Catalyzed electrochemical gasification of carbonaceous materials at anode and electrowinning of metals at cathode

The electrochemical gasification reaction of carbonaceous materials by anodic oxidation in an aqueous acidic electrolyte to produce oxides of carbon at the anode and metallic elements at the cathode of an electrolysis cell is catalyzed by the use of an iron catalyst.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the use of an iron catalyst in the 
electrochemical gasification of carbonaceous materials in an aqueous 
acidic electrolyte. 
2. Prior Art 
It is known in the art that carbonaceous materials when mixed with an 
aqueous acidic electrolyte in an electrochemical cell through which a 
direct current passes electrochemically react oxidizing the carbonaceous 
material to oxides of carbon at the anode and reducing water to hydrogen 
or metallic ions to metallic elements at the cathode. 
U.S. Pat. No. 4,268,363 teaches the electrochemical gasification of 
carbonaceous materials by anodic oxidation which produces oxides of carbon 
at the anode and hydrogen or metallic elements at the cathode of an 
electrolysis cell. 
U.S. Pat. No. 4,226,683 teaches the method of producing hydrogen by 
reacting coal or carbon dust with hot water retained as water by 
superatmospheric pressure. The pressure is controlled by the use of an 
inert dielectric liquid which washes the electrodes and, while doing so, 
depolarizes them by absorption of the gases. 
U.S. Pat. No. 4,233,132 teaches a method wherein electrodes are immersed 
within oil which forms a layer over a quantity of water. When current is 
passed between the electrodes, water is caused to undergo 
electrodecomposition. Gaseous hydrogen is collected in the sealed space 
above the oil-water layers, and the oxygen is believed to react with the 
constituents in the oil layer. 
As acknowledged in U.S. Pat. No. 4,226,683, the principal problem in the 
past use of this principle for commercial production of hydrogen, was the 
slow rate of the electrochemical reaction of coal or carbon and water. It 
has now been found that iron, when added to an aqueous acidic electrolyte 
containing the carbonaceous material, and preferably iron in the +3 
valence state, catalyzes the rate of reaction and assists in obtaining 
more complete oxidation for the electrochemical oxidation of the 
carbonaceous material at the anode thus making the commercial production 
of hydrogen or method of electrowinning commercially feasible. 
SUMMARY OF THE INVENTION 
As described above, it is well known that carbonaceous material such as 
coal can be oxidized at the anode of an electrochemical cell containing an 
aqueous acidic electrolyte with the simultaneous production of oxides of 
carbon at the anode and that this anodic half-cell reaction may be used in 
combination with the cathodic half-cell reaction of electrodeposition of a 
metal M from an aqueous solution of its ions M.sup.m+. For example, 
focusing on the carbon in coal and representing it by C, this anodic 
reaction can be written according to the stoichiometry: 
EQU C.sub.(s) +2H.sub.2 O.sub.(l) .fwdarw.CO.sub.2(g) +4H.sup.+ +4e.sup.-(I) 
in combination with the simultaneous cathodic reaction 
EQU M.sup.m+ +me.sup.- .fwdarw.M (II) 
The net reaction, that is the sum of equations (I) and (II) [for case m=1] 
is: 
EQU C.sub.(s) +2H.sub.2 O.sub.(l) +4M.sup.+ .fwdarw.CO.sub.2(g) 
+4M+4H.sup.+(III) 
In these equations, the symbols (g), (s) and (l) symbolize the gaseous, 
solid and liquid states, respectively. Equation (III), the reaction 
between coal and water, caused by impressing an appropriate potential on a 
suitable electrochemical cell, is what is referred to in U.S. Pat. No. 
4,268,363 as the electrochemical gasification of coal which reference is 
incorporated totally herein by reference. 
In the case of copper, for example, the coal-assisted electrodeposition of 
copper would take the form: 
EQU C.sub.(s) +2H.sub.2 O.sub.(l) +2Cu.sup.2+ .fwdarw.CO.sub.2(g) +2Cu.sub.(s) 
+4H.sup.+ (I) 
A problem with this prior art method is the relatively slow rate of 
reaction and the incomplete combustion of the carbonaceous material at the 
anode. 
It has now been found that the addition of a sufficient amount of iron in 
the elemental, +2 or +3 valence state or mixtures thereof, to the 
carbonaceous material undergoing oxidation in an aqueous electrolyte at 
the anode will increase the rate of reaction of the oxidation process. The 
iron catalyst assists the oxidation of carbonaceous material in going to 
completion and increases the amount of current produced at the anode per 
given operating voltage. 
