Fire Assay Principle, Method And Equipment

The mysterious, ancient, irreplaceable fire assay

Tiandao Research Institute 2018-01-26 Author: Zhou Xing Yu

The fire assay method is a classic analytical method that applies the principles and techniques of metallurgy to analytical chemistry and is one of the oldest methods in analytical chemistry.

The fire assay method is to quantitatively determine the content of precious metals by adding flux to smelt ores and metallurgical products. This method has the advantages of good sampling representativeness, wide applicability, and good enrichment effect. It is an important means for the chemical analysis of gold, silver, and precious metals.

Features of The Fire Assay

Fire assay is not only an ancient way of enriching gold and silver, but also an important way of gold and silver analysis. Geology, mines, gold and silver smelters use it as the most reliable analysis method and widely used in production. Many countries have set this method as a national standard method for the determination of gold in gold concentrates, copper concentrates, jewelry gold, and alloy gold. With the development of science and technology, there are more and more new techniques for analyzing gold and silver, and analytical instruments are becoming more and more advanced. Compared with other methods, the fire assay has more operating procedures and requires skills. There are many analysts who try to use other analysis methods instead of fire assay. However, the fire assay method is irreplaceable. For the determination of gold content in high-content gold raw materials or pure gold, its accuracy are inferior to other direct determination methods. In the arbitration analysis of gold and silver content, fire Assay analysis can give convincing results for all parties to the dispute. This is because the fire assay has many unique advantages that other analytical methods do not have:

(1) Good sampling representativeness. Gold and silver are often present in samples unevenly in the order of less than gram per ton. The fire assay requires a large sample amount, generally 20-40g, and even samples up to 100g or more can be taken. Therefore, the good representativeness of the sample can reduce the sampling error to a minimum.

(2) Wide adaptability. It can adapt to almost all samples, from ore, gold concentrate to alloy gold, the fire assay method can accurately determine gold and silver, including those antimonites that cannot be solved by wet analysis. For the analysis of the main components of pure gold, the fire assay can also obtain satisfactory results. Except for very few samples, this method can be adapted to almost all minerals.

(3) The enrichment efficiency is high, reaching more than ten thousand times. Fire assay can quantitatively enrich a small amount of gold and silver from dozens of grams of samples containing a large amount of matrix elements into the assay button. Even if micrograms of gold and silver are enriched, the loss is very small, generally only a few percent. Due to the simple composition of the compound granule (or enrichment slag), it is beneficial to use various test methods to determine in the future.

(4) The analysis result is reliable and accurate. South Africa Rand Company's routine analysis of pure gold (>99.9%), 74 analysis results of the same sample, standard deviation (S) 0.0058%. The S of the 10 analysis results of similar domestic products is also about 0.005%. Over the years, some scholars at home and abroad have tried to completely replace the fire assay with new wet chemical analysis or instrumental analysis, but they have not succeeded so far. Werbicki et al. compared the three analytical methods of Au in solution-AAS, ICP-AES and the assay method, and gave the standard deviation S of each method analyzed by 18 laboratories. The result is ICP-AES and AAS. Basically the same, but they are all slightly worse than the trial gold method. Wall pointed out that the fire assay is suitable for samples with a gold content of <1μg~1g, and its accuracy and precision are better than other instrumental analysis.

