Low-residue high-extraction production of sodium dichromate

The invention relates to a process for the low-waste production of sodium dichromate from the mineral chromite with simultaneous recovery of low-carbon ferrochromium.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for the low-waste production of sodium 
dichromate from the mineral chromite with simultaneous recovery of 
low-carbon ferrochromium. 
Of the various minerals which contain chromium, only the chromium spinels, 
particularly chromite (chrome ironstone, idealized: FeCr.sub.2 O.sub.4), 
are of economic importance. 
Sodium dichromate is by far the most important starting material for the 
production of chromium compounds. 
Accordingly, the conversion of chromite into sodium dichromate is the 
crucial step from the minerals to the chromium chemicals with their broad 
range of applications. 
2. Description of the Prior Art 
The only process carried out on an industrial scale is based on the 
alkaline digestion of chromite with soda or sodium hydroxide and air in 
the presence of a leaning agent. This process which is described in detail 
elsewhere is attended by two serious disadvantages, namely: 
a) The chromium is never completely dissolved out, the yield reaching just 
under 90% of the original content of Cr.sub.2 O.sub.3 ; 4 to 6% Cr.sub.2 
O.sub.3 remain in the residue. 
b) The elements accompanying the chromium as oxides and hydroxides form the 
residue which has to be discarded and disposed of. The principal 
constituent of the residue is iron oxide although it cannot be put to any 
further use in this impure form. The chromate obstinately remaining 
therein has to be rendered inert by an aftertreatment. 
c) The digestion process is very slow so that the starting materials have 
to be very finely ground to achieve economically acceptable reaction 
times. 
d) The excess of alkaline digesting agent, for example sodium carbonate or 
sodium hydroxide, has to be limited to minimize digestion of the 
approximately 5 to 28% aluminum oxide present in the chromite. The 
quantities of valuable alkali used for the digestion of aluminium oxide 
are not only lost, their presence in the sodium chromate solution produced 
from the digested material can also lead to serious problems in the 
further course of the process. The aluminate passing into solution despite 
the limitation of the quantities of alkaline digesting agent has to be 
precipitated with acidic agents, preferably with dichromate solution, 
during the actual dissolution process. 
More recently proposed processes, for example oxidation in the melt, have 
done little to eliminate these disadvantages. In addition, they impose 
exacting demands on the furnace material so that these problems have never 
been completely solved. 
The accumulation of large quantities of residue also cannot be avoided by 
digestion with potassium hydroxide or potassium carbonate; to the 
contrary, voluminous aluminum hydroxide precipitation sludges are 
obtained. 
The digestion of chromium ore with acid does not exceed 90% either. Iron 
and aluminum accumulate in the form of impure and hence worthless sulfates 
or ammonium double sulfates. 
The chloridizing of chromium ore to form basically separable Fe(III) 
chloride and chromyl chloride is accompanied by extensive chloridizing of 
the secondary constituents and hence is a burden at the working up stage 
due to the accumulation of solutions of the metal chlorides. 
Numerous attempts have also been made to use ferrochromium for the 
production of chromium chemicals by oxidation with air and with electrical 
current or chlorine or sulfuric acid. Despite a possible increase in the 
yield of chromium to 96% of the theoretical, none of these processes is 
capable of easing or even solving the problem of waste, particularly iron 
in the form of worthless iron hydroxides. 
The only industrially practiced method mentioned above for the production 
of sodium dichromate as the most important starting material for all 
chromium chemicals essentially comprises three stages, namely: 
oxidizing digestion of chromium ore or chromium ore concentrate under 
alkaline conditions 
leaching out the sodium chromate formed 
converting the sodium monochromate into dichromate by acidification of the 
solution. 
In addition to chromite and sodium alkalis, particularly sodium carbonate, 
substances which are intended to maintain the porosity of the furnace 
charge during the digestion process ("leaning agents") are added to the 
burden. Porosity is necessary for forming a sufficiently large surface for 
the reaction with oxygen. The yield of chromium where chromite is used is 
between 74 and 90% of the chromium present in the chromium ore, depending 
on its composition. 
The soluble monochromate is removed by filtration, mainly through drum 
filters, after cooling and leaching at a pH value adjusted by addition of 
acids or dichromate solution. The insoluble residue is repeatedly leached 
to reduce the content of Cr(VI). Part of the residue may be dried so that 
it may be reintroduced as leaning agent into the furnace burden. 
The residue remaining is subjected to a reduction process in order to 
insolubilize the Cr(VI) remaining. This is done by treatment with reducing 
agents, for example Fe(II) sulfate (see also Ullmann's Encyclopedia of 
Industrial Chemistry, 5th Edition, Weinheim, 1986). 
