Method of using a porous Fe.sub.3 O.sub.4 drilling mud additive

An iron oxide having the ideal composition Fe.sub.3 O.sub.4 has a unique high porosity, substantially spherical particle structure. Used in oil and gas well drilling muds, it scavenges hydrogen sulfide. It also improves the rheological properties of the mud, maintains its stability under high temperatures, and functions as a mud weighting material. Upon completion of drilling, the drilling mud containing the porous Fe.sub.3 O.sub.4 additive may be left in place between the inner casing and the formation wall or the outer casing as a packer fluid.

BACKGROUND OF THE INVENTION 
Hydrogen sulfide is frequently encountered in drilling oil and gas wells. 
It is corrosive to the drill pipe and casing. When a drill pipe breaks due 
to hydrogen sulfide embrittlement, the drilling operation must be 
interrupted and the drill pipe string repaired. It is also a pollutant to 
the environment and a risk to the health and lives of the drilling 
personnel. Low concentrations of hydrogen sulfide produce irritation of 
conjunctiva and mucous membranes, headackes, dizziness, nausea, and 
lassitude. Exposure to high concentrations can result in death. 
Magnetite has been used as a weighting material in drilling muds, as shown 
in U.S. Pat. No. 2,276,075; but without effectively scavenging hydrogen 
sulfide or substantial improvement in mud rheology. 
OBJECTS OF THE INVENTION 
It is an important object of the present invention to provide a type of 
Fe.sub.3 O.sub.4 having a large surface area which is useful in drilling 
muds as an effective scavenger for hydrogen sulfide, thereby reducing 
corrosion or embrittlement of the drill pipe and casings and avoiding the 
health hazard mentioned. 
Other objects are to provide a porous Fe.sub.3 O.sub.4 which in drilling 
muds improves the rheological properties and thermal stability thereof, 
while also serving as a weighting agent; and to provide a porous Fe.sub.3 
O.sub.4 whose concentration in drilling muds is easily monitored by 
magnetic techniques. 
SUMMARY OF THE INVENTION 
The present invention provides a new additive iron oxide having the ideal 
phase composition of substantially Fe.sub.3 O.sub.4 whose particles have a 
sponge-like porosity with a surface area many times as great as that of 
magnetite. Used in oil well drilling mud, it is exceptionally effective in 
scavenging hydrogen sulfide. It also improves the rheological properties 
and thermal stability of the mud, and has further utility as set forth 
hereinafter. 
DETAILED DESCRIPTION OF THE INVENTION 
The porous iron oxide of this invention is prepared by oxidizing iron 
containing carbon under controlled oxidative conditions. For example, iron 
borings containing about 3.5 percent carbon and somewhat less silicon and 
other minerals may be ground to a powder to pass through a 10-mesh screen. 
So powdered, it is subjected to oxidation at relatively low temperatures 
of about 400.degree. F. to 450.degree. F., under conditions so 
conventionally controlled as to form an oxide having an ideal composition 
Fe.sub.3 O.sub.4 without proceeding to Fe.sub.2 O.sub.3. The initial 
presence of such carbon, silicon and accompanying elements in the iron 
provides such a structure that the resultant Fe.sub.3 O.sub.4 particles 
will have the exceptional spongy porosity hereafter described. 
To be useful for the purposes of this invention, the oxide must be further 
processed, assuming that the oxidation process has left it agglomerated, 
the oxidized mass is first de-agglomerated by crushing, without grinding 
into fine particles. Sufficient water and base, usually sodium hydroxide, 
is added so that the mixture consists of about 50 to about 90 percent by 
weight water, but preferably 70 percent, and has a pH essentially 7.0. If 
the initial pH of the mixture is less than 5.5, it is preferred that 
ammonia be used as the base. The neutralized water-oxide mixture is 
mechanically agitated to produce a slurry, and is passed through a 140 
mesh screen, removing larger unreacted iron particles. 
Excess water is then removed from the slurry. The recovered material, which 
contains about 50 percent by weight water, is dried at about 400.degree. 
F. until the iron oxide contains about 10 to 20 percent by weight of 
water. Higher drying temperatures should be avoided since the Fe.sub.3 
O.sub.4 may further oxidize. It is preferred that the dried material 
contain no less than about 10 percent by weight of water inasmuch as the 
product is otherwise dusty and difficult to redisperse in water. If the 
drying process utilized should result in caking, any gentle 
de-agglomeration step will restore the material to its screened particle 
size. 
