Before gold and other precious metals can be converted into jewelries and used for various other purposes. They have to go through a refining process to get them into a useful form. This is to ensure enhanced purity and fluidity to make it easier to form into different shapes.
Smelting is an integral part of the refining process of gold and other precious metals. It is the major part of refining where the precious metal (gold in this case) is melted out of its ore. Metal ores often contain a lot of other elements and impurities, smelting is therefore important in removing these.
Gold smelting flux is often added to the ore during the smelting process. In this post, we will be discussing the importance of Gold smelting flux and its uses in smelting. Most importantly, you will learn how to select the right gold smelting flux for your smelting projects.
Traditionally, smelting is carried out between 1150 and 1450oc for about 2 hours. After this, the gold and silver alloy settles to the bottom since it is heavier while the slag floats and is taken off. Despite these extreme conditions, however, it still remains difficult to separate out some impurities.
It is often the case that some of the gold gets trapped within the slag which floats. Conversely, some of the impurities and associated elements may also stick to the pure metal, sinking along with it. These two anomalies are sure to lead to situations in which impure ingots are produced. They may also lead to loss of the pure metal along with the slag.
This is why gold smelting flux is important. gold smelting flux is basically a chemical substance or a mixture of chemical substances. Which are added in regulated amounts to the precious metal ore charge. It helps to combat all the problems mentioned previously and ensure that the ingots are as pure as possible. Below are some of the ways in which the use of gold smelting flux makes the smelting process more efficient:
There are various types of gold smelting flux available on the market. Beyond just gold smelting, flux is also used in other areas of metallurgy and metal joining. Below are some types of flux that are in use generally:
All of these chemicals perform various functions in the smelting of Gold. Each of them contribute their own specific quota in ensuring that the resulting gold bullion is pure. Below are some of the unique roles played by each kind of flux.
A white crystalline mixture which is in powdered form at room temperature. Borax helps to lower the smelting point temperature of the ore. This means that lower temperatures that smelting will occur quickly and allow the precious metal to settle out.
Also, Borax is very effective in capturing metallic oxides. This is necessary because many of the other metals that constitute impurities are highly reactive metals. They will therefore readily form oxides in the presence of borax.
Gold on the other hand is majorly an inert element, it will not take part in the oxidation process. The gold will therefore sink down easily in its molten form. The oxidized impurities (aided by Borax) will float up as slag and get collected.
Therefore, the efficiency of the smelting process is highly increased and there is improved separation. The advantage of this is that a purer ingot will be gotten. Also, there is reduced loss of precious metal alongside the slag.
Silica is quite similar to Borax in its gold smelting flux properties. It is selectively soluble with Gold and silver and similarly allows for the easy separation of pure metal from slag. By forming silicates with metal oxides, Silica ensures the stability, homogeneity, and low viscosity of the slag.
Also, some ores already contain silica due to the use of diatomaceous earth during the extraction process. Extra silica as flux must therefore be added with caution. Silica also has the effect of increasing the viscosity and fluidity of the charge, allowing the molten gold to flow down easily.
This is an alkaline flux which has the ability to react with and remove sulphides from pure gold. Sodium carbonate may also be used in synergy with another flux. Silica for example when mixed with Sodium Carbonate leads to the formation of Sodium silicate which can also form oxides with impure metals and get them out.
It should be noted that adding Sodium Carbonate as flux in excessive amounts may become counter-productive. This is because it may lead to the formation of sticky slag which will not flow of easily. Care must therefore be taken to determine the optimum amount of Sodium Carbonate used as flux in any smelting process.
This chemical majorly helps with lowering the smelting point temperature of the charge. It is often combined with Sodium Carbonate (which has been previously mentioned). Thus combining the flux effects of both substances and giving an improved performance.
Some types of flux are good at oxidizing specific metals. Sodium Nitrate belongs to this class of flux elements. Sodium nitrate oxidizes metals like iron, copper, and zinc by liberating oxygen at a temperature of about 500oC.