MgO, Fe.sub.2 O.sub.3 and Al.sub.2 O.sub.3 participate to a very limited 
extent, if at all, in the digestion process. Nevertheless, they are 
passed through the furnace, 
expensively separated and 
finally aftertreated and disposed of as fine-particle, reactive, oxidized 
water-rich sludge containing residual chromium. 
The SiO.sub.2 introduced and the aluminum oxide partly take place in the 
digestion process by reaction with the alkali carbonate (soda consumption 
by binding into the residue as aluminosilicate). 
Accordingly, the problem addressed by the present invention was to provide 
a process for the production of sodium dichromate which 
1. would enable the chromium present in the chromite to be almost 
completely reacted, 
2. would reduce the input of accompanying elements into the digestion 
process and, hence, reduce or even totally avoid the amount of residue for 
disposal, 
3. and at the same time would put the iron present in the chromite to some 
use, 
4. would largely reduce or even totally eliminate the expense involved in 
keeping troublesome impurities, such as aluminium, away from the alkaline 
sodium chromate solution. 
SUMMARY OF THE INVENTION 
The problem stated above is solved by a process for the low-waste, 
high-extraction production of sodium dichromate from chromite, comprising 
in a first stage reducing chromite to an iron/chromium alloy, in a second 
stage reacting the iron/chromium alloy with oxygen to form a low-carbon 
ferrochromium melt and a slag insoluble therein, which slag is rich in 
chromium oxide (&gt;80% Cr.sub.2 O.sub.3), reacting the slag with sodium 
carbonate, circulating leaning agent and oxygen to form sodium chromate, 
and converting the sodium chromate into sodium dichromate. 
Any chromates ranging from approximately 40% to approximately 60% in their 
Cr.sub.2 O.sub.3 content may be used as starting material for the process 
according to the invention. Both lump ore and concentrate may be used. 
In a first step, the chromium ores mentioned are reduced to ferrochromium 
and a slag in typical furnaces using coke or coal and electrical current. 
Processes for the reduction of lump chrome ore are well documented (see 
Ullmann, Vol. A7, pages 48-58). Numerous furnaces have recently been 
developed for processing concentrate. These processes either operate with 
plasma burners or pre-reduce the chromite in a rotating tube. 
Reduction is preferably carried out in such a way that the silicon content 
in the ferrochromium formed is extremely low, preferably below 3%. This 
can be achieved in known manner by low reduction temperatures and by 
producing a low-viscosity, low-melting slag. 
This reduction step gives two phases at temperatures of approximately 
1500.degree. C. or higher: 
A slag which contains the secondary constituents MgO and Al.sub.2 O.sub.3 
present in the chromite and which may be conditioned for use as building 
materials or fillers either as such or by addition of Ca-containing 
substances or quartzite. 
The second phase is the metallic phase, the ferrochromium, which contains 
the largest part of the chromium and iron bound in the spinel. In 
addition, the ferrochromium contains residual carbon (5 to 7%) and 
fractions of Si through reduction of silicates present in the ore. 
The slag and the liquid ferrochromium are separated. The largely 
chrome-free slag is used as a building material or as an inert, 
non-oxidizable and non-leachable, low-volume filler. 
In a second step, the ferrochromium is fed to a converter in which the melt 
is freshened with oxygen or oxygen-containing gases. The oxygen may be 
introduced by lances or by variously arranged tuyeres or by a combination 
of both methods. 
In the treatment of a melt of the type in question with oxygen or 
oxygen-containing gases, the carbon bound as carbide to the chromium and 
the silicon preferably undergo oxidation in an initial phase. With 
increasing decarburization, chromium is also oxidized. In general, 2 to 
15% of the chromium present in the ferrochromium are oxidized in this 
step, although basically the chromium may even be oxidized to a greater 
extent during freshening. In general, freshening is carried out after 
addition of additives which bind silicon dioxide, such as lime or 
dolomite. In the process according to the invention, the addition is 
preferably limited to no more than twice the stoichiometric quantity, 
based on the silicon content of the ferrochromium. The oxides formed 
accumulate in the slag on account of their non-metallic character. On 
completion of the freshening step, two phases are again present, namely: 
a slag containing increased levels of oxidized chromium 
and a low-carbon residual metal phase which is free from silicon and which 
consists: essentially of iron and the non-oxidized chromium (low- or 
medium-carbon ferrochromium). 
The two phases are separated. This may be done by tilting the converter or 
by tapping via a runner and skimmer bar. More slag rich in chromium oxide 
can be obtained from the waste gas of the freshening step through the 
precipitation of entrained solids from the gas stream. 
