) to be removed by the soil (James & Bartlett, 1983a).
However, the oxidation of Cr(III) by dissolved oxygen has been reported to be very slow. Manganese oxides are considered the only naturally occurring oxidant of Cr(III), especially in soil environments (Eary & Rai, 1987). The oxidation of Cr(III) with manganese oxides is controlled by the surface characteristics of the oxides and the availability of Cr(III) to the surface.
At pH > 4, the conversion of Cr(III) propionate to chromate/dichromate was found to be > 90%.
Ozone
- Characteristics of ozone
- General chemistry of ozone
- Ozone generation
- Mass transfer of ozone into water
- The uses of ozone into water
A single initiation step can cause the breakdown of hundreds of ozone molecules before the chain is completed (Beltrán, 2003). In general, the activation of ozone produces several radicals, all of which help to oxidize compounds. The process of direct oxidation of ozone occurs quite slowly, but the concentration of aqueous ozone is relatively high.
This mass transfer of ozone from the gas bubbles occurs through the gas/liquid interface to the water and is dependent on a number of factors which are themselves influenced by the design and operation of the systems. When designing an ozone contact system, there should be maximum solubility of ozone in the water, with minimum off-gas production (Rajagopaul et al., 2008). The amount of dissolved ozone will depend on the temperature of the water (see Table 2.4) and the pressure at which the gas is applied (Degrémont, 2007).
Wiggins Water Works in Cato Manor has the capacity to produce 90 kg·h−1 of ozone to 350 Mℓ·day−1 of raw water.
Conclusions
EXPERIMENTAL
- Reagents
- General chemicals
- Samples
- Apparatus and measurement methods
- Sample preparation
- pH measurements
- Particle size analyses
- Scanning electron microscope (SEM)
- Chemical analyses
- Ozone generation
- UV/visible spectra
- Cr(VI) analysis
- Experimental setup
- Managing statistical data
All parts of the pulverizer, which made contact with the actual sample material, were made of tungsten carbide (Figure 3.1b). A Malvern Mastersizer 2000 (Figure 3.2) was used to determine the particle size distribution of the pulverized samples (Etxebarria et al., 2005). X-rays are created as a result of the ejection of an inner-level (low-energy) electron from the sample by an energetic electron from an electron column.
Scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) was used to detect potential surface chemical composition changes that occurred as a result of ozonation of the Cr-containing materials. The intensity of this emission is indicative of the concentration of the element in the sample (PerkinElmer, 2004). A Spectro Ciros Vision ICP-OES was also used to characterize the chemical composition of the Cr-containing materials used in this case study.
The absolute value of the difference between the doubtful value (xq) and the closest value (xn) is divided by the difference between the largest value and the smallest value (w) of the entire data set to obtain the estimated Q value (Mendham et al al., 2000).
- Case study material characterisation
- SEM–EDS characterisation
- Chemical characterisation
- Particle size distribution
- Effect of pH on Cr(VI) generation
- Effect of ozonation contact time
- Temperature effect
- Effect of solid loading
- Effect of gaseous O 3 concentrations
- Mechanism of Cr–liberation
- O 3 formation and decomposition
- Conclusions
SEM–EDS surface chemical analysis (Par. 3.3.4) of the polished sections of the two case study materials is presented in Table 4.1. The average particle size (Par. 3.3.3) distribution of the pulverized slag and UG2 samples is shown in Figure 4.3. In this particular study, higher Cr(VI) levels were measured for the ferrochrome slag compared to the UG2 ore (Figure 4.4), despite the ore having a higher total chromium content (Tables 4.1 and 4.2).
All results presented in the previous sections were performed with the maximum setting of the O3 generator used, i.e. to assess the effect of the O3 gas concentration on the formation of Cr(VI) during aqueous ozonation, two lower settings of the O3 were used. generator used. were used. Figure 4.12 shows the time dependence of the decomposition of O3 at the two pH values.
Other parameters such as contact time, water temperature, solid loading, ozone concentration, and characteristics of the Cr-containing material also affect the formation of Cr(VI).
RESULTS: Liberation of chromium from waste
- Introductory remarks
- Case study material characterisation
- Effect of pH on Cr–liberation
- Conclusions
The term Cr2O3–release was chosen, since the Cr content of feed materials for the ferrochrome industry is mainly expressed in terms of Cr2O3 content. The effect of pH on the release of Cr2O3 during the ozonation of ferrochrome slag is shown in Figure 5.1, while the release of Cr2O3 during the ozonation of UG2 ore is shown in Figure 5.2. From the results presented in Figure 5.1 it can be seen that an increase in pH resulted in increased Cr2O3 release.
Cr2O3 release from the UG2 was less successful (Figure 5.2), with a maximum Cr2O3 release of only about 5.5% achieved at pH 7. The slag, where better Cr2O3 release was achieved (Figure 5.1), passed through a pyrometallurgical means. process (liquid temperature of molten ferrochrome slag varies from 1620 to 1750°C) so the chromite structure was destroyed. However, the Cr2O3 release achieved for the ferrochrome slag is an indication that further work should be continued.
Given the Cr release mechanism observed based on particle size (Section 4.7), finer solids will most certainly result in better Cr2O3 release values.
PROJECT EVALUATION
Project evaluation and future perspectives
In addition to the removal of suspended chromium-containing materials prior to the use of ozonation, it became apparent that by controlling the pH of the water, the oxidation of Cr–. From the results presented in Chapter 4, it was clear that Cr(VI) can be formed during aqueous ozonation of Cr-containing materials. As mentioned above (point iv), relatively poor recovery (expressed as released Cr2O3) of Cr-units from waste materials was experimentally established.
These included the use of a better ozone generator (which was a limitation of this study) and finer grinding of waste materials. The Cr release mechanism observed in this study (Chapter 4) suggested that finer particles would be more suitable for the Cr recovery process. Moreover, the results showed that ozonation as a tool for the release of chromium from waste materials is highly dependent on the nature of the investigated Cr-containing wastes.
In addition, it is suggested that Cr(VI) formation via ozonation be investigated according to classical kinetic mechanistic studies to determine a possible chemical reaction mechanism for Cr(VI) formation with ozonation.
BIBLIOGRAPHY
Treatment of Cr(VI)-containing wastes in the South African ferrochrome industry - An overview of current applied methods. Review studies on the primary step of the reduction of chromium(VI) by iron(II). Kinetics and mechanisms of reactions of chromium(IV) and iron(II) species in acidic solution.
Activated carbon and low-cost adsorbents for the remediation of tri- and hexavalent chromium from water. Oligomerization of chromium(III) and its impact on the oxidation of chromium(III) by hydrogen peroxide in alkaline solutions. Conversion of chromium(III) propionate to chromium(VI) by the advanced oxidation process pretreatment of a biomimetic complex for metal analysis.
Comparison of the efficiency of OH radicals formation during ozonation and the advanced oxidation processes O3/H2O2 and UV/H2O2.
