Reduction of ferric chloride

The invention relates to a method for the reduction of ferric chloride to produce ferrous chloride. The method comprises using gaseous sulphur or a gaseous sulphur chloride in which the atomic ratio of sulphur to chlorine is more than 1:1 as the reducing agent. The reaction is conveniently performed in a fluidised bed. According to a particular aspect, the ferric chloride reduction forms part of a process for the recovery of chlorine values from iron chloride by-produced by industrial processes such as the chlorination of a titaniferrous or aluminous material.

This invention relates to a method for the reduction of ferric chloride. 
More particularly this invention relates to a method for the partial 
dechlorination of ferric chloride to ferrous chloride in the presence of 
one or more suitable reducing agents. 
According to a particular aspect of this invention the partial 
dechlorination of ferric chloride is a step in a method for the recovery 
of the chlorine values from iron chloride obtained as a by-product, for 
example, from the chlorination of a titaniferrous material containing more 
than 5% by weight iron oxide such as ilmenite, or obtained as a by-product 
from the chlorination of an aluminous material, such as bauxite. 
U.S. Pat. No. 4,140,746 describes a process for the recovery of the 
chlorine values from iron chloride obtained as a by-product from the 
chlorination of a titaniferrous material containing more than 5% by weight 
iron oxide which comprises the steps of: 
(a) subjecting ferric chloride to partial dechlorination in the presence of 
one or more suitable reducing agents to produce ferrous chloride and a 
chloride compound; 
(b) subjecting ferrous chloride to an oxidation reaction in the presence of 
oxygen or a molecular oxygen-containing gas at a temperature between 
300.degree. C. and 1200.degree. C. to produce ferric chloride and ferric 
oxide; and 
(c) recycling the resulting ferric chloride to the partial dechlorination 
step (a). 
Similar methods for recovering the chlorine values from iron chloride are 
disclosed in U.S. patent application Ser. No. 37,718, filed Aug. 10, 1979 
relating to iron chloride by-produced in the chlorination of an aluminous 
material e.g. bauxite. 
In those processes, a suitable reducing agent for stage (a) is defined as 
one which meets the two following conditions: first that it is effective 
in dechlorinating ferric chloride to ferrous chloride; second, that in 
reaction with ferric chloride, it produces a chloride compound which, 
directly or after further processing, is either suitable for recycle to 
the chlorination process (if appropriate) or has other industrial utility. 
One such suitable reducing agent described in the U.S. Pat. No. 4,140,746 
and U.S. patent application Ser. No. 37,718, filed Aug. 10, 1979 is 
sulphur, and the method of using it was summarised in the following 
equation (1): 
EQU Fe.sub.2 Cl.sub.6 (s)+2S(l).fwdarw.FeCl.sub.2 (s)+S.sub.2 Cl.sub.2 (g) (1) 
(where (s) represents solid, (l) represents liquid and (g) represents gas). 
Thus in equation (1) liquid sulphur is reacted with solid ferric chloride 
to produce solid ferrous chloride and gaseous sulphur monochloride. 
The method of using sulphur as the reducing agent for ferric chloride, 
which is summarised in equation (1), has been established experimentally 
as an effective and successful procedure. However, it has two major 
drawbacks. First, it requires that the ferric chloride produced in the 
oxidation stage (b) is condensed from the gas stream, which is a costly 
and elaborate procedure. Second, the equipment required for contacting the 
mixing liquid sulphur with solid ferric chloride is, again, costly and 
elaborate. 
It has now surprisingly been found that the reaction represented by 
equation (1) can be carried out much more conveniently and advantageously 
using gaseous sulphur or a gaseous sulphur chloride in which the atomic 
ratio of the sulphur to chlorine is more than 1:1. 
The fact that this reaction can be successfully carried out is surprising 
because the literature suggests that sulphur monochloride is increasingly 
decomposed into sulphur and chlorine as its temperature rises, until at 
the boiling point of sulphur (444.degree. C.) it is fully decomposed. Thus 
the use of gaseous sulphur or gaseous sulphur chloride in which the atomic 
ratio of sulphur to chlorine is more than 1:1 to produce sulphur 
monochloride and ferrous chloride by reaction with ferric chloride did not 
appear promising. 
Nevertheless, it has been found that sulphur or gaseous sulphur chloride in 
which the atomic ratio of sulphur to chlorine is more than 1:1 is an 
effective reducing agent for gaseous ferric chloride, particularly when 
used in a gas fluidised bed of solid ferrous chloride. 