Examination of the dried Fe.sub.3 O.sub.4 with a scanning electron 
microscope shows the product to comprise porous, spronge-like, somewhat 
spherical particles. Examination with a Coulter counter showed that 98 
percent of the particles of a typical batch have an average diameter 
within the range of about 1.5 to about 60 microns. This is considered a 
desirable size range for subsequent use in drilling muds; and to have 95 
percent of the particles within this size range (or other desired size 
range) is commercially satisfactory. 
While 60 microns corresponds theoretically to a 240 mesh screen, a screen 
not larger than 100 mesh and preferably about 140 mesh will satisfactorily 
remove most larger particles including those of unreacted iron. 
Analysis of the particles by emission spectroscopy showed that they were 
iron oxide having an ideal composition of substantially Fe304. The porous 
iron oxide was further characterized by a hardness of about 6 and a 
density of about 4.55 gm/cm.sup.3. The material was ferrimagnetic at room 
temperature, having a curie point at 575.degree. C., a saturation 
magnetism of 480 cgs/cm.sup.3, and a remanent magnetism approximately 5 
percent of saturation. 
The iron powder from which the Fe.sub.3 O.sub.4 is derived usually contains 
trace amounts of minerals other than iron. On analysis by emission 
spectroscopy these materials in a typical batch are as follows, 
percentages being expressed as percent by weight of the sample: 0.04 
percent aluminum, 0.003 percent barium, 0.08 percent calcium, 0.2 percent 
chromium, 0.1 percent copper, 0.1 percent magnesium, 0.7 percent 
manganese, 0.1 percent molybdenum, 0.1 percent nickel, 2.0 percent silicon 
dioxide, 0.2 percent sodium, 0.07 percent titanium, and 0.02 percent 
vanadium. 
The dried, porous Fe.sub.3 O.sub.4 is ideally suited for use in drilling 
muds, ordinarily in combination with clay and other conventional additives 
such as weighting agents, caustic for pH adjustment, calcium compounds for 
conditioning in calcium formations and for pH control, hydrocarbons for 
fluid loss control and lubrication, sealants, thinners such as 
lignosulfonates and tannins for dispersion of mud solids, and 
bactericides. 
In the drilling mud the porous iron oxide is an effective scavenger for 
hydrogen sulfide. The reaction product is mainly FeS.sub.2 with smaller 
amounts, if any, of Fe.sub.3 S.sub.4. This reaction is pH dependent to the 
extent that H.sub.2 S preferentially reacts with caustic, e.g. NaOH or 
other alkaline materials, in the known manner. Since the reaction of 
H.sub.2 S with the porous iron oxide of the present invention is nonionic, 
the initial reaction of the H.sub.2 S is with the caustic until it is 
substantially exhausted and the pH lowered below about 10.5. The FeS.sub.2 
and the lesser amount of Fe.sub.3 S.sub.4, if any, unlike other forms of 
iron sulfides, are extremely stable and will not give up sulphur 
regardless of the subsequent pH of the mud, nor will they regenerate 
hydrogen sulfide. 
The FeS.sub.2 is similar to a natural pyrite. Its particles appear to 
agglomerate, so as to be removed along the drill cuttings when the mud is 
screened before re-circulation. The exceptional reaction characteristics, 
in conjunction with the favorable rheological properties hereafter 
described, result in the capability of treating far greater concentrations 
of H.sub.2 S than is now possible using other agents. 
For the purpose of scavenging hydrogen sulfide, about 2 to 20 pounds of 
porous Fe.sub.3 O.sub.4 are added per barrel of mud, depending on the 
hydrogen sulfide present, and sufficient other additives are included to 
provide a mud in the above weight range. The above range is not critical 
and more or less oxide may be used as desired. 