DETAILED DESCRIPTION 
The carbonaceous materials suitable for use in accordance with the present 
invention include a wide variety of material such as bituminous coal, 
chars made from coal, lignite, peat, active carbons, coke, carbon blacks, 
graphite; wood or other lignocellulosic materials including forest 
products, such as wood waste, wood chips, sawdust, wood dust, bark, 
shavings, wood pellets; including various biomass materials as land or 
marine vegetation or its waste after other processing, including grasses, 
various cuttings, crops and crop wastes, coffee grounds, leaves, straw, 
pits, hulls, shells, stems, husks, cobs and waste materials including 
animal manure; sewage sludge resulting from municipal treatment plants, 
and plastics or the scraps or wastes formed in the production of plastic 
such as polyethylene, cellulose acetate and the like. Thus, it is seen 
that substantially any fuel or waste material whether a liquid, such as 
oil, a gas, such as methane or other hydrocarbon or waste material 
containing carbon, with the exception of CO.sub.2, provides a suitable 
source of carbonaceous material for use according to this invention. 
The particular apparatus used to carry out the electrolytic oxidation of 
carbonaceous materials is not critical. Substantially, the same apparatus 
and techniques that are utilized in the electrolytic deposition of metals 
as well as those described in U.S. Pat. No. 4,268,363, which reference is 
incorporated herein by reference, can be used with the method of this 
invention. Any selection of appropriate changes in use of materials and/or 
technique is well within the skill of those versed in the art to which 
this invention applies. 
The cells described in U.S. Pat. No. 4,268,363 including the use of acidic 
aqueous electrolyes, selection of anode and cathode materials and the 
optional but preferred use of cell membranes to keep the carbonaceous 
material on the anode side are most preferred. 
While the electrode materials described in U.S. Pat. No. 4,268,363 are 
suitable for use in the method of this invention, anode materials which 
were found to perform especially well include a mixture of RuO.sub.2 
/TiO.sub.2 on a Ti substrate and a mixture of IrO.sub.2 /TiO.sub.2 on a Ti 
substrate, which anodes are both commercially available. 
The preferred acidic aqueous electrolytes that can be employed have a pH of 
less than 3 and include solutions of strong acids such as sulfuric acid, 
nitric acid, hydrochloric acid, phosphoric acid and the like. 
While temperatures from above the freezing point of water and greater may 
be used, temperatures of from about 25.degree. C. to 350.degree. C. are 
preferred. Temperatures of from 120.degree. C. to 300.degree. C. are most 
preferred especially when using solid carbonaceous materials such as coal. 
At temperatures below 140.degree. C. the reactivity of solid carbonaceous 
materials such as coal steadily decreases as the electrochemical oxidation 
proceeds. This decreased reactivity is believed to be caused by surface 
oxides building on the surface of the coal which hinders further sustained 
reactivity of the coal. At temperatures of about 140.degree. C. and 
greater, the reactivity of the coal is sustained and no substantial 
decrease is observed. 
Since it is desired to maintain the reaction in a liquid phase, it is of 
course necessary, that at elevated temperatures, the reaction be carried 
out at elevated pressure. Generally, pressures of from about 2 to 400 
atmospheres are satisfactory. 
It has also been found that the addition of the iron catalyst to a solid 
carbonaceous material such as coal will sustain the activity of the coal 
longer at temperatures below 120.degree. C. as compared to systems not 
containing the iron catalyst. Also, the catalytic effects of the iron 
catalyst are more pronounced at the higher temperatures. 
While iron may be used in its elemental state, iron in its +2 and +3 
valence, i.e. ferrous and ferric states, respectively, are preferred. Most 
preferred is the use of iron in the +3 valence state. Thus, inorganic iron 
compounds such as iron oxides, iron carbonate, iron silicates, iron 
sulfide, iron oxide, iron hydroxide, iron halides, iron sulfate, iron 
nitrate, and the like, may be used. Also, various organic iron salts and 
complexes, such as salts of carboxylic acids, e.g., iron acetates, iron 
citrates, iron formates, iron glyconates, and the like, iron cyanide, iron 
chelate compounds, such as chelates with diketones as 2,4-pentanedione, 
iron ethylenediaminetetracetic acid, iron oxalates, and the like. 
While the iron catalyst may be used at a concentration up to the saturation 
point in the aqueous electrolyte, the preferred range of iron catalyst is 
in the range of from 0.04 to 0.5 molar and most preferably from 0.05 to 
0.1 molar concentration. 