Thus, the present invention provides a method of reducing ferric chloride 
to ferrous chloride wherein ferric chloride in the gaseous or solid state 
is partially dechlorinated in the presence of gaseous sulphur or a gaseous 
sulphur chloride in which the atomic ratio of sulphur to chlorine is more 
than 1:1 to form ferrous chloride in the solid state in accordance with 
the following reaction equations: 
EQU Fe.sub.2 Cl.sub.6 (g/s)+S.sub.2 (g).fwdarw.2FeCl.sub.2 (s)+S.sub.2 Cl.sub.2 
(g) (2) 
EQU 21/2Fe.sub.2 Cl.sub.6 (g/s)+S.sub.5 Cl.sub.2 (g).fwdarw.5FeCl.sub.2 
(s)+21/2S.sub.2 Cl.sub.2 (g) (3) 
In these equations, the second term in equation (3) represents an example 
of a sulphur chloride on which the atomic ratio of sulphur to chlorine is 
more than 1:1. In practice a gaseous mixture of S.sub.2 Cl.sub.2 and S may 
be employed as the sulphur chloride. 
The method according to the present invention is particularly suited to be 
the partial dechlorination step (a) of a method for the recovery of the 
chlorine values from iron chloride. 
Thus the ferric chloride input to the method summarised in equations (2) 
and (3) preferably derives from the processes disclosed in U.S. Pat. No. 
4,140,746 and U.S. patent application Ser. No. 37,718, filed Aug. 10, 
1979. However, the present invention is not limited to ferric chloride 
derived from any particular source. 
The reaction between gaseous sulphur/gaseous sulphur chloride and ferric 
chloride is preferably carried out in a fluidised bed of (product) ferrous 
chloride. The bed temperature should be at least sufficient to maintain 
the sulphur or sulphur chloride in the gas phase. The actual minimum 
temperature for this purpose will vary with the composition of the source 
of sulphur values and with the quantity of any inert gases present in the 
reactor. Thus the overall temperature limits for the reaction are between 
120.degree. C. and 650.degree. C., preferably between 200.degree. C. and 
450.degree. C. and more particularly between 300.degree. C. and 
450.degree. C. 
Alternatively, a stirred bed reactor or a rotary kiln could be used. The 
reaction is preferably carried out continuously. 
The sulphur values, either as elemental sulphur or as a sulphur chloride in 
which the atomic ratio of sulphur to chlorine is more than 1:1, are 
preferably introduced to the reactor as a gas, but may also be introduced 
as a liquid in which case the liquid is immediately gasified by contact 
with the hot reaction bed prior to reaction of the sulphur chloride with 
the ferric chloride. 
The ferric chloride is preferably introduced to the reactor as a gas, but 
may also be introduced as a solid. 
The off-gas from the reactor is preferably condensed to a liquid and 
centrifuged to remove solid particles (ferric and ferrous chloride). The 
resulting sulphur chloride, which is predominantly S.sub.2 Cl.sub.2, may 
then be treated by various methods depending on the industrial 
circumstances to the plant. If the ferric chloride feed to the process 
according to the invention has been obtained by a sulpho-chlorination 
process, for example a bauxite sulpho-chlorination process, it is 
preferred to recycle the S.sub.2 Cl.sub.2 to the sulpho-chlorination 
process. 
However, if the ferric chloride feed has been obtained by a 
carbo-chlorination process, for example an ilmenite carbo-chlorination, it 
is preferred to pass the S.sub.2 Cl.sub.2 to a fractional distillation 
process. By fractional distillation, chlorine is recovered as the overhead 
product and a sulphur chloride with between 60% and 80% atomic percent 
sulphur is obtained as the bottom product which can then be recycled to 
the reduction process according to the invention. Alternatively, the 
S.sub.2 Cl.sub.2 obtained from the off-gas may be reacted with carbon 
disulphide to produce carbon tetrachloride which can be used either for 
recycle to the carbo-chlorination reaction or for sale to other industrial 
processes, with the by-product sulphur being recycled to the reduction 
reaction: or the S.sub.2 Cl.sub.2 product may be reacted with carbon 
monoxide to produce phosgene which may be recycled to the 
carbo-chlorination reaction or passed for sale to other industrial 
processes, with the by-product sulphur or sulphur chloride being recycled 
to the reduction reaction. 
