Process for chlorine recovery

Details Process for chlorine recovery Process for chlorine recovery

2001-02-23

2004-08-03

Nguyen, Ngoc-Yen (Department: 1754)

Chemistry of inorganic compounds

Halogen or compound thereof

Elemental halogen

C423S502000, C423S504000, C423S492000, C423S493000, C423S148000

Reexamination Certificate

active

06770255

ABSTRACT:

The present invention is concerned with a process for the recovery of chlorine from metal chlorides. More specifically, the invention relates to a process for dechlorinating iron chloride mixed with several other metal chlorides to produce chlorine gas. The process has the ability to recover chlorine from these chlorides in the presence of carbon and other materials such as oxides which are present in typical wastes generated during the chlorination of synthetic rutile.
Titanium tetrachloride is conventionally produced from either ilmenite or rutile (including synthetic rutile) in a fluidised bed at around 1000° C. Chlorine, a carbon rich material such as petroleum coke and a titanium-bearing material are fed into the fluidised bed chlorinator and titanium tetrachloride vapour leaves the system in the gas phase. It is subsequently condensed, purified and used in the production of either pigment or titanium metal. Chlorine is recycled to the chlorinator.
Impurities such as Mg, Mn, Al in the feed are chlorinated to varying degrees. Iron in the feed chlorinates readily and is sufficiently volatile to leave the reactor with the effluent gas. The chlorides of these metals are condensed from the gas phase at temperatures around 200° C., while titanium tetrachloride, which has a lower boiling point, remains as a vapour. Non titanium metal chlorides are therefore amenable to separation by differential condensation. They are recovered as solids in the chlorinator off gas cooling system.
The condensed stream from the chlorinator typically contains chlorides such as FeCl
2
, MnCl
2
, MgCl
2
and AlOCl, as well as large quantities of coke and synthetic rutile which are blown over from the chlorinator. This material is referred to as the chlorinator waste.
The chlorinator waste is subsequently disposed of by whatever means are most acceptable from an environmental point of view. Techniques include direct dumping in mineshafts, blending into concrete for low-strength marine applications and wet chemical treatment to produce iron oxide and HCl (or NaCl with usage of caustic soda). In some pigment plants the waste is currently disposed of by mixing it with CaO and water which react with the chlorides to form oxides and CaCl
2
. The oxides and other solids are thickened, de-watered and returned to the mine. The CaCl
2
liquor is discharged into the ocean.
These forms of disposal mean that valuable chlorine present as chlorides in the chlorinator waste is not recovered. These options are also undesirable because they are either environmentally sensitive or costly in terms of consumables and generally call for minimisation of iron chloride production. For this reason, rutile is the feedstock of choice for chlorination. It contains less iron than ilmenite. In commercial processes, therefore, ilmenite is first converted to synthetic Futile by removing substantially all of the iron by appropriate pre-treatment processes and the synthetic rutile is subsequently used in the chlorination process.
An alternative to simply disposing of the chlorinator waste is to react the waste directly with oxygen to form oxides and recover the chlorine as shown schematically in FIG.
1
. The oxides generated could be useful as landfill or smelter feed, or returned to the mine and the chlorine recycled to the chlorinator. Such a dechlorination process has the potential to reduce the cost of pigment production by reducing the quantity of fresh chlorine required, and by significantly reducing the consumption of water.
The chlorination industry has a long-standing need for a process which can convert iron chloride into chlorine and iron oxide. In the production of titanium tetrachloride the chlorine could be directly recycled to the chlorinator, thereby decreasing the need for chlorine-make-up. For such a process to be effective, the iron oxide produced would need to be sufficiently free of chlorine to allow disposal as landfill or smelter feed with little or no further treatment. The availability of such a process would create the potential for processes such as:
Direct chlorination of ilmenite in the case of “clean” (low Ca, Mg, Mn, Al) deposits. This simultaneously obviates the need for synthetic rutile production and offers a solution to the iron chloride disposal problem.
Ilmenite conversion to synthetic rutile by partial chlorination with subsequent alkali removal in the case of dirty (high Ca, Mg and/or Mn) deposits. This would present an alternative to more conventional methods of impurity removal by reduction and leaching and would lead to reduced waste disposal requirements.
A general waste disposal route for undesired iron chloride and other chlorides which yields chlorine for direct recycle to chlorination systems. This includes the use of skid mounted plants to process accumulated iron chloride waste at existing chlorinator sites, particularly in Europe and USA.
PRIOR ART
The proposed technologies for chlorine recovery, as disclosed in the literature, are not sufficiently selective and/or have unattractive scale-up features.
The Du Pont recirculating fluidised bed approach (U.S. Pat. Nos. 3,793,444, 4,144,316 and 4,174,381) claims greater than 95% chlorine removal, leaving a nominal 3-5% leachable chloride in the solid phase and rendering it unsuitable for direct disposal.
The Mitsubishi vapour-phase approach (S Fukushima and Y. Sugawara, Light metals, AIME 1974), claims 90% chlorine removal and in addition, has perceived scale-up limitations.
The Mineral Process approach (U.S. Pat. No. 4,140,746) comprises partial dechlorination of ferric chloride to ferrous chloride in the presence of a reducing agent such as sulfur or chlorine polysulfides to produce a chloride compound in the first step. In the second step the ferrous chloride is oxidised to ferric chloride and ferric oxide and the ferric chloride is recycled to the first step. The chlorine values were recovered as compounds containing chlorine but not as chlorine gas.
SCM Chemical's U.S. Pat. No. 4,624,843 (inventor M Robinson, 25 Nov. 1986) refers to control of the proportions of the iron chloride and carbon in the blown over material from the chlorinator by controlling the iron oxide concentration in the feed to the chlorinator. The iron oxide is claimed to be introduced through the addition of ilmenite and/or rutile slag. The chlorinator is therefore being used as a selective chlorinator or beneficiator to upgrade other Ti bearing materials. The blown over carbon is controlled between 7.5 and 20% carbon based on the carbon plus iron chloride only, the blow over of rutile is not specifically mentioned. It is claimed that the iron chloride is formed as ferric chloride under the conditions used in the selective chlorinator.
The dechlorinator claimed in U.S. Pat. No. 4,624,843 is based on the introduction of more than one oxygen stream into the reactor. A first stream introduced at the base of the fluidised bed is intended to react with the carbon content of the feed and maintain the bed temperature at between 500 and 1050° C., “preferably at least at 600° C.”. This step seems to be intended to vaporise the ferric chloride although the patent is not very clear on this aspect. A second or more oxygen streams, which appear to need preheating, are introduced above the bed to react with the ferric chloride vapour.
With the dechlorination reaction occurring in the gas phase above the bed between ferric chloride vapour and oxygen, the conversions are unlikely to be complete because of residence time constraints. This implies that iron chloride recycle levels will be significant. Moreover, the iron oxide will be produced as fine dust which increase the probability of accretion formation at the bed exit.
We have discovered that waste obtained from typical chlorinators a) the iron is present as ferrous chloride, b) the conversion of ferrous to ferric chloride is reasonably rapid during dechlorination and c) the conversion of chloride to oxide is helped by using a fluidised bed of particles. The need for the type of pseudo multi-stage operation claimed in U.

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