Chloride process

In the SCALE project we investigate the recovery of Sc from waste streams originating from the chloride process of TiO₂ production. Both wastes (acid slurry and filter cake)  originate from the TiO₂ production facility of TRONOX in Botlek, the Netherlands. The input materials used are natural rutile from operations in Empangeni, KwaZulu- Natal and Brand-se-Baai, Western Cape (Namakwa Sands), South Africa as well as from Perth, Western Australia. Additionally, titanium slag from Namakwa Sands and synthetic rutile from Western Australia are used as feedstock, whereas those materials make up > 75% of the input (status 2018) [2].

"The chloride process can use a wide range of feedstock, such as rutil, synthetic rutile, high-grade ilmenite or slag, depending of the industry, however the main sources are  rutile and synthetic rutile (90%-95% TiO₂). The chloride process begins with the mixing of raw materials with gaseous chlorine at a temperature of around 900˚C-1000˚C in a fluidized bed reactor in the presence of coke as a reducing agent. The resulting gas stream contains titanium tetrachloride (TiCl₄), oxides of carbon and all the impurity metals from the feedstock in the form of metal chlorides, but impurities such as silica and zirconium may not chlorinated and remain accumulated in the reactor. Significant quantities of gas chlorine are required for low TiO₂ content feedstock. The gas stream is contacted with recycled liquid TiCl₄ which cools it to a level in at which the other metal chlorides separate out as solids by condensation and chemical treatment. The purified TiCl₄ goes forward with further cooling to be condensed as a liquid and then fed to a high temperature oxidation reactor where it is reacted with oxygen, above 1500˚C, either in a plasma arc furnace or in a toluene-fired furnace to form titanium dioxide and release the chlorine which is recycled back to the beginning of the reaction. Residual chlorine associated with the solid TiO₂ is removed by aqueous hydrolysis. Finally the pure titanium dioxide is subjected to a range of chemical surface treatments, milling and drying. The wastes generated by the chloride process are mainly coke and ore solids that remain un-reacted during the chlorination process. In addition a waste acid solution, usually called iron chloride waste acid, is also generated when the combined stream of un-reacted coke and ore solids, metal chloride solids, is acidified using water or waste hydrochloric acid (HCl) from the reaction scrubber. The metal chloride impurities are generally environmentally harmful, especially the iron chloride, which are removed and neutralized with lime or limestone, and finally sent for disposal via landfill.

About one ton of chlorine is required to produce 5 to 6 tons of titanium dioxide pigment. Currently, the chloride process offers tighter product control, is less labor intensive, and is environmentally safer. Currently about 60 percent of the 4.5 million tons of pigment production world-wide is generated by the chlorine process. Although declining in response to concerns about environmentally unacceptable waste, many sulfate plants have introduced innovative techniques deferring their closure." [1]

Chemical reactions of the process:

2 TiO₂ + 3 C + 4 Cl₂ → 2 TiCl₄ + 2 CO + CO₂

TiCl₄ (impure gas) → TiCl₄ (pure liquid)

TiCl₄ + O2 → TiO₂ + 2 Cl₂

Therefore, two different waste streams can accumulate during the chlorination process: 

Metal chlorides, non-reacted ore and coke dusts which are mixed and dissolved in water form an acidic, liquid waste stream with a certain amount of solid bottom sediment which will further be referred to as acid slurry (AS). This AS can be treated with lime whereby it is neutralized and metals precipitate as hydroxides.

The precipitates and the undissolved solids are filtered and filter pressed resulting in a second waste stream, the filter cake (FC).

In the TRONOX facility in the Netherlands currently only small amounts of the acid slurry and filter cake (FC) are stored on-site, most of the residues are converted to FC and are landfilled offsite. Both materials are of course also prone to changes in the original feedstock which can influence the general chemical composition specifically with regard to trace elements [2].

Both wastes are investigated in SCALE as potential Sc resources.