In addition to functioning as a hydrogen sulfide scavenger, the porous 
Fe.sub.3 O.sub.4 may also be used for other purposes, principally 
improving the rheological properties and thermal stability of the mud and 
serving as a weighting agent therein. In order to appreciate these 
additional functions it is important to understand the function of a 
drilling mud in serving as a lubricant. The mud is circulated down the 
interior of the drill pipe, through the drill bit and up the annular space 
between the drill pipe and the formation wall or casing to the surface. It 
thus removes the heat produced by the cutting action of the bit as well as 
the cuttings themselves, carrying them from the drill bit and up the 
annulus to the surface, where they are separated. In the annulus, the mud 
reduces the drag of the rotating drill pipe and provides sufficient 
hydrostatic pressure to contain any liquid or gaseous component within the 
formation being penetrated. The substantially spherical shape of the 
present iron oxide particles eases the flow of the mud. Finally, the mud 
should be thermally stable and therefore capable of withstanding 
bottom-hole pressures and temperatures. The present oxide particles do not 
deteriorate under the extreme pressures and temperatures so encountered. 
Materials are frequently added to the mud to decrease its viscosity, thus 
improving its lubricating qualities. Lignosulfonates and tannins, the 
conventional additives for this purpose, tend to break down in use at 
commonly encountered bottom-hole temperatures; the mud viscosity then 
increases, and more additive must be added to overcome the increased 
viscosity. Using the present porous Fe.sub.3 O.sub.4 any stiffening of the 
mud in use may be usually overcome merely by the addition of water. 
The porous Fe.sub.3 O.sub.4 of this invention is useful as a weighting 
agent; and, unlike other iron oxides, its substantially spherical shape is 
rheologically advantageous. No other prior art iron oxides (or other 
conventional weighting agents such as barite) are useful for improving the 
lubricating qualities of the mud. Instead, barite tends to grind down into 
fines during use, thus changing the flow characteristics of the mud, and 
magnetite is abrasive to the bit, drill pipe and casing. When the porous 
iron oxide of this invention is added as a weighting agent, as much as 500 
pounds of oxide per barrel of mud may be used, to provide about an 18 
pound mud. 
When the drilling mud contains between about 2 and about 20 pounds of 
porous Fe.sub.3 O.sub.4 per barrel of mud, the drilling mud may be left in 
place as a packer fluid with little or no additional chemical treatment 
when the drilling is completed. This effects considerable savings in 
materials and labor. In a packer fluid the suspended particles should not 
pack down, and the porous iron oxide described herein is a very effective 
suspending agent. Further, since hydrogen sulfide present has been reacted 
to form a stable pyrite-like substance, it offers no danger of gradually 
corroding the well casing.

EXAMPLE 1 
In this example the surface area of the porous Fe.sub.3 O.sub.4 of this 
invention was determined by the Brunauer-Emmett-Teller equation for 
determining surface areas by the surface adsorption of nitrogen. This 
classical method is described in the Journal of the American Chemical 
Society, 60, 309-319 (1938). These results are compared with those for a 
similar determination of surface area for magnetite having a particle size 
similar to that of the porous oxide. Briefly described, a sample 
comprising 11.2094 g. of porous Fe.sub.3 O.sub.4 was placed in an 
adsorption bulb for the determination of surface area with a 4-4680 Aminco 
Adsorptomat. The sample was preconditioned for 4 hrs. at 93.degree. C. and 
5 .times. 10.sup.-6 torr. The "dead space factor" (D.sub.f) was determined 
by the introduction of helium into the sample section and found to be 
0.5931 cc. STP/cm. Hg. After the helium was evacuated, nitrogen was 
introduced in dose sized (D.sub.a) of 2.034 cc. STP and an equilibrium 
time of 5 min. was allowed after each dose. The saturated vapor pressure 
(P.sub.o) was found to be 76.1 cm. Hg; hence, the factor (P.sub.o D.sub.f) 
was calculated as 45.135. From the following data in Table I, 
##EQU1## 
versus P/P.sub.o was plotted as shown in FIG. 1 in the drawing. The 
surface area of the Fe.sub.3 O.sub.4, as reported in the table, was then 
determined from the drawing. 
