Nya Lapphyttan - The trial to produce iron august 11-18
1996.
Summary, observations and comments.
The data obtained from the trial described in this report was very instructive.
We gained answers to many questions regarding the reconstruction of the
Lapphyttan medieval blast furnace. In that sense, the project was a great
success. If, however, the success of the experiment depended on the tonnage
of iron that was produced, then it would be rather disappointing.
The quantity of metallic iron that was collected was about 15 kg; the
rest was lost with the slag in an oxidised form. It is now obvious to all
who participated in the work that it is not easy to produce iron in a full
scale model of a medieval blast furnace.
We spent a total of one week on the experiment which was inadequate.
In retrospect, it would have been better if we spent two weeks on the project.
Before the test started, the available iron ore was analysed. Two lots of
ore, Klackberg ore and Kolningsberg ore, had been already roasted and crushed.
By checking the non-metallic parts of the ore it was established that the
Kolningsberg ore could be used without any additives whereas we had to add
a little quartz and a little lime to the Klackberg ore. The intention was
to adjust the mix so that the liquidus temperature of the slag would not
be higher than 1300º C.
The experience gained from the previous test conducted in November, 1994
resulted in a number of recommendations for modifying the furnace. Some
of the suggestions were such that extensive rebuilding would have been necessary
and were therefore postponed. One of several recommendations adopted led
to widening the hole for the tuyere. Unfortunately only the outermost part
was widened. This led to problems which are discussed later in this article.
The suggestion that the furnace should be 80 - 100 cm higher was not accepted.
Several other changes were made as a result of earlier testing. These
changes include:
Rebuilding the uppermost part of the shaft.
A constriction of the uppermost part of the shaft effectively prevented
air leaking down and burning charcoal from the top of the charge.
Modification of the hole for the tuyere.
This change was made in an attempt to have better control of the air
flow. Unfortunately, the change was not dramatic enough since the distance
between the inner wall and outer wall was too large when compared to the
cross section of the hole.
Air duct under the crucible.
This change would have been very important if the heating of the crucible
had been sufficient. This in turn highlighted the fact that the air blast
could not be controlled sufficiently.
On July 27 firing with wood commenced and continued until August 11,
when the furnace was filled with charcoal. The week before starting, one
of the supporting stones in the tapping hole cracked. To avoid a total collapse,
particularly when the temperature increased in this part of the furnace
within a day or two, a relieving steel frame was put in place. Traverse
cracks, originating from the original, appeared at the end of the week.
It was clear at the conclusion of the experiment that this steel frame was
necessary.
Another important conclusion was that coarse crystalline rock, such as
granite, was not a good choice of material so close to the intense heat
in this part of the furnace. It would have been much better if this part
of the furnace had also been made of mica schist.
Sunday, August 11, at 5 p.m., the furnace was ready to start working
and the tapping hole was closed. The furnace was filled with charcoal and
a charge of one shovel of Kolningsberg ore was added to the top. One bellows,
calculated to give sufficient blast had been mounted to the furnace and
was put into operation immediately. From the beginning the temperature of
the furnace was insufficient. Several more days with only charcoal in the
furnace would have been helpful. However, that would have diminished our
charcoal stock drastically. Calculations showed that the charcoal available
was sufficient for one week's operation. Even then, during that week another
5 m3 of charcoal was purchased to ensure enough fuel for the
experiment.
After a couple of days a reasonable distribution of temperature in all
parts of the furnace shaft was achieved.
Despite intense efforts, including placing a guiding plate in the blast
hole, we never succeeded in reaching the desired crucible temperature. On
Tuesday night we tried to place a second bellows alongside the first. The
aim was to increase the blast and hence the temperature. With this arrangement
it became impossible to reach the blast hole for cleaning. Consequently,
bellows number two was taken away after a couple of hours. To be able to
increase the blast with only one bellows, it has to be changed to a double
action mode. During the last day of operation, we used the blowing machine
"Hugin", a newly invented apparatus based on my vacuum cleaner
which had a motor rating of 850 W. Again, the goal was to raise the temperature
in the crucible. The highest recorded temperature at the bottom of the crucible
was 930º C, which was probably the result of a piece of fallen slag
coming into contact with the thermocouple. Other measurements taken at the
bottom of the crucible indicated that the temperature was between 650º
C and 750º C.
During the third day we began to have serious problems with excess slag
running down. Since the amount of slag was not in proportion to the iron
ore charge we concluded that parts of the furnace wall had loosened and
was melting down. An analysis of the plaster that was used for the coating
of the shaft walls (25 % clay + 75 % sand) showed a melting temperature
of about 1000º C. The calculation was based on the fact that the sand
used incorporated 30 - 40 % potassium feldspar (microcline).
Several consecutive hangings in the furnace resulted in delays of up
to one hour to clean out the solidified slag. On occasions we were forced
to use heavy rock drilling equipment to remove the obstruction.
The fact that the temperature in the crucible never reached the correct
level can be explained by the ineffective blast direction. The present construction
of the blast hole determines the direction of the air flow, which is upwards,
as opposed to the desired direction which is marginally downward. The reason
for the upward slant is the slag flowing down the wall of the furnace and
solidifying just below the blast hole. Endless efforts to keep this area
clean did not result in any improvement. When the furnace was operating
at its best efficiency, the airflow into the furnace was horizontal. A suggestion
to reconstruct the blast hole, similar to the construction made by Emanuel
Swedenborg and Carl Joh. Garney, is given below.
Almost immediately one suspected there was a "hanging" a little
higher in the furnace. To investigate this, a 5 m long steel rod was used
as a probe, pushed into the charge from the top of the furnace shaft. This
check was inconclusive. However, after blowing down the furnace we could
see that a "hanging" had formed on the sloping plane above the
crucible. At the side away from the blast hole, was a big "lump"
of solidified slag and iron ore. On the sloping plane above the blast hole
a large amount of material was seen to be burnt out. This was the reason
why the half molten rock fragments had been flowing together with excessive
amounts of slag.
The small amount of iron produced consisted of various types. Grey pig
iron (4,3 % C), white pig iron (3,8 % C), low carbon iron (< 0.05 % C)
and "steel" with a carbon content of 0, 40 % were produced. The
pig iron had flowed, down to the bottom of the crucible, and solidified.
Other iron was taken higher up before it had oxidised in the oxygen rich
blast. {It has been refined.}
If we succeed in controlling the temperature in the crucible I am confidant
that this furnace will produce very good pig iron.
Suggestion for the reconstruction of the blast hole.
Present construction.
Altered construction.
Fagersta 96-08-23
Bengt Högrelius
tf masmästare