Cover Flux

This Salt Flux is of very high purity and it offers several advantages over the standard salt fluxes presently used in the majority of reclamation plants.
1. Lower flux consumption.
2. Higher metal recoveries.
3. Lower energy costs.
4. A reduction of black dross generated; therefore less disposal costs.
5. The high purity of this flux reduces the possibility of contaminating the metal with unwanted elements.
6. Safety - mixing under strict control ensures the absence of oxidizing agents.
Salt flux is used for recovering aluminum from high surface area or dirty aluminum scrap and dross.
The furnaces used may be reverberatory with side well or fixed and tilting rotary salt bath furnaces, each of these furnaces is trying to accomplish the same thing; that is to melt scrap with minimal losses arising from the oxidation of the aluminum.
Using a salt flux in these furnaces provides several beneficial actions:
1. It protects the aluminum from further oxidation during melting.
2. It strips the dirt and oxides away from the surface of the molten aluminum.
3. It suspends the dirt and oxide materials thereby preventing them from contaminating the aluminum metal.
Usually a mixture of Sodium Chloride (NaCl) and Potassium Chloride (KCl) is used to make a salt flux. It may be seen from the phase diagram that at a weight ratio of 42% NaCl and 58% KCl the binary mixture gives a minimum eutectic melting point of 658°C.
Pure NaCl melts at 800.4°C and pure KCl at 790°C. (Pure aluminum melts at 660°C and its alloys will melt at lower temperatures).
The benefits of using a mixture close to the eutectic melting point is that less heat energy would be required to melt it and so less oxidation of the aluminum would occur.
However, salt flux mixtures with larger proportions of NaCl are frequently used because NaCl is cheaper to buy than KCl. These mixtures will melt at higher temperatures but the decrease in metal recovery and increased energy cost due to the higher melting point of the flux is balanced against the less expensive salt flux.