Nitre is a "necessary evil". If you tame your metal oxides to neutral, then your crucibles will survive. The trick is to slag your metal oxides with silica and keep your carbonates high.
It also kicks hell out of the furnace. But, if you don't use it... You need to use MnO2. Do the stiochiometry.
Also although silica is dandy, it can be very viscous and too much can soak gold into the slag by not allow sufficient decomposition of the bases.
Borax allows fluid slags but too much can make too tough a slag.
1 Fe + 1 MnO2 = 1 FeO + 1 MnO
56 + 87
100 grams charge equals 90 grams Fe
MnO2 = 155 grams
Silica in charge 90 grams
Borax = 15 grams.
2 Fe0 : SIO2
144: 60 = 37.5 grams silica.
2 MnO : SIO2
140: 60 = 64.5 grams silica.
Satisfaction of silica = 102.1
So you need to add an additional 2 grams of silica and 1 grams of Borax. Not bad, but MnO is not a strong oxidizer. You may need stronger stuff, and nitre is the place to get it.
What if you had Na2CO3 ? It is usual to have fewer oxides at the end of the day, and thus use sodium carbonate to balance the silica in a 50=50 ratio. Here your metal oxides predominate, as one is an oxidizer. This could be a danger to crucibles So you can see going to Sodium nitrate is a good idea.
GAW Ratios for oxidization reaction:
2 Fe + 2NaNO3 = 2FeO + 1Na2O + 2NO2 gas/>
112 + 170 = 144 + 62 + gas
Which produces two bases to satisfy the silica.
So now its
100 grams charge = 90 grams Fe/Ni/Cr
Sodium Nitrate = 137 grams
Sllica = 90 grams
Borax = 15 grams
Silica balance:
2Fe0: SiO2
144 : 60
2Na2O: SiO2
124 : 60
Base amounts produced from reagants:
Fe0 = (out of 90 gms) 72/56 X 90 = 115.71 gms
Na2O (out of 137 grams Na2NO3) = 62/170 X 137 = 50 grams.
Here you use the actual GAW weights of bases produced in
the melt to satisfy the silica as you are not trying to
find out what to add, just what weights of product will
act in the formula to balance known silica. This tells
us whether the bases are fluxed to silicate or not.
115.71 FeO = 60/144 X 115.71`= 48.21 silica matched
50 grams Na2O = 60/124 X 50 = 24.19 silica matched
Silica satisfaction = 72.40 grams
Needed to satisfy: 27.60 grams silica.
Use 2Na2C03 == > 2Na20 : 1 SiO2 + CO2 Gas
212 124: 60
Use
Sodium Carbonate = 212/60 x 27.60 = 97.5 grams
So there is not much silica and borax needed to keep the
bases from being too high and temp too high.
Your charge is now finally:
100 grams charge
100 grams silica
20 grams borax glass
137 grams Sodium Nitrate - perhaps 15 grams more.
97.5 grams Baking Soda.
5 grams Calcium Flouride. (CaF (59) --> CaO goes to 2 CaO: 1
SiO2 or 118:60) -- I add 5 grams borax to balance) Note you
use the CaF weight here to balance as you satisfy 1 silica
with two CaF. The CaO just tells you how many molecules you
need to get the oxygens to balance with the resultant base.
This is subject to experiment. The other charges asked for
150 grams of Oxidizer. Perhaps the nickel and chrome are a
problem. A few experiments with small crucibles would be in
order.
Another subject is, borax glass (not hydrated borax), to
silica equivalence. Na2B4O7 is seven acid oxygens and one
base. Making 5 effective acid oxygens.
So 1 Na2B4O7 = 2.5 SiO2.
Or 201.24: 150.
5 grams Borax = 3.72 grams silica for monosilicate charge.
Nitre has to be there to chase the sulfides and make the steel into FeO, nickel into NiO and Chromium to CrO. It your charge is 75% steel, and some random metals from the cyanide solution, then you basiclly need to oxidize the whole damn thing. Otherwise your gold will end up with the nickel which is a very efficient collector. In fact if you add only a bit of lead to balance the silica and some reducer you can collect platinum in the nickel button like a hot damn.
See the amount of nitre or sodium nitrate they add to that stainless wool charge of 100 grams is 150 grams NaNO3? Steel, nickel and chrome are not to taken lightly. Have to work them back to silicates. Perhaps the extra oxidizer is necessary. Ni and Cr might be 20% of the wool. NiO and CrO are pretty close to iron in weight, 58 and 61 GAW, but they may have differing oxide potential, so need more oxidization. 10 more grams of this stuff could easily add 15 grams of sodium nitrate to your charge.
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