Wednesday, June 25, 2008

ERROR CORRECTION - Ore Analog Composition

(with assistance by Neil Peterson)

It turns out there was a major error on my earlier postings on the DARK Dirt One ore analog. This was suggested by Jesus (on Early Iron Group), who also had worked with the Spanish Red iron oxide material. He questioned (correctly) my published figure of 96.5 % Fe2O3 content.

It turns out this was the result of the typical transfer of lab test into full sized production. When Gus did his initial bench tests, he was focusing on material availability and on gross physical properties. What we could purchase, and how easy it was to mix and handle the resulting mixed material. He had purchased a number of iron oxides from a local pottery supply company (Pottery Supply House).

The company sells :
Black Iron Oxide - Fe3O4 = 99.3 % (as Fe2O3 103.5 %) / SiO2 = .2 % (@ $5 / kg)
Yellow Iron Oxide - Fe(OH3) = (typical as Fe2O3 86 % ) / SiO2 = ? (@ $12.25 / kg)
Red Iron Oxide - Fe2O3 = 96.5 % / SiO2 = 2 % (@ $12.50 / kg)

It had been decided at the test stage to use the red oxide as it most closely resembled the iron chemical form created after roasting a natural ore.

These are typically much finer powders ( listed as average 1.5 microns) than are really useful in the smelter. Others who have attempted to use oxide powders have reported the venting of gas out of the smelter tends to blow this fine material right back out again. Our solution was to mix a small amount of whole wheat flour (regular baking flour) as an organic binder. The decision to add a small amount of silica was based on the almost total lack of that component in the sample material. (Required for generating the required slag bath).

DARK Dirt One:
Oxide - 80%
Silica - 10%
Flour - 10%

When it came to purchasing full sized lots (as 25 kg bags), the raw cost of the material became a major factor. Gus had also supplied the first materials for a full scale test The error was in not checking that the cheaper 'Spanish Red'
had the same chemical composition as the 'red oxide' used for the bench tests:

'Spanish' Red Iron Oxide - = 81 % / SiO2 = 5 % (@ $5 / kg)

The dry powders were mixed roughly by hand, then enough water added to make a dough like paste. This required the addition of about 50 % by weight of water. The paste was dried before it was then broken up for addition to the smelter.

As added to smelter:
Solids - 90%
Water - 10%

So in correcting the composition at the various stages of mixing DARK Dirt One, we need to refer back to the specifications of the 'Spanish Red' source material (product number - MIOR) :

Fe203 - 81 %
SiO2 (silica) - 5 %
Al2O3 (alumina) - 2.6 %
CaO (calcium oxide) - 2.3 %
MgO (magnesium Oxide) - 2 %
LOI * ('loss on ignition' - water & C02) - 7 %

At the mixing phase (not considering water) :

Fe203 - 64.8 % (81 % of 80 %)
SiO2 (silica) - 14 % (10 % plus 5 % of 80%)
Al2O3 (alumina) - 2.1 % (2.6 % of 80 %)
CaO (calcium oxide) - 1.84 % (2.3 % of 80 %)
MgO (magnesium Oxide) - 1.6% (2 % of 80 %)

That suggests the Fe total is on the low side. With only 64.8 % Fe2O3, that means only 45.4 % Fe available.


Our original samples of primary bog ore (from L'Anse aux Meadows and St Lunaire) were tested :

St Lunaire (by M. Burnham):

Fe203 - 64.04 % (80.7)
SiO2 (silica) - 2.24 % (2.8)
Al2O3 (alumina) - 3.35 % (4.2)
CaO (calcium oxide) - .69 % (.9)
MgO (magnesium oxide) - .08 % (.1)
Mn0 (manganese oxide) - .62 (.8)

On this sample, there was LOI of some 26 %. That suggests for easy comparison, the numbers should be really be adjusted upwards (the second figures). If considering the LOI potential in the analog (about 20 % in flour and water) the DARC Dirt One is a very close mimic of the St Lunaire material.

L'Anse aux Meadows (by R. Hansen)

Fe203 - 89.5 %
SiO2 (silica) - 1.24 %
Al2O3 (alumina) - 2.45 %
CaO (calcium oxide) - .47%
MgO (magnesium oxide) - .05 %
Mn0 (manganese oxide) - 5.33


There is another variable to be considered when comparing total ore additions to a furnace and comparing the use of DARC Dirt One with other ore types. As the material is not roasted before use, there is a significant amount of the recorded weight contained in volatiles - in this case water and the flour binder. I would suggest we should be at least subtracting the water weight (about 10 %) from our yield calculations. So we really only are putting in 90 % of the 20 kg as actual 'ore' - and of that only 81% was actually the Fe2O3.
From the working (charge) weight of ore - the total iron oxide content is only 64.8 %.

We should be amazed we got anything, as this is right at the bottom limit on Fe concentration normally considered suitable for our type of smelting equipment and process.

From our last test (June 14, 2008) thats:

(Chemistry less water)
Fe2O3 = 64.8/.9 = 72% (Fe = 50.4%)
SiO2 = 14/.9 = 15.5%
Al2O3 = 2.1/.9 = 2.3%
CaO = 1.84/.9 = 2%
MgO = 1.6/.9 = 1.8%

(90 % of 20 kg) = 18 kg (removing weight of water)
(80% of 18 kg) = 14.4 kg (working weight of Spanish Red)
(81 % of 14.4 kg) = 11.7 kg total Fe203 added
(70 % of 11.7 kg) = 8.2 kg as actual iron added
Of which we got back 1.8 kg as bloom.
We normally compare ore weight against bloom, so our 'relative' yield would be 1.8 bloom from 16 kg working ore = 11 %.
If we wanted to spin this, we could compare Fe in against Fe out - 1.8 bloom from 8.2 available iron = 22 %

I am quite happy, that even despite this error in record keeping, the analog is a splendid success. It has done exactly what we wanted - provided a material which mimics primary bog ore in field tests, and which at the same time can be easily changed to duplicate specific chemical contents. The cost is relatively cheap (about $50 per smelt), and the source materials widely and easily available.

* (Thanks to Tim for gently pointing out what 'LOI' means to me - thats 'loss on ignition'.)

Special thanks to Gus Gissing of Harder-Gissing Machining for doing the initial materials samples and donating the first raw materials.

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