The metal phase is put to another use in the steel industry while it is 
still liquid or after it has solidified. According to the invention, the 
slag with its high Cr.sub.2 O.sub.3 content is subjected to oxidizing 
digestion under alkaline conditions for the production of sodium chromate. 
The freshening of the ferrochromium melt may of course also be carried out 
in two stages in order, for example, to reduce the silicon content by 
oxidation in a first freshening stage and to separate a slag rich in 
silicon dioxide (a suitable starting material for the production of 
ferrosilicochromium) and to produce the slag rich in chromium oxide 
suitable for the process according to the invention in a second freshening 
stage. 
The reaction of the slag with sodium-based alkaline digesting agent, 
particularly sodium carbonate, may take place in accordance with the 
following scheme: 
EQU Cr.sub.2 O.sub.3 +2 Na.sub.2 CO.sub.3 +3/2 O.sub.2 .fwdarw.2 Na.sub.2 
CrO.sub.4 +2 CO.sub.2 
The reaction of Cr slag and sodium carbonate may take place, for example, 
in exactly the stoichiometric ratio. Where a slag containing 90% Cr.sub.2 
O.sub.3 is used, 100 parts slag have to be reacted with 125 parts sodium 
carbonate in the presence of approximately 380 parts inert material 
(leaning agent). Sodium carbonate may of course also be used in more than 
or less than the stoichiometric quantity. An excess generally results in 
acceleration of the digestion process. 
The sodium chromate produced is extracted from the clinker by leaching with 
water. The alkaline sodium-based digesting agent used in excess and sodium 
silicate dissolve together with the sodium chromate. This aqueous solution 
may be removed without pH adjustment, i.e. without acids or dichromate 
solution having to be added to precipitate aluminate. In this way, all the 
sodium usefully remains in the solution. Silica is precipitated therefrom 
by addition of acid, preferably carbonic acid, to the filtrate for pH 
adjustment to 4-8. 
Carbonic acid is particularly preferred for the pH reduction because, in 
the event of subsequent conversion of the sodium chromate into sodium 
dichromate, all the sodium used can be recovered as sodium bicarbonate by 
pressure acidification with carbonic acid. The chromium-containing silica 
removed may be used as a raw material in the production of 
ferrosilicochromium. The chromate-containing filtrate is further processed 
in known manner to sodium dichromate whereas the residue may be reused 
without further careful washing as inert material/leaning agent in the 
oxidizing alkaline digestion either directly or after drying, for example 
with hot waste gases, but preferably by mixing with hot chromium oxide 
slag from the freshening process. It is only periodically that part of the 
residue has to be removed commensurate with the amount of substances which 
are introduced with the slag of the freshening process and which are not 
Cr.sub.2 O.sub.3, and either returned to the ferrochromium production 
process or leached with carbonic acid for the selective removal of calcium 
carbonate or used for another purpose or disposed of after washing out 
and, optionally, reduction. 
This partial removal may also take place continuously from a small 
sidestream. In all events, the quantity of fine-particle, reactive 
digestion residue to be removed is drastically reduced in relation to the 
direct use of chromite. 
Another way of reacting slag and sodium carbonate without having to use 
leaning agent brought in from outside is to replace the leaning agent by 
Cr slag. In this case, sodium carbonate is only added to the burden in 
such a quantity that only pan of the slag reacts to form sodium chromate 
and the solid-to-melt ratio in the furnace remains intact in contrast to 
separate leaning, for example with Fe.sub.2 O.sub.3. 
For example, 500 parts of a slag containing 90% Cr.sub.2 O.sub.3 are mixed 
with 140 parts sodium carbonate and reacted in an excess of oxygen at 
approximately 1070.degree. C. in accordance with the following scheme: 
EQU Cr.sub.2 O.sub.3 +2 Na.sub.2 CO.sub.3 +3/2 O.sub.2 .fwdarw.2 Na.sub.2 
CrO.sub.4 +2 CO.sub.2 
The advantage of leaning the slag with more slag lies in the increased 
availability of chromium minerals in the burden. By comparison with the 
leaning of chromium ore with an inert material introduced from outside, 
much more chromium is available to the sodium carbonate added in the case 
of the slag. This results in a considerably higher reaction rate of the 
sodium carbonate with the chromium in the slag and hence in the complete 
reaction of the sodium carbonate. 
Where chromium oxide slag is used, pressure, temperature, gas phase 
composition and fineness of grinding have to meet far less exacting 
requirements than where chromite is used in the oxidizing alkaline 
digestion. 