The Principle of Fire Assay

5.2 Basic Principles of Fire Assay
Fire assay analysis is actually a gold assay method with crucible or cupel as a container. There are many types and different operating procedures. There are lead assays, bismuth assays, tin assays, antimony assays, nickel sulfide assays, Copper sulfide test, copper-iron-nickel test, copper test, iron test, etc. However, the smelting principles and reactions in the various new assay methods still have many similarities with the lead assay method. Among all the fire assays, the most commonly used and most important is the lead assay, which has the advantage that the resulting lead can be cupellation. The combination of lead assay and cupellation technology can enrich the precious metals in dozens of grams of samples in aggregates weighing several milligrams. In the lead assay, the collection rate of Au is >99%, and the recovery rate of Au as low as 0.2~0.3g/t is still very high. The analysis accuracy of the lead assay for both macro and trace precious metals is very high. The following is a brief description of the principle of the fire assay with the lead test method as an example.
The lead fire assay is mainly divided into 3 stages:
(1) Smelting. It uses solid reagents to mix with rocks, ore or smelted products, heat and melt in a crucible, and use lead to capture gold, silver and precious metals in a molten state to form lead alloys (generally called lead buttons, also called precious lead). Due to the high specific gravity of the lead alloy, it sinks to the bottom of the crucible. At the same time, the base metal oxides and gangue in the sample react with fluxes such as silica, borax, and sodium carbonate to form slag such as silicate or borate, which floats on it due to its small specific gravity. This separates the gold and silver from the sample. Therefore, during the fire assay process, both functions of decomposing samples and enriching precious metals are played.
(2) Cupellation. Put the obtained lead alloy in a cupel at an appropriate temperature for cupellation to remove lead. During cupellation, the lead is oxidized to lead oxide and penetrates into the porous cupel, thereby removing the lead in the lead button and a small amount of base metal. Gold silver and precious metals are not oxidized and remain in the cupel to form gold and silver particles.
(3) Divide. Use nitric acid to dissolve the gold and silver alloy particles to dissolve the silver while the gold remains solid. After quenching, the obtained gold particles are weighed to calculate the gold content. The silver content can be calculated according to the difference between the quality of the gold and silver particles and the quality of the gold.
After the fire assay method completes the separation and enrichment of gold, silver and precious metals, in addition to the above-mentioned gravimetric method for determining gold and silver, after dissolving the gold and silver particles with aqua regia, a variety of chemical analysis methods can be used to determine gold, silver and other precious metals.