Litharge is basically a monoxide of lead (often yellow in color) which is used as gold smelting flux. Just like Sodium carbonate, litharge is also useful in removing sulphides from the pure metal and getting it to flow out with the slag.
Fluorspar is neither acidic nor basic; a neutral flux that is highly applicable especially in reducing the loss of precious metal in refractory ores. In a manner similar to Silica, Fluorspar increases viscosity and allows free flow of both pure molten gold and slag. It also aids decomposition; thereby allowing for more fluidity and again contributing to the overall flow of slag and pure metal.
Despite the benefits listed above, theres a note of warning to be sounded in the use of Fluorspar. It may begin to react with and corrode the crucible when added in uncontrolled proportions. Fluorspar also has the tendency to interfere with the oxide formation property of borax when used together.
These substances are often useful in helping to break down Litharge, thereby extracting the metallic lead. With the lead freed, oxygen molecules are liberated to react with the impure metals. This makes it easier to form metallic oxides through an oxidation process aided by Borax, Silica, and others.
It is worthy of note to state here that Mercury used to be utilized as a gold smelting flux in the past. However, due to hazards to the environment and rising concerns about environmental protection and climate change, it has been made obsolete. Apart from these, mercury also has adverse health effects on workers who are exposed to it, it is therefore safer to do away with it.
As can be seen from the previous section, various flux perform different specific functions in the gold smelting process. There are however various types of concentrate, and each one has unique needs that can be met by various gold smelting flux. In most cases, the effect of more than just one kind of flux is required.
For example, a charge may have high smelting point which needs to be lowered by a flux like Potassium Carbonate for example. This same charge may also have metallic impurities. Therefore also requiring the use of a flux like Borax to form metal oxides and remove impurities.
There are various flux recipes which depend majorly on the kind of ore or concentrates being smelted. It is advisable to consult standard texts and determine the calculation for the effective weights and ratios of flux combinations. However, below are a couple of basic recipes that will at least point you in the right direction to start checking.
Traditionally, for most ores, the composition of the flux mixture should contain between 40-50% Borax, 10-15% Soda Ash, and 5-1-% Silica sand. In most scenarios however, the properties of the ore will vary and so will require different proportions.
In neutral ores, a combination of 25g Litharge, 20g Sodium Carbonate, 1g flour, and 8g silica will be appropriate in Smelting 25g of concentrates. Of course, all of these weight values will have to be adjusted for increasing or decreasing ore weights.
As recommendedhere, a good recipe for ores sourced from Black sand will include 30g litharge, 30g Sodium Carbonate, 25g borax, 5g flour, 8g fluorspar, 30g silica and 20g ore sample. Again, it is important to note that these are just starting points and not hard and fast rules.
You have to make appropriate adjustments for differences in weight. Also, ensure that you carry out tests to determine the optimal flux combination for your specific ore sample. This is because even black sand gold ores may have slight differences in characteristics, differing from one location to the other.
This is another common recipe that is used not only for Gold, but also as Silver smelting flux or in a combination of both. The recipe contains 40% Borax, 20% each of Soda Ash, Manganese Dioxide, and Silica. (As offered for salehere)
Generally, experience is often the most important guiding element in selecting a flux recipe. If you dont have much experience yourself, you can always take advantage of previous work done by other people. Using the points listed here as a starting point, ask questions and make findings from a variety of sources.
More importantly, run a test smelting process on any recipe you wish to try out. This should be done before applying it on a large scale to the whole body of precious metal concentrates. So as to avoid wastes and loss of precious metal.
Higher Profit Margins:The use of gold smelting flux ultimately increases the purity of the precious metal to be casted into ingots or bars. Therefore, buyers are dealers are definitely going to be willing to pay higher prices for your gold bars. When compared with the increased revenue associated with higher purity, the cost of purchasing gold smelting flux is definitely a wise and worthy investment.
Apart from just short term monetary gains, a long term economic benefit of using gold smelting flux is the reputation. When your plant or foundry becomes popular for producing only high purity bars, you will have steady business and increased profits.