Both pure oxygen and air and enriched oxygen and also mixtures thereof with 
fuel gases may be used as the oxygen source. These various gas mixtures 
may be used both in directly heated rotating tube furnaces and annular 
hearth furnaces and in indirectly heated tube furnaces and also in 
fluidized bed furnaces. In the last case, the mixture of slag rich in 
chromium oxide, alkaline digesting agent and, optionally, leaning agent to 
be reacted with the oxygen has to be convened by compacting, for example 
by pelleting or granulation, into a form suitable for the fluidized bed, 
solutions containing sodium-alkaline components, such as sodium hydroxide, 
or binders, such as molasses solution or a phenol/formaldehyde mixture, 
being added for paste formation and combustible solid components, such as 
coal dust, being added to increase porosity. Whereas the directly heated 
digestion units are preferably operated under normal pressure, methods of 
operation involving elevated pressure of the oxygen-containing gas are 
advisable for indirectly heated digestion mixtures. 
In general terms, therefore, the process according to the invention is 
carried out as follows: 
1. Production of a ferrochromium melt by reduction of chromium ore in lump 
form or as concentrate 
2. The melt is treated with oxygen (so-called freshening) in a suitable 
vessel (converter). This treatment may be carried out in two ways: 
blowing the oxygen onto the melt with a lance 
injecting the oxygen into the melt through nozzles arranged at the side or 
at the bottom of the vessel 
or by a combination of both methods. 
In both cases, the oxygen may be mixed with fuel gases or inert gases. 
3. The treatment with oxygen is continued until a certain carbon and/or 
chromium content is reached in the metal bath. 
4. On completion of freshening, the slag and the metal phase are removed 
from the treatment vessel. 
5. The metal phase recovered, which consists of low-carbon ferrochromium, 
is used as a starting material for stainless steel. 
6. The slag is subjected to oxidizing digestion under alkaline conditions 
for the production of sodium chromate; the digestion step may be carried 
out at the same time as freshening or after cooling and grinding. 
7. The aqueous sodium chromate solution produced by leaching of the 
digested material with water or an aqueous solution is converted into 
sodium dichromate solution by acidification and, if desired, the sodium 
dichromate is isolated therefrom. 
The process according to the invention affords the following advantages 
over the prior art: 
1. The separation of Cr and Fe in the converter by freshening with oxygen 
enables chromium oxide slag with high Cr.sub.2 O.sub.3 contents of up to 
100% to be produced. 
2. Through the use of a ferrochromium melt as starting material for a 
synthetic raw material for the production of sodium chromate, the input of 
the inert constituents MgO, Al.sub.2 O.sub.3, FeO and Fe.sub.2 O.sub.3 
into the oxidizing alkaline digestion process is considerably reduced. 
3. In the production of chromium oxide slag by the described method, the 
iron does not accumulate as oxide made worthless by impurities, but 
instead in the form of low-carbon ferrochromium. This alloy is in great 
demand as a raw material for the production of stainless steel. 
4. Since, in the alkaline digestion of the slag rich in chromium oxide, the 
quantity of inert material (MgO, Al.sub.2 O.sub.3, FeO and Fe.sub.2 
O.sub.3) reintroduced is small compared with the quantity of circulated 
leaning agent and since only that quantity of inert material introduced by 
the slag rich in chromium oxide has to be removed from the circuit, both 
the leaning agent and the inert material pass repeatedly through the 
digestion process. This multiple digestion of the same material results in 
a high extraction of Cr from the raw material used and hence in low 
Cr.sub.2 O.sub.3 contents in the small quantity of residue removed from 
the circuit. 
5. Whereas, where chrome ore is used, the Cr.sub.2 O.sub.3 to be digested 
is bound into the thermally and chemically resistant structure of the 
spinel which can only be digested under the most rigorous conditions, the 
chromium oxide slag produced in the described manner can surprisingly be 
digested with far less effort. This is also beneficial to the complete 
reaction of all the chromium present in the chromium oxide slag to sodium 
chromate. 
6. Surprisingly, however, the time required for digestion of the 
chromium-containing starting material can also be considerably shortened 
by replacing chromite with the chromium oxide slag typical of the process 
according to the invention in the oxidizing alkaline digestion process. 
This surprising effect provides for considerably better volume/time 
utilization of the digestion furnaces and is technically advantageous. 
The easier digestability of the chromium oxide slag typical of the process 
according to the invention may of course also be utilized to reduce the 
digestion temperature or to lower the oxygen partial pressure in the 
furnace atmosphere--in either case by comparison with the digestion of 
chromite. However, the preferred measure is the increased conversion. 
7. The absence of aluminum from the digestion mixture enables the roasted 
material to be subjected to alkaline leaching so that all the sodium may 
readily be recovered as sodium bicarbonate.

It will be understood that the specification and examples are illustrative 
but not limitative of the present invention and that other embodiments 
within the spirit and scope of the invention will suggest themselves to 
those skilled in the art.