Where fractional distillation is employed, it is preferred to perform the 
distillation in two stages. In a first stage S.sub.2 Cl.sub.2 is fed to a 
first distillation column which operates at atmospheric pressure with an 
overhead temperature of between 20.degree. C. and 60.degree. C. and a 
bottom temperature of between 140.degree. C. and 180.degree. C. such that 
there are produced a sulphur-rich bottom product and a chlorine-rich 
overhead product. The bottom product which preferably contains between 60 
and 80% atomic percent of sulphur is recycled for the reduction of ferric 
chloride. in a second stage, the chlorine-rich sulphur chloride overhead 
product from the first distillation column is fed to a second distillation 
column which is operated at a pressure of about 10 atmospheres with an 
overhead temperature of between 20.degree. C. and 60.degree. C. and a 
bottom temperature of between 160.degree. C. and 220.degree. C. Pure 
elemental chlorine is produced as the overhead product and a sulphur 
chloride in which the atomic ratio of sulphur to chlorine is approximately 
equal is produced as the bottom product. The chlorine overhead product 
can, for example, be recycled to the chlorination process from which the 
ferric chloride was obtained and the sulphur chloride bottom product may 
be recycled to the first distillation column. 
Instead of the two-stage distillation process, just described, a 
single-stage distillation process may be employed although this is more 
energy intensive. In the case of a single-stage distillation process, it 
is preferred to use an elevated pressure of about 10 atmospheres, with an 
overhead temperature of between 20.degree. C. and 60.degree. C. and a 
bottom temperature between 200.degree. C. and 240.degree. C. to produce 
the same products as in the two-stage process. 
The ferrous chloride bed overflow from the reduction reactor is preferably 
converted to ferric chloride and ferric oxide by reaction with a 
controlled quantity of oxygen or air, as disclosed for stage (b) of the 
process, described in United States Patent No. 4,140,746 and in U.S. 
patent application Ser. No. 37,718, filed Aug. 10, 1979 relating to the 
recovery of chlorine values from iron chloride derived from various 
sources. The resulting iron chloride is preferably recycled to the 
reduction reaction which is the subject of this invention.

The invention is further illustrated by the following Examples: 
EXAMPLE 1 
A 100 mm diameter bed of ferrous chloride was fluidised at 380.degree. C. 
using a gas mixture analysing: 
4 l/min ferric chloride gas 
2 l/min nitrogen 
4 l/min sulphur chlorine mixture 
(atomic ratio sulphur to chlorine:--2:1) 
The ferric chloride and sulphur/chlorine mixture entered the bed through 
separate orifices and the off-gas was found to have reacted completely to 
give a sulphur monochloride off-gas with negligible residual ferric 
chloride. The ferrous chloride reaction product reported to the reaction 
bed. 
EXAMPLE 2 
The distillation of S.sub.2 Cl.sub.2 produced according to the equation (3) 
was carried out in a 80 mm diameter Inconel column, which was used for 
both stages as set out below. For convenience, the quantities of feed 
material used in each stage were not matched since the critical constraint 
was the heat load on the condenser. Samples were collected for analysis 
after three hours running time, and the results are given below. The 
theoretical heats (i.e. the heat input at the bottom and the heat load on 
the condensers) were up to 50% greater in practice, at the feed rates 
specified. 
__________________________________________________________________________ 
Stage 1 Stage 2 
__________________________________________________________________________ 
Pressure: Atmospheric 10 Atmospheres 
Packing: 1.06 meters of Knitmesh 
1.6 meters of Knitmesh 
Multifil packing (equiv- 
Multifil packing (equiv- 
alent to 20 theoretical 
alent to 30 theoretical 
plates) plates) 
Location Center of Column 
60% of packing below 
of Feed and 40% above. 
Plate: 
Theoretical 
47 Kcal/mole/feed 
14 Kcal/mole/feed 
Heat Input 
at Bottom: 
Heat Load on 
Condenser: 
34 Kcal/mole/feed 
8.5 Kcal/mole/feed 
Feed Analysis: 
100% S.sub.2 Cl.sub.2 
90 mole % SCl.sub.2 10 mole % 
S.sub.2 Cl.sub.2 
Top Product 
Analysis: 90 mole % SCl.sub.2 10 mole % 
100% Cl.sub.2 (99.7 .+-. 0.3%) 
Bottom Product 
Analysis: 80 mole % S 20 mole % 
100% S.sub.2 Cl.sub.2 (98 .+-. 2%) 
S.sub.2 Cl.sub.2 
Reflux Ratio: 
5 3 
Feed Rate & 
Temperature: 
27 moles/hour at 20.degree. C. 
170 moles/hour 20.degree. C. 
Top Production 
Rate & Temp. 
22 moles/hour at 50.degree. C. 
76 moles/hour 30.degree. C. 
Bottom Production 
Rate & Temp. 
25 moles/hour at 160.degree. C. 
93 moles/hour 210.degree. C. 
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