'Espelund' Two Stage Process - Overview

(The following is converted from a posting to EARLY IRON)

Skip Williams wrote:

Espelund has had the idea for several years that his 'dust ore' was smelted in a two step process. It makes sense that the first step would be to sinter the ore in a furnace at an orange temperature and a low air rate until it sticks together. The second step would be to break up the sintered ore to 'normal' smelting sized particles and then smelt it in a bloomery furnace.

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Some maniac from Canada volunteered for a first test of that Espelund two stage process at the Heltborg event (in Denmark). And no, I have not crunched the numbers and formatted the images or written a report (yet).

As some of you know, Arne can be a bit difficult for us 'thick headed Poles' (sorry - in joke from the Seminar) to effectively communicate with. He is firmly entrenched in his viewpoint of analytical chemistry. Which of course explains much - but is often quite hard to convert to direct physical action. For the working iron maker, the answer to 'how much air' has to be answered in terms of at least litres per minute - not moles per smelt event. (In most cases, thats really more like 'pumps per minute' on an individual bellows. Arne often states that he finds our experimental process sloppy and lacking in critical measurements. He is most certainly correct - from the viewpoint of strict analytical science. Us working grunts however have such limited resources and are trapped by working in the dirty field, not a sterile laboratory.
Our efforts are ore to IRON - and the slag is just a waste product. Few of us even bother to weigh slag after the smelt, and there are extremely good practical reasons why NOT to bother. (more on that in a separate posting).

So - given what I could understand was the framework of the two step process, the sequence goes like this:
Step one is ore to iron rich slag mass
Step two is broken slag to bloom

If I got his point of evidence correct, he has examined a number (?) of Viking Age (?) iron smelting sites in Norway that show considerable volumes of materials (multiple smelts). At these locations he found remains of broken up slags, which looked like they were being intentionally prepared from larger slag blocks, and being collected for possible use as furnace charges.

I may have missed the point, but he did keep referring to 'broken slag' - not 'partially sintered ore'.

I saw two large potential problems with the process as he described it:
1) a duplication of effort and resources. Why run what was 'almost' a complete furnace operation (set up, cycle time, charcoal expended) just to produce a slag? Why not just run that operation (correctly) complete straight through to metal bloom?
2) breaking the slag mass from step one to prepare for step two is a HUGE amount of work. We all know how difficult it is to physically smash up slag compared to the effort required to prepare ore itself.

Anyway, without being able to provide (at this point) the exact details, the rough sequence of the experiment at Heltborg was like this:

The smelter used was a pretty standard 'short shaft' furnace:
Interior roughly 25 cm
working height about 30 - 35 cm above tuyere (55 total from base)
modified front plate (bellows tube tight to plate surface)
tuyere set about 10 cm above floor of smelter
provision for slag tapping, but not required

consumption rate of charcoal averaged about 10 minutes per kg (8 - 12 minute variable)
ore was added at roughly 2 kg per 1 kg of charcoal
total ore added was 14 kg
ore used was the 'Guldager' material (from Michael Nissen)

total production of slag was 11.2 kg
main slag mass was 7.5 kg
fragments recovered were 3.7 kg

The furnace was re-set, and Arne broke up the slag mass into typical sized pieces (roughly 'half walnut' or less) and sorted by eye.
The second firing:

working height about 50 cm above tuyere
tuyere angle at 22 down
(otherwise identical)

consumption rate of charcoal averaged about 7 minutes per kg (6 - 8 variable)
slag was added at roughly 1.5 kg per 1 kg of charcoal
total slag added was 7.75 kg

only a few small fragments of metal were recovered

So - right off the top, remember that this was the first working attempt at a brand new method. Two entirely new processes, plus some integration between these two - all three parts unknown.
On part A we got iron rich slag. How iron rich awaits tests by Arne of the samples he took.
On part B we did not achieve a viable iron bloom.

As Arne said ' Even a negative result is a valuable experiment '. I certainly would expect that to get this whole linked process working correctly, the development process will take as least as long as for two different smelt methods. (We all remember how long it took us to finally get any iron when we started!)

There are a number of what I feel are extreme challenges to get this process to work correctly. (This assuming that I got Arne's instructions on the chemistry correct.) What I recorded in my notes (please forgive me if I have this wrong!)

On the first step, the temperature range is extremely tight:
Require at least 1100 C to produce a working slag
But no more than 1200 C - which is "the threshold for CO production" (keep below to keep ore from reducing).
This is just way too tight a working range for the primitive equipment of an ancient smelter.

Anyway, that is the bare bones of the experiment. I'll let everyone know here when I get the experimental sequence formated and posted, plus the photographs of the process ready on the web site.

Darrell