TABLE I 
__________________________________________________________________________ 
SURFACE 
No. of 
1 2 3 4 5 6 7 AREA, 
Doses (2-3) (4.times.5) 
(1.div.6) 
V.sub.M* 
M.sup.2 /G 
__________________________________________________________________________ 
N.sub.d 
P/P.sub.o 
N.sub.d D.sub.a 
##STR1## 
D.sub.f V.sub.a 
##STR2## 
##STR3## 
##STR4## 
##STR5## 
##STR6## 
__________________________________________________________________________ 
12 0.046 
24.408 
2.076 
22.332 
0.954 
21.305 
0.0022 
13 0.061 
26.442 
2.753 
23.689 
0.939 
22.244 
0.0027 
14 0.079 
28.476 
3.566 
24.910 
0.921 
22.942 
0.0034 
15 0.100 
30.510 
4.514 
25.996 
0.900 
23.396 
0.0043 
17 0.142 
34.578 
6.409 
28.169 
0.858 
24.169 
0.0059 25.0 9.8 
19 0.187 
38.646 
8.440 
30.206 
0.813 
24.557 
0.0076 
21 0.232 
42.714 
10.471 
32.243 
0.768 
24.763 
0.0094 
23 0.279 
46.782 
12.593 
34.189 
0.721 
24.650 
0.0113 
24 0.302 
48.816 
13.631 
35.185 
0.698 
24.559 
0.0123 
__________________________________________________________________________ 
##STR7## 
Following the procedure used for the determination of the surface area of 
the porous Fe.sub.3 O.sub.4 of this invention, 16.0525 g. of magnetite, a 
mineral form of Fe.sub.3 O.sub.4, was preconditioned for 4 hrs. at 
93.degree. C. and 5 .times. 10.sup.-6 torr. The D.sub.f was found to be 
0.5462 cc. STP/sm. Hg, the D.sub.a was 2.034 cc. STP, the P.sub.o was 
76.0 cm. Hg, and the P.sub.o D.sub.f was calculated as 41.511. From the 
data in Table II, the BDT equation was plotted as shown in FIG. 2 in the 
drawing, and the surface area, which is also reported in the table, was 
determined: 
TABLE II 
__________________________________________________________________________ 
SURFACE 
No. of 
1 2 3 4 5 6 7 AREA, 
Doses (2-3) (4.times.5) 
(1.div.6) 
V.sub.M* 
M.sup.2 /G 
__________________________________________________________________________ 
N.sub.d 
P/P.sub.o 
N.sub.d D.sub.a 
##STR8## 
D.sub.f V.sub.a 
##STR9## 
##STR10## 
##STR11## 
##STR12## 
##STR13## 
__________________________________________________________________________ 
2 0.055 
4.068 
2.283 
1.785 
0.945 
1.687 
0.033 
3 0.098 
6.102 
4.068 
2.034 
0.902 
1.835 
0.053 
4 0.142 
8.136 
5.895 
2.241 
0.858 
1.923 
0.074 
5 0.184 
10.170 
7.638 
2.532 
0.816 
2.066 
0.089 
6 0.229 
12.204 
9.506 
2.698 
0.771 
2.080 
0.110 2.2 0.6 
7 0.273 
14.238 
11.333 
2.906 
0.727 
2.112 
0.129 
8 0.319 
16.272 
13.242 
3.030 
0.681 
2.063 
0.155 
__________________________________________________________________________ 
##STR14## 
By comparing the results of Table I with those of Table II, it is seen 
that the porous Fe.sub.3 O.sub.4 particles of this invention have a 
surface area at least sixteen times as great as magnetite. With other 
batches of material, the porous Fe.sub.3 O.sub.4 particles may have a 
surface area from about 10 to 25 times as great as magnetite, as 
determined by the BET method, but preferably are within the range of from 
In this example, varying amounts of porous iron oxide were added to a 
number of drilling muds and the effect on viscosity was measured. The muds 
tested in this example were non-Newtonian; that is to say, viscosity 
varied with shear rate. The water-based mud before the addition of the 
porous iron oxide of this invention consisted of 10 pounds per barrel 
(ppb) of bentonite and 0.5 pound per barrel of caustic. The mud was 
stirred for 30 minutes and amounts of porous iron oxide, up to 100 pounds 
per barrel, were added as reported in the table which follows. The mud 
viscosity was determined with a rotational viscometer at rotor speeds of 
600 and 300 rpm. Using the dial readings d.sub.600 and d.sub.300, a number 
of parameters were determined as follows: 
apparent viscosity (AV) = 1/2 d.sub.600 
plastic viscosity (PV) = d.sub.600 - d.sub.300 
Yield point (YP) = d.sub.300 - plastic viscosity 
The plastic viscosity reflects the inherent interparticle friction and its 
magnitude is a function of particle size and concentration. The yield 
point reflects the attraction of the solid particles to one another. 