Lower Energy Costs:Since the use of gold smelting flux helps to lower the smelting point temperature. Less energy is utilized in bringing the ore up to smelting point. The decrease in costs expended on either electricity or fuel leads to a corresponding increase in profit levels.
Removal of Residual Mercury:For gold ores that contain mercury, they pose a relative amount of danger to operators. Mercury can cause severe harm like lung damage or even death if exposure occurs leading to inhalation. Although most Gold refining process use the process of retorting to get rid of most of the mercury in the charge.
There often still remains some that must be gotten rid of by the smelting process. The addition of flux speeds of the removal of this residual mercury thereby ensuring the health and safety of operators and personnel.
Adding flux to a precious metal ore charge can be considered a separate process in itself. There are quite a number of important things to put into consideration. This is to ensure that the addition of flux fulfils its purpose and does not become counterproductive.
Having mentioned the advantages of using Gold smelting flux. It is almost unimaginable to carry out a gold smelting process without making use of flux. We will however still outline the disadvantages of melting gold without flux.
The use of Gold smelting flux in the gold smelting process is considerably intricate. It may even be considered a kind of science in its own right. It should be noted that the information contained here is not everything you need especially in choosing the right flux combinations.
This is however a good starting point to point you in the right direction and help you to select the appropriate Gold smelting flux. With the right combination of flux, your gold smelting process will definitely bring huge returns. The resulting pure bars and ingots will make your brand a force to reckon with in the market.
The extraction of raw minerals begins with the mining of rich ores, which are then cut up in crushers and grinders. The pieces of rock initially weighing tonnes are ground down to a few tenths of a millimetre. This grinding process, which often covers six decimal orders of magnitude of the particle size, is carried out in several steps. Classic crushers are used for the coarse grinding process. The primary and secondary grinding takes place in autogenous (AG) or semi-autogenous (SAG) mills and in ball or rod mills. If the raw material is sufficiently finely distributed, it is classified in wet-chemical flotation cells according to reusable material and waste rock (residual mineral). The material substrate obtained is called concentrate.
The particle size is of particular importance in this process. If the particles are too large, unwanted accessory minerals cannot be separated from the ore. The ore concentrate obtained only has a low purity. Overgrinding, on the other hand, results in high milling costs and low throughput as well as an increased need for chemicals in flotation. Furthermore, flotation cells are sensitive to solid overloading. Another relevant process parameter is therefore the solid concentration of the ore suspensions fed to the flotation cells.
Compliance with the ideal grain size and the solids load in the transition from the last grinding stage for the flotation cell requires monitoring of the ore suspension in real time. This measurement task places high demands on the technology used. It is therefore important, on the one hand, to record relevant and representative sample quantities from the huge streams of the main product. On the other hand, the abrasive mineral slurries flowing at a high speed require robust and wear-resistant sensors.
The OPUS measuring system based on ultrasonic extinction fulfils these requirements and provides simultaneous real-time analysis of both the particle size distribution and the solid concentration. The acoustic measurement method allows the ultrasonic measuring probe to be immersed directly into the volume flow and, in this way, analyses up to 300 litres of undiluted and unconditioned mineral slurry within minutes.
In practice, a combination of primary samplers and a MULTIPLEXER is frequently applied in order to monitor several grinding lines using only one OPUS sensor. Initially, partial streams are diverted from up to four main product streams and led to the MULTIPLEXER. This fully automatically ensures sample feeding to the OPUS-sensor, which is seamlessly integrated in the MULTIPLEXER Use of the MULTIPLEXER also provides a reduced product flow in the OPUS sensor measurement zone, which in turn significantly increases the already very good service lives.
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In hydrometallurgy, fusion techniques are considered key in the processing of chromite ore. Salts containing easily ionized elements are often used as flux, which results in solutions with complicated matrices. A matrix correction approach was developed to remove sodium and phosphate ions from a solution obtained from the fusion of chromite using sodium phosphate salt as flux. The method was first tested on the chrome reference material SARM 131 (Cr2O3 41%) and validated using a chromite sample (Cr2O3 67%). The matrix effects were resolved by first precipitating the phosphates followed by sodium ions using group (II) salts (MgCl2, CaCl2, SrCl2 or BaCl2). Elemental analysis of major elements using ICP-OES revealed average percentages of 40.73% (Cr2O3), 28.45% (Fe2O3), 10.40% (MgO) and 15.45% (Al2O3) in SARM 131, which were within the acceptable range of the certified values, and 65.72% (Cr2O3), 22.95% (Fe2O3), 2.45% (Al2O3) and 3.36% (SiO2) for the chromite sample.