The theoretical basis of fire assay can be summarized into five aspects.
(1) the correct use of chemical reagents reduces the melting point and ensures that minerals with good fluidity can be obtained at the temperature reached by the assaying electric furnace.
(2) metallic lead melted at a high temperature has a great ability to collect gold and silver and precious metals, and can completely melt the gold and silver exposed in the molten state in the lead.
(3) the specific gravity of metallic lead and molten slag are different. The lead sinks to the bottom during melting to form a lead button, and the slag floats on it, realizing a good separation between the lead button and the molten slag.
(4) lead is easily oxidized at a certain temperature, and at the same time, lead oxide can be absorbed by the dense and porous cupel, and gold and silver cannot be oxidized to form composite particles and remain in the ash pan.
(5) the difference in the solubility of gold and silver in nitric acid is used to separate gold and silver. Silver forms silver nitrate and enters the solution. The gold grade can be calculated by weighing the gold.
Utensils and equipment commonly used in fire assays
(1) Assay crucible
The crucible used for assault smelting is generally called the assault crucible, and the material is refractory clay. The general requirement for the assay crucible is: it has sufficient refractory degree, that is, the crucible does not soften or collapse when heated at high temperature; it can still maintain sufficient pressure when heated, and it will not break when it is picked up or fork out. ; It can resist the chemical action of the melt, and will not be corroded by various melts including strong acid, strong alkali or containing a large amount of lead oxide, causing the crucible to leak.
(2) Cupel
Cupel is a porous refractory ware used to absorb lead oxide (or bismuth oxide) when cupellation lead buttons (or bismuth buttons). There are three commonly used cupels: cement cupel, ashes-cement cupel and magnesia cupel.
①Cement cupel uses 400, 500 Portland cement, add 8-12% water, mix well, and press on the cupel machine. The composition of Portland cement is CaO 60~70%, A12O3 4~7%, SiO2 19~24%, Fe2O3 2~6%. Cement is an inexpensive common material. The cement cupel is hard and not easy to crack, but the loss of precious metals during ash blowing is larger than the latter two.
②Cream ashes and ashes-cement ashes. The ashes are obtained by burning, grinding and then burning the bones of cattle and sheep, and all organic matter must be removed. Its composition is 90% calcium phosphate, 5.65% calcium oxide, 1% magnesium oxide, and 3.1% calcium fluoride. The fineness of ashes should be less than 0.147mm, of which 0.088mm should account for more than 50%. The cupel made of pure ashes is looser and can be used for cupellation of crude gold and alloyed gold. Assay analysis generally uses a mixed cupel of ashes and cement. The ashes and cement are mixed in different proportions, and 8-12% water is added and pressed on the cupel machine. Different people have different results. Some think that 3:7 is good, and some think that 4:6 or 5:5 is good. The ashes-cement ashes are harder than the pure ashes, but softer than the cement ashes. Using ashes-cement cupel to cupellation, the loss of gold and silver is smaller than that of cement cupel. The preparation of ashes is more troublesome and requires several processes of burning and grinding.
③ Magnesia cupel Grind the calcined magnesia finely, and it is required that more than 63% pass through a 0.074mm sieve, and the particles with a particle size of 0.2~0.1mm should not exceed 20%. The finely ground magnesia should be pressed within a few days, otherwise it will agglomerate after being left for a long time. Take 85 parts of finely ground magnesia and 15 parts of No. 500 cement, mix them, add 8-12% water and press to make a cupel. The loss of precious metals during ash blowing in the cupel made of magnesia is smaller than that of the first two.
The main component of magnesia is magnesium oxide, which is a good refractory material and can resist corrosion by alkaline flux. The lead oxide produced during lead ash blowing is a very strong alkaline flux. At high temperatures, lead oxide has a strong affinity with silicon dioxide and can invade the silicate in the cupel. The ashes-cement cupel contains more silicates. After blowing with this cupel, small pits will appear on the surface of the cupel, and precious metals will be lost due to this. Using magnesia cupel, there is no such phenomenon after ash blowing, and the surface is very smooth.
Gold and silver are blown in three kinds of cupels. The weight method is compared in the literature [23], which proves that the loss of magnesia cupel is the smallest, followed by pure ashes and ashes-cement (1+1) cupels, and cement cupels have the largest loss. . In recent years, some people have made more intuitive experiments with Ag110 and Au198 isotopes. Literature [24] reported that using Ag110 isotope and 5mg of non-radioactive silver in ashes and magnesia cupels (895 ℃), measuring Ag110 in the ashes, the results are shown in Table 5-1, the loss of silver in ashes is more than magnesium The sand cupel is 25% larger.
Literature [25] reported the use of Au198 isotope as a test to compare the loss of gold in magnesia and ashes. Cupellation at 960°C, the results obtained show that the loss of gold in the cupel is much greater than the loss in the magnesia cupel. The results are shown in Table 5-2.