As shown in Table III below, adding the Fe.sub.3 O.sub.4 of this invention 
greatly changes the rheological properties of the mud. Where added in 
quantities up to 50 pounds per barrel, both the apparent viscosity and the 
plastic viscosity are much improved. The outstanding improvement in yield 
point persists even where much greater quantities are added. 
TABLE III 
__________________________________________________________________________ 
Test No. 1 2 3 4 5 
Porous iron oxide (ppb) 
0 25 50 75 100 
d.sub.600 13.5 7 6 11 15 
d.sub.300 10 4 4 6 8 
AV 6.7 3.5 3 5.5 7.5 
PV 3.5 3 2 5 7 
YP 6.5 1 2 1 1 
__________________________________________________________________________ 
EXAMPLE 3 
The principal purpose of this example is to demonstrate that the porous 
Fe.sub.3 O.sub.4 is less abrasive than barite. To do this, in Test 12 the 
wear of a 14.0 pound per gallon slurry of barite on the metal blades of a 
Waring blender after stirring for 10 minutes, measured in grams lost by 
abrasion, was compared with that of Test 13 wherein the slurry comprised a 
14.0 pound per gallon slurry of porous Fe.sub.3 O.sub.4. The results are 
reported in Table LV below: 
______________________________________ 
Test No. 12 13 
Blade Wear 0.0488 g. 0.0198 g. 
______________________________________ 
The reduced abrasiveness of the porous Fe.sub.3 O.sub.4 as compared to 
barite is unexpected inasmuch as the porous Fe.sub.3 O.sub.4 has a 
hardness of 6.0 on the Mohs' scale while barite has a hardness of 3.3. 
While applicant does not wish to be bound by any theory, it is believed 
that this difference is the result of: (a) the spherical nature of the 
Fe.sub.3 O.sub.4 particles, which contrasts with the sharp particle edges 
present in barite; and (b) the high porosity which allows particle 
deformation. Furthermore, even though the present porous particles deform, 
they do not appear to grind down into fines like barite. 
EXAMPLE 4 
In this example it is demonstrated that the porous Fe.sub.3 O.sub.4 
described herein reduces the shear strength and gel strength of drilling 
muds. Shear strength and gel strength are measures of the thixotropic 
character of the mud. They indicate the ease or difficulty with which a 
column of mud at rest can be brought into motion. 
Reduction in shear and gel strength after aging is important, for example, 
when the circulation of the mud in the well is interrupted as when a 
drilling rig is down during repair. If the mud "sets up" it is difficult 
to resume its circulation. Avoidance of mud "set up" is important also 
when the drilling mud is used as a packer fluid; otherwise removing the 
inner tubing for repair is very difficult. 
Gel strength (gels) in pounds per 100 sq. ft. was measured by reading the 
dial of a rotational viscometer at 3 rpm. This was done at 0 and 10 
minutes following vigorous stirring at 600 rpm. and is reported in pounds 
per 100 square feet. Shear strength was also determined in a conventional 
manner and is also reported in pounds per 100 square feet. 
In treating the mud with the present porous iron oxide, it was added at the 
rate of 9 pounds per barrel. Aging was at 350.degree. F. for 65 hrs. 
According to the following table, the addition of the porous iron oxide 
greatly lessened the shear strength after aging (from 600 psi to 250 psi). 
Similarly gel strength developed after 10 minutes was markedly reduced. 
TABLE V 
__________________________________________________________________________ 
Test No. 14 15 16 17 
POROUS IRON OXIDE 
UNTREATED MUD 
TREATED MUD 
__________________________________________________________________________ 
BEFORE 
AFTER BEFORE 
AFTER 
AGING 
65 HRS. 
AGING 65 HRS. 
__________________________________________________________________________ 
Weight (ppg) 10.6 10.6 10.7 10.7 
d.sub.600 44 103 30 70 
d.sub.300 31 77 19 55 
PV 13 26 11 15 
YP 18 51 3 40 
Gels - immediately on stirring 
15 5 3 35 
after ten minutes 
57 97 21 75 
Shear Strength (psi) 
-- 600 -- 250 
__________________________________________________________________________ 
EXAMPLE 5 
One of the purposes of this example was to investigate whether that the 
porous iron oxide has better lubricating qualities than barite. Two 
slurries were prepared, one was a 14.0 pound per gallon mixture of barite 
and bentonite and the other was a 14.0 pound per gallon mixture of porous 
iron oxide and bentonite. The friction coefficient for each slurry was 
determined with a Baroid tester and is reported in Table VI below. 