Chiweshe, T.T., Welman-Purchase, M. & Deysel, LM. Fusion of Chromite Ore Using Sodium Phosphate Salt as Flux and the Effects of Sodium Ions in Wet Chemical Analysis. JOM 73, 13441352 (2021). https://doi.org/10.1007/s11837-021-04632-y
YAG:Ce3+ phosphors were synthesized by mechanical method with the addition of YF3.The YAG:Ce3+ phase was obtained at the maximum vessel temperature of 230C.YAG phosphor exhibited maximum internal quantum yield of 55%.
In this study, the effect of YF3 as a flux addition on the mechanical processing low temperature synthesis of Ce3+-doped Y3Al5O12 (YAG:Ce3+) phosphors for white light emitting diodes of next generation lighting was investigated. The YAG phosphors were synthesized by the mechanical method using an attrition-type mill without any extra-heat assistance. When YF3 was added at 10mass% to the raw powder materials and 10min processed, the synthesis of YAG:Ce3+ was favorably achieved at the vessel temperature of 230C. The internal quantum yield of YAG:Ce3+ phosphor was evaluated by a quantum yield measurement device. The synthesized YAG:Ce3+ phosphor revealed the maximum internal quantum yield of 55%.
A coupled thermal/fluid/chemical/ablation method is proposed.Oxidation of pyrolysis gases in boundary layer have influences on surface ablation.The coupled method can more accurately determine the surface recession rate.
The surface ablation of charring composites is critical for estimating the performance of the thermal protection system of a hypersonic vehicle during reentry. A coupled thermal/fluid/chemical/ablation method is proposed to solve the surface ablation of charring composites with the pyrolysis. Comparing to the previous method, it considers that the chemical reactions between the pyrolysis gases and the oxidative gases in the boundary layer have further influences on the oxidation of surface char in the Parks model. The new mathematical models are discretized by using the center and up-wind formats, and solved by the FORTRAN and MATLAB codes written. The numerical results indicate that the coupled method shows a validation in solving the surface ablation by comparing to the experimental data, and it can more accurately determine the surface recession rate of charring materials. This study will be helpful for the design of the thermal protection systems in hypersonic reentry vehicles.
Earth crust is the source of many elements. Out of these elements, 70% are metals. Aluminium is the most abundant metal of earth crust and iron comes second. The percentage of different elements in earth crust are
Elements which have low chemical reactivity or noble metals having least electropositive character are not attacked by oxygen. moisture and CO2 of the air. These elements, therefore, occur in the free state or in the native state, e.g., Au, Ag, Pt, S, O, N, noble gases, etc.
Removel of unwanted materials (e.g., sand. clays, etc.) from the ore is known as ore concentration, ore dressing or ore benefaction. It can be carried out by various ways depending upon the nature of the ore.
The process by which lighter earthy impurities are removed from the heavier ore particles by washing WIth water is called levigation. The lighter impurities are washed away. Thus. this method is based on the difference in the densities (specific gravities) of ore and gangue.
This method is used for the concentration of sulphide ores. This method is based on the preferential wetting of ore particles by oil and that of gangue by water .. As a result. the ore particles become light and rise to the top in the form of froth while the gangue particles become heavy and settle down. Thus. adsorption is involved in this method.
Depressants These are used to prevent certain types of particles from forming the froth with air bubbled, e.g., NaCN can be used as a depressant in the separation of ZnS and PbS ores. KCN is an another depressant.
This method of concentration is employed when either the ore or the lmpurities associated with it are magnetic in nature. e.g., chromite, FeCr2O4, containing magnetic SiliCIOUS gangue and wolframite FeWO4,Containing cassiterite, 8nO4 (non-magnetic impurities) can be separated by this method.