While certain carbonaceous material, such as coal, may contain iron as an 
impurity, an iron-containing catalyst from an external source is generally 
required in order to increase the rate of reaction, at least initially to 
acceptable levels for commercial use. The iron catalyst can conceivably be 
generated in-situ by oxidizing sufficient iron-containing coal to generate 
an effective amount of iron catalyst in the electrolyte. 
Of course, essentially iron-free carbonaceous materials, such as carbon 
black, requires an iron catalyst to be added from an external source. 
Thus, in one embodiment of this invention, sufficient iron catalyst is 
added from an external source in order to supply the preferred range of 
catalyst, namely, 0.04 to 0.5 molar. 
In a second embodiment, an effective amount of iron catalyst can be 
generated in-situ by oxidizing sufficient iron-containing coal, albeit 
initially at a slower rate, to supply the preferred range of catalyst. 
The catalyst generated would then be freed from the coal and be able to 
function in a similar manner as externally-supplied iron catalyst. 
In a third embodiment, a combination of externally-supplied iron catalyst 
and in situ generated catalyst can be used to supply the preferred range 
of catalyst, i.e., 0.04 to 0.5 molar. 
The concentration or amount of carbonaceous material present in the 
electrolyte may vary over a wide range depending upon whether it is a 
liquid, solid or gas and depending on particle size, however, the 
preferred range is from about 0.1 gm to 0.7 gm per ml. 
As in the case described in U.S. Pat. No. 4,268,363, it is possible to 
electrowin, electroplate or electrodeposit any element that can be 
cathodically reduced from aqueous solution with simultaneous 
electrochemical anodic oxidation of carbonaceous material. Typical 
metallic elements often deposited in practice from aqueous electrolyte 
include, Cr, Mn, Co, Ni, Pb, Cu, Sn, Zn, Ga, Hg, Cd, Ir, Au, Ag, Os, Rh, 
Ru, Ir, Pd and Pt. Preferably the metallic elements are Cu, Zn, Ag, Ni, 
and Pb. 
The following examples will serve to illustrate the invention.

EXAMPLES 
EXAMPLE 1 
Electrodeposition of Cu was conducted at constant voltage in a cell with a 
Nafion.TM. membrane and catholyte and anolyte solutions being pumped. 
The anolyte in this first embodiment contained no coal and no added iron 
and was pumped through an external circulation system. The catholyte was 
pumped through a system similar to the anolyte. Aqueous electrolyte was 
0.5 M H.sub.2 SO.sub.4 with the cathode also containing 0.5 M in 
CuSO.sub.4. Total volume was 500 ml; temperature was run at 95.degree. C. 
and 120.degree. C.; anode was 55 cm.sup.2 of iridium oxide/titanium 
dioxide (TIR) coated titanium; cathode was copper sheet. 
______________________________________ 
95.degree. C. 120.degree. C. 
Cell Cell 
Potential (V) 
Current (mA) Potential (V) 
Current (mA) 
______________________________________ 
1.25 370 1.22 600 
1.00 9 1.00 35 
______________________________________ 
EXAMPLE II 
Apparatus and conditions were the same as those in Example I except that 
the pumped anolyte contained 0.5 g/cm.sup.3 of coal (WOW 3932). The 
results were as follows: 
______________________________________ 
95.degree. C. 120.degree. C. 
Cell Cell 
Potential (V) 
Current (mA) Potential (V) 
Current (mA) 
______________________________________ 
0.40 103 0.40 170 
0.60 330 0.60 520 
1.00 370 1.00 627 
______________________________________ 
EXAMPLE III 
In the third experiment the conditions and apparatus were the same as in 
Example II except that the anolyte was made 0.04 M in Fe.sup.3+ by the 
addition of Fe.sub.2 (SO.sub.4).sub.3. Results were as follows: 
______________________________________ 
95.degree. C. 120.degree. C. 
Cell Cell 
Potential (V) 
Current (mA) Potential (V) 
Current (mA) 
______________________________________ 
0.40 180 0.40 370 
0.60 530 0.60 1140 
1.00 620 0.65 1310 
______________________________________ 
At 120.degree. C. the Fe.sup.3+ concentration was increased to 0.1 M with 
all other conditions and apparatus remaining constant. The results were: 
______________________________________ 
120.degree. 
Cell Potential (V) 
Current (mA) 
______________________________________ 
0.35 260 
0.40 418 
0.60 1230 
______________________________________