Table 5-1 Loss of silver in various ash dishes
Ashtray type                       Cupel weight (g)  Loss of silver in the grayware (%) Average (%)
Magnesia (1 inch diameter) 25 2.2 2.2 2.6 2.3
Magnesia (1 inch diameter) 25 2.3 2.4 2.4 2.4
Ashes (1 inch diameter)                 25 2.9 2.9 3.2 3.0
Magnesia (1.5 inches in diameter) 45 2.4 2.4 2.4 2.4
Table 5-2 Loss of gold in various ash dishes
Cupel type Magnesia Magnesium Magnesium   Magnesium   Magnesium Ashes
British system British system British system British system  French system
Number of measurements 18              18 17 18 18
Average loss (%) 0.821 0.396 0.908            0.754 3.432
Standard deviation           0.220 0.097 0.260 0.156 1.731
Coefficient of variation (%) 26.8          24.6 28.7 21.1 50.4
(3) Baking Crucible
Rectangular porcelain crucible, used for sample roasting to remove S and As, length 120mm, width 65mm, height 20mm, generally 20-40g sample, up to 50g.
(4) Assay furnace and cupellation furnace
The high-temperature cupellation furnace used for assaying is generally called the muffle furnace. The materials of various countries have been introduced to a certain extent and have certain technical requirements. Literature [22] pointed out that " cupellation furnace—a muffle type furnace. This furnace should have air inlets and outlets for air circulation. It is best to preheat the air and allow it to pass stably. The furnace temperature can be evenly heated from room temperature to 1100 ℃. According to South African data, the assay furnace used in its laboratory can be completed when placing the cupel at one time, and the same is true for placing the lead button in the cupel. After the cupellation is completed, all the cupel can be completed at once take out.
(5) Balance and weight
The fire assay method is a quality analysis method, which has strict requirements on the assay balance. Early Japanese double-arm swing-type gold test balances, with a maximum weighing of 1-2g, had more stringent requirements for weights, and they had to be made of platinum-iridium alloy. Assaying and analysis laboratories use precision analytical balances weighing 20g and a sensitivity of 0.01mg, and many units have used precision analytical balances with a sensitivity of 0.001mg. Balances and weights require frequent calibration. According to the workload, the calibration cycle should be one month or one quarter.
(6) Divider
Each country has specific regulations for the gold divider dedicated to assay analysis. Japan uses platinum or porcelain plates; the former Soviet Union uses platinum; India uses platinum or quartz frames, which are composed of many small sleeves. These small sleeves are made of porous platinum cups based on platinum frames or fused silica frames. Porous fused silica cups; Chinese assay chamber is made of platinum or stainless steel plates.
(7) Cupel machine and tablet mill
Most of the domestic and foreign countries have not made clear requirements for the cupel machine and the flake milling machine, but it is required that the cupel molding pressure should be consistent when making the cupel, and the gold and silver alloy sheet should be formed uniformly during the milling to avoid increasing the analyze error.
The main reagents used in fire assays and their functions
The fire assay requires the addition of various reagents to separate the precious metal to be measured from the matrix components in the sample through high-temperature melting. The roles of the various reagents added are not the same. Some can trap the precious metals in the sample after chemical action at high temperature, which is called a trapping agent; some can melt the sample and combine with the matrix components to generate silicate, borate and other slag. It is called flux or flux, slagging agent. According to the role played by the reagents in the smelting process, the reagents used in the test are divided into seven categories: flux, reducing agent, oxidizing agent, desulfurizing agent, vulcanizing agent, trapping agent and covering agent. Some reagents have only one purpose. For example, SiO2 is only used as an acidic flux, but other reagents have several different uses. For example, PbO is both an alkaline flux, a trapping agent and a desulfurizing agent.
1. Flux
The function of the flux is to melt the refractory Al2O3, CaO or silicate in the sample and generate a good slag, thereby decomposing the sample. According to the chemical properties, the flux is divided into three types: acidic, alkaline and neutral.
(1) Silica (SiO2), or quartz powder, is a strong acidic flux.
(2) Glass powder (the main component is xNa2O·yCaO·zSiO2) is a commonly used acid flux, which can be used to replace silica powder. In addition to the acidic SiO2, the glass powder also contains CaO, Na2O and other alkaline components, so its acidity is weaker than that of quartz powder. Generally, 2 to 3g of glass powder is equivalent to 1g of SiO2. Usually flat glass is used as the raw material, washed and dried in water and crushed to 0.246mm~0.175mm in a mill.