TABLE VI 
______________________________________ 
Mud System Friction Coefficient 
______________________________________ 
Barite-bentonite 0.32 
Porous Iron Oxide-bentonite 
0.28 
______________________________________ 
EXAMPLE 6 
This example demonstrates the effectiveness of the porous iron oxide of 
this invention in reacting hydrogen sulfide to form FeS.sub.2. It is also 
demonstrated that hydrogen sulfide is not regenerated from the FeS.sub.2 
once it is formed. 
In drilling an oil well, hydrogen sulfide gas was emitted at the well head 
from the circulating drilling mud. The drilling mud turned green and 
blackening of the drill pipe due to sulfide corrosion was observed. The 
porous Fe.sub.3 O.sub.4 of this invention was then added into the 
circulating mud, in the proportion of approximately 10 pounds per barrel 
to slightly basic water-based mud weighing approximately 10 pounds per 
gallon. 
On a single re-circulation of the mud containing the Fe.sub.3 O.sub.4 
additive, the emissions of hydrogen sulfide ceased; and even the odor of 
hydrogen sulfide was no longer noted at the well head. Within 2 days the 
drill pipe was observed to be cleaned of the sulfide effect. 
A sample of cuttings from the mud, then taken, was examined and compared 
with cuttings taken immediately before treating with Fe.sub.3 O.sub.4. The 
treated mud sample was found to contain FeS.sub.2 whereas the untreated 
mud contained no substantial amount of FeS.sub.2. 
EXAMPLE 7 
This example also demonstrates the reactivity of the porous iron oxide with 
hydrogen sulfide. A drill stem test was conducted in a zone known to 
contain hydrogen sulfide. 300 feet of open hole had been tested; and 600 
parts per million H.sub.2 S was measured to surface. Subsequently after 
the drill stem test, mud circulated from the bottom of the hole and 
containing the present Fe.sub.3 O.sub.4 was observed to be severely gas 
cut for 35 minutes. This fluid showed no evidence of hydrogen sulfide, 
proving that all had been reacted by the Fe.sub.3 O.sub.4. 
A sample of this mud was observed to contain FeS.sub.2 as the result of the 
reaction with H.sub.2 S. This mud was acidified to a pH of 3 without 
release of hydrogen sulfide, as proved by lack of hydrogen sulfide odor. 
This test confirmed the stability of FeS.sub.2, once formed, in acid; that 
is, that the removal of hydrogen sulfide reacted by the present additive 
was permanent and the gas would not be subsequently released. 
EXAMPLE 8 
This example demonstrates that the porous iron oxide of the present 
invention, when reacted by hydrogen sulfide, forms a coarser agglomerated 
pyrite which substantially removes itself by being screened out with drill 
cuttings. 
In an oil field where great quantities of gas had been encountered, 
concentrations of hydrogen sulfide were so great that, in a previous well, 
hydrogen embrittlement of the drill pipe was experienced, with subsequent 
failure and loss of the hole. 
An offset well was then drilled, with the mud treated with 13 pounds per 
barrel of the present porous Fe.sub.3 O.sub.4. Gas flow was encountered in 
which the drilling mud weight was reduced from 12.4 pounds per gallon to 
10.3 pounds per gallon. Over a period of approximately 48 hours the mud 
density was increased to approximately 13.3 pounds per gallon while 
continuously flaring gas and holding 100 psi back pressure on the well 
annulus. Nevertheless H.sub.2 S could not be detected at the surface and 
no evidence of pipe embrittlement or corrosion was observed. 
That large quantities of hydrogen sulfide had been reacted was evident from 
large amounts of agglomerated pyrite screened out with the drill cuttings, 
with measurable reduction in the concentration of Fe.sub.3 O.sub.4 in the 
drilling mud. 
The present invention thus contributes a substantial improvement to the 
drilling mud art. It will be readily apparent to those skilled in the art 
that various changes and modifications may be made without departing from 
the scope of the invention as it is more precisely defined in the 
subjoined claims. In them the phrase "substantially Fe.sub.3 O.sub.4 " is 
to be taken to include FeO.Fe.sub.2 O.sub.3.