This method is used for the separation of lead sulphide (good conductor) which is charged immediately in an electrostatic field and is thrown away from the roller from zinc sulphide (poor conductor) which is not charged and hence, drops vertically from the roller.
Leaching is the process in which the ore is concentrated by chemical reaction with a suitable reagent which dissolves the ore but not the impurities, e.g., bauxite is leached with a hot concentrated solution of NaOH which dissolves aluminium whileother oxides (Fe2O3, TiO2, SiO2), remain undissolved and noble metals (Ag and Au)are leached with a dilute aqueous solution of NaCN or KCN in the presence of air.
(ii) Roasting It is the process of converting an ore into its metallic oxide by heating it strongly. below its melting point m excess of air. This process is commonly used for sulphide ores and is carried out in blast furnace or reverberatory furnace. Roasting helps to remove the non-metallic impurities and moisture.
Metals which are low in the activity series (like Cu, Hg, Au) are obtained by heating their compounds lD air: metals which are in the middle of the activity cries (like Fe. Zn, Ni, Sn) are obtained by heating their oxides with carbon while metals which are very high in the activity series (e.g., Na, K, Ca, Mg, Al) are obtained by electrolvtic reduction method.
During smelting a substance. called flux is added which removes the non-fusible impurities as fusible slag. This slag is insoluble in the molten metal and is lighter than the molten metal. So, it floats over the molten metal and is skimmed off.
(iv) Auto reduction This is used for reduction of sulphide ores of Pb, Hg, Cu, etc. The sulphide ore is heated in a supply of air at 770-970 K when the metal sulphide is partially oxidised to form its oxide or sulphate which then reacts with the remaining sulphide to give the metal.
vi) Electrolytic reduction or electrometallurgy It is the process of extracting highly electropositive (active) metals suchas Na, K, Ca, Mg, Al, etc by electrolysis of their oxides, hydroxides or chlorides in fused state, e.g., Mg is prepared by the electrolysis of fused salt of MgCl2 (Dows process).
2. A metal will reduce the oxide of other metals which lie above it in Ellingham diagram, i.e., the metals for which the free energy of formation (Gf) of their oxides is more negative can reduce those metal oxides which has less negative Gf
(i) Liquation This method is used for refining the metals having low melting points (such as Sn. Pb, Hg, Bi) than the impurities, The impure metal is placed on the sloping hearth and is gently heated. The metal melts and flows down leaving behind the non-fusible impurrties.
(i) Poling This method is used when the impure metal contains impurities of Its own oxide, e.g., CU2O in blister copper and SnO2 in impure Sn. The molten impure metal is stirred with green wood poles. At this high temperature. wood liberates gases such as CH4 which reduces any oxides present in the metal.
On passing electricity, the pure metal gets deposited on the cathode while the insoluble impurities settle down below theanode as anode mud or anode sludge. Metals like Cu, Ag, Au, Cr, Zn, Ni, etc are purified by this method.
(iii) Zone-refining This method is based upon the principle of fractional crystallisation, i.e., difference in solubilities of impurities in molten and solid state of metal. Semiconductors like silicon, germanium, gallium arsenideand indium antimonide are purified by this method. Elements of very high purity are obtained by this method.
(iv) Vapour phase refining In this method, crude metal is made free from impurities by first convertmg it Into its volatile compound by heating with a chemical reagent at low temperature. After this, the volatile compound is decomposed by heating to some higher temperature to give pure metal.
(v) Chromatographic method Adsorption chromatography is generally used. The impure metal is dissolved in a suitable solvent and the solution is allowed to run slowly into an adsorbent column packed with alumina (Al2O3). The metal and the impurities present are adsorbed at different rates. These are then eluted with suitable eluent (solvent). In this method.
Wrought iron or malleable iron is the purest form of commercial iron and is prepared from cast iron by oxidising impurities in a reverberatory furnace lined with haematite. This haematite oxidises carbon to carbon monoxide.