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Optimal Metallurgical Flux Recipe for Primary Refining of Go ... (10 replies and 1 comment)

Carmen Ibanz
9 months ago
Carmen Ibanz 9 months ago

We use the Zadra strip/EW process to produce gold sludge, which is then sent to our furnace, after drying, to be refined. I was curious to know how one would determine the best flux recipe (constituents/amounts) for primary refining. Additionally, what are some control strategies to determine when the molten metal should be poured?

Victor Bergman
9 months ago
Victor Bergman 9 months ago

A lot of things will determine your flux recipe and depending on your slag viscosity and metal content you may have to add other fluxes such as Nitre-(an oxidising agent) or fluorspar (also reduces slag viscosity). Also you may need to work on your ratio depending on your smelt and slag results

Below is an example:
Borax 40% of the total calcine weight and is used to make the slag more fluid at the furnace operating temperature – reducing viscosity and dissolving metal oxides. The melting point of gold is 1064℃, by adding borax to the heavy mineral concentrate, the melting point temperature decreases.

Soda Ash 16% of the total calcine weight; it also acts as a de-sulphurising and oxidising agent and possesses a very high fluidity.

Silica 10% of the total calcine weight. Silica forms the basis of most fluxes because it has the capability of dissolving most metal oxides. It has a high melting point and forms a viscous slag which is reduced by the addition of borax and soda ash.

There a couple schools of thought on the pour time- one is when you pull the metal stirrer out of the pot and it sparkles like a sparkler the gold is ready to pour. Another is when the charge has melted in the pot it is ready to pour.

9 months ago
Obergruppenfuhrer 9 months ago

I agree with you, what you need to remember is that flux mix are also almost tailor made for your individual process therefore you need to experiment a bit, in our application we are on 50% Borax, 10% Soda Ash and 5% Silica, with resulting bullion purities of 92-94%.

Ace Levy
9 months ago
Ace Levy 9 months ago

Agree flux ratio tailored to suit individual processes. Do trials within provided ranges for different fluxes and you may find your optimum combination. For pouring, when the molten charge flows freely back into the furnace upon lifting the metal stirrer, it’s ready.

Maya Rothman
9 months ago
Maya Rothman 9 months ago

What is chemical comp of sludge? Most sludge can be loaded with base metals but the industry has a mind- set for induction furnaces. That is because the people designing the system know nothing about refining. Result is high shipping cost for base metals pndore and high refiner charges. Induction furnaces really limit what you can do with fluxing.

Most people I have encountered in the industry do not know how to calculate a flux. Induction makes fluxing out impurities in order to get a decent dore is difficult. Nitre should never be used in induction furnaces due to the SiC crucible. Borax must be kept to only what is required to complex Si.

First thing you need to know is if you are going to have to deal with base metals in sludge, if so go with reverb technology. I do refinery audits and training and there are a lot of places that have a "proprietary flux". Translation - "They have no idea what the different fluxes do or how to calculate the correct one for their precipitate/sludge. Refiners buying their dore love it because they kill them with refining charges.

8 months ago

How does one calculate the correct flux ratio`s for the sludge.

9 months ago
Gruppen 9 months ago

In tests should be noted that if we have more oxides, further increase the composition of silica, but this will increase the viscosity. To reduce viscosity, increase Na2CO3, fluidizing, deoxidizing and desulfurizing. When the pot is so transparent, it is also indicative. I have worked with 50% borax, 16% Na2CO3 and 5% silica.

Maya Rothman
9 months ago
Maya Rothman 9 months ago

This suggestion is to general to be of much value. What is sludge composition and furnace type? Are you using diatomaceous earth as filter aid? If you have base metals present such as copper this flux will drive the Cu into the Dore rather than retain it in the slag. The correct flux recipe will provide the same recovery as the flux suggested here but will also result in a Dore with much higher fines value.

Carl Jenkins
9 months ago
Carl Jenkins 9 months ago

What mine are you working at? A simple bi-silicate flux is usually needed.

9 months ago
Oberfuhrer 9 months ago

If you are using stainless steel cathodes, the sludge should be fairly easy to melt and pour once dried. There is no steel wool to oxidize and base metals are usually limited to copper. I seem to recall we used some silica flour and borax at the last operation I worked at.

Maya Rothman
9 months ago
Maya Rothman 9 months ago

Steel wool isn't an issue anyway; it’s the copper that is a problem. You need to know the composition of the sludge to even suggest a flux! A point that cannot be understated is that the flux to sludge ratio can be just as important the flux recipe.

Sandeep Bisht
9 months ago
Sandeep Bisht 9 months ago

Totally agree but before proceeding for refining, complete analysis is required to know which will help in choosing correct fluxing constituents.

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