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Gold Smelting Procedure
A tilting type barring furnace is used. It is heated by L.P. gas and the specified maximum rated output of the burner is achieved with a gas pressure of 140 kPa. It is presently operated on 135 kPa, i.e. the maximum attainable pressure from our supply system.
The advantages of using this type of furnace relate mainly to safety and ease of handling. The crucible is fully enclosed even during the pour hence the risk of injury to the operator during a pour is considerably lessened in the event of a mishap. Pouring is relatively easy, a virtual one man operation provided that the furnace has been properly counterbalanced. (See fig. 18)
The only real problem with this type of furnace is that the changing of crucibles is a major task requiring the removal of the furnace lid and extensive fireclay work. Smelting is normally done of a Tuesday. Should a crucible failure occur during the first smelt it would be virtually impossible to put in a new crucible in time to get the gold out on Wednesday as the furnace must be allowed to cool before doing any repair work.
A 100 graphite based crucibles are currently being used. These have been good for about 20 to 25 smelts in the past but they are prone to attack by nitre. Of recent months around 50% of the gold has come from the gravity circuit and whilst the acid digestion of the concentrate from the Wilfley table has improved, it has nevertheless been necessary to use more nitre when smelting that concentrate. With the refinement of the intensive cyanidation system this problem will be avoided as most of our gold will be plated out on cathode wool.
However a silica based crucible will shortly be installed. These are twice as expensive as the graphite based crucibles but with them as much nitre as is required may be used provided that there is an excess of silica in the charge. Any iron in the charge will combine with silica used in the flux to form ferrous silicate. If insufficient silica is used the iron will combine with the silica in the crucible and eventually cause its failure.
The normal charge (cake + flux) is usually no more than 30 kgs., this assuming that the standard 2:1 (cake: flux) mix is used. It is good practice to vary the size of the charge as this helps to prolong the life of the crucible.
The size of the bullion bars is limited by the capacity of the molds. The molds used here have a capacity of around 23 kgs. However for ease of handling within the despatch system, it is desirable that the bars do not exceed 19 kgs.
The fineness of the bars poured here is very good, there being very little difference between the Mint assays for CIL bars and gravity bars in recent months. The main fluxes used here are:
- Borax: helps collect iron and a good fluidizing agent.
- Boric acid: helps collect iron but gives a thicker smelt than borax.
- Silica Flour: provides silicon for slag and helps collect iron.
- Sodium Nitrate: Strong oxidising agent – oxidises iron but very damaging on crucibles if used too generously.
- Soda Ash: Fluidizes slag but works against nitre.
With the development of the intensive cyanidation system in the future it will be possible to use a standard flux mix. In the past it has been necessary to use more sodium nitrate and silica flour when smelting gravity gold.
- Fire up the furnace at least one hour before charging the crucible. For the first thirty minutes keep the gas pressure below 30 kPa and the air addition regulator closed. Fifteen or so minutes before charging turn the gas up to maximum and open up the air regulator fully.
- Divide up the total cake into convenient smelting lots. Avoid having very small smelts.
- Mix the fluxes according to desired percentages of each, making sure that they are thoroughly blended and all of the lumps broken up. Wear a mask and gloves when handling flux.
- Mix the cake and flux together (currently 2:1 mix) and then pour the total charge into paper bags. This helps to prevent loss of fine gold and makes it easier to load the crucible when the furnace is hot.
- Turn off the flame – do not attempt to load the crucible whilst the flame is burning.
- Load the crucible and put the cap over the charging hole.
- Put the gas on maximum and open the air regulator up fully. Put the stirring rod on top of the furnace.
- Monitor the smelt regularly. When the charge liquifies it will be necessary to stir it at regular intervals. Make sure that the rod is red hot beforehand.
- Warm up the mould – do not over heat – then position it on the sand tray so that it is quite level.
- Coat the mould with castor oil just before the pour. This prevents the gold from sticking to it.
- The smelt will be ready for pouring when the surface goes flat and there is no movement, i.e. the charge is in a state of “quiet fusion’ . Beware of bubbling around the edge of the crucible – it could mean that the charge needs stirring and it could also mean that the nitre is eating into the crucible wall. The charge will be quite fluid when ready to pour. On an average the first smelt takes ninety minutes and every one thereafter around sixty minutes. (The furnace has by the end of the first smelt achieved its maximum heat.)
- Put on the safety clothing and face shield and then pour the smelt in a steady stream. There must be two people present at each pour. (For safety and security reasons)
- Allow the loaded mould to cool until the slag has set.n. Put on gloves and glasses — then break the bar out of the mould.
- If necessary, immerse the bar in nitric acid 50:50 (nitric:water) mix to aid the cleaning process.
- Scrub the bar with a wire brush, using soda ash and water.
- Stamp with ‘MY-MINE’ on the top of the bar and the bar number, e.g. ‘0696’ on both top and bottom.
- Weigh the bar and lock it up in the strongroom.
- For each smelt fill in the appropriate details in the ‘Process Record’ book.
- Clean away the slag after each pour and check for prills. Any prills found should go into the next smelt.
- After the final smelt record the bar details in the ‘Gold Movement’ book and inform the office so that the administration representatives can check the weights and lock the bars up in the bullion safe.
- Put the smelting safety gear in the strongroom and then lock both combinations.
- At the end of each smelt day, check out the crucible thoroughly for wear. Use the calipers to compare the wall thickness with that of a new crucible.
Points to Watch
- Beware of fumes – these can be extremely toxic. Fortunately The CAKE is very clean.
- Check out the oxygen respirator before commencing smelting.
- Keep liquids away from the furnace. Keep the floor dry.
- Keep the goldroom doors locked and make sure that all visitors are signed in. Make sure that they fully understand the rule, ‘hands off’.