(3) Borax (Na2B4O7·10H2O) is a lively and fusible acid flux. It starts to lose its crystal water at 350°C during smelting and expands rapidly. Therefore, excessive use of borax in the ingredients can easily cause material overflow during smelting and cause loss of samples in the crucible. Borax can form borates with many metal oxides, and their melting points are lower than the corresponding silicates. For example, the melting point of CaSiO2 is 1540°C, the melting point of Ca2SiO4 is 2130°C, and the melting point of CaO·B2O3 is only 1154°C. The addition of borax to the ingredients can effectively reduce the melting point of slag.
(4) Boric acid (H3BO3) is an acidic flux, which can replace borax. After boric acid is heated, it loses water and generates B2O3 with strong slagging ability.
(5) Sodium carbonate (Na2CO3) is a cheap and commonly used alkaline flux. It is easy to react with alkali metal sulfide to form sulfate when it is melted. Sometimes it plays a role of desulfurization or oxidation. Anhydrous sodium carbonate starts at 852℃ Melting, when heated to 950°C, it begins to emit a small amount of carbon dioxide and slightly decomposes.
Na2CO3 →△Na2O+CO2
The generated sodium oxide combines with acidic substances to form salts,
(6) Potassium carbonate (K2CO3) has similar properties to sodium carbonate and is also an alkaline flux. Its price is more expensive than sodium carbonate.
(7) Lead oxide (PbO), also known as Huang Dan powder, is a strong alkaline flux, as well as an oxidizer, desulfurizer and a collector of precious metals, so it is widely used in lead assays. Lead oxide has a strong affinity with silicon dioxide, and combines with silicon dioxide at a lower temperature to form lead silicate with good fluidity. The purpose of using lead oxide in the fire assay is to collect gold and silver, and the added lead oxide is quantitatively reduced to lead. The content of gold and silver must be checked before use of lead oxide. The content of gold should be less than 20×10-6%, and the content of silver should be less than 2×10-5%. Otherwise it cannot be used.
(8) Lead tetroxide (Pb3O4), also known as red lead powder, has the same properties, use and quality requirements as lead oxide, but its oxidizing power is much stronger than lead oxide.
(9) Calcium oxide (CaO) is an infrequently used alkaline flux. It is low in price and can reduce the specific gravity of slag and increase the fluidity of slag. Some testers advocate testing in chromite and copper-nickel ore Add a certain amount of calcium oxide to gold.
(10) Calcium fluoride (CaF2) is an uncommon neutral flux, which can increase the fluidity of slag. Calcium fluoride should be added to the ingredients of some chromite and copper-nickel ore.
(11) Cryolite (Na3AlF6) is a neutral flux that is rarely used. When testing samples with high alumina content, adding cryolite can lower the slagging temperature.
2. Reducing agent
The role of the reducing agent is to reduce the metal oxides added in the ingredients to metals or alloys, thereby trapping precious metals. Another function is to reduce high-priced oxides to low-priced oxides, which is beneficial for slagging with silica.
Commonly used reducing agents in assay analysis are carbohydrates, carbons and metallic iron. Carbohydrates include wheat flour, rye flour, corn flour, sucrose, starch, etc. The most commonly used is wheat flour. The more commonly used carbon reducing agents are charcoal powder and coke powder. Metal iron is both a reducing agent and a desulfurizing agent.
Flour (C6H10O5) is a commonly used reducing agent in gold assay analysis. It loses moisture after being heated and produces fine-grained amorphous carbon, which can be evenly distributed in the crucible material. The reduction reaction starts at less than 500°C, when 600°C Its response speed is the fastest at that time. The theoretical value of the reducing power of flour is 15.3, that is, 1g of flour can reduce 15.3g of lead, but in fact only 10-12g of lead can be reduced.
3. Oxidizer
The purpose of adding the oxidant is to partially or completely oxidize the sulfide in the sample to oxide, so that the metal oxide enters the slag, while avoiding the formation of matte (the mutual solution of various metal sulfides) from the sulfide. Precious metals are lost.
(1) Potassium nitrate (KNO3), also known as saltpeter, is a strong oxidant. It decomposes and releases oxygen at high temperature, oxidizes sulfide and arsenide into oxides, and controls the reduction ability of sulfide to lead oxide in order to obtain a lead button of suitable quality. When using potassium nitrate, the sample must be tested for oxidizing power first, and then the required amount of potassium nitrate must be calculated. Generally, it is calculated based on the oxidizing capacity of 4g of metallic lead per gram of potassium nitrate.
(2) Sodium nitrate (NaNO3) is similar in nature to potassium nitrate, and it is cheap and can replace potassium nitrate.
(3) When lead oxide (PbO) is heated with heavy metal sulfides, it can easily release oxygen, oxidizing the sulfides into oxides (except for precious metals and lead sulfides), and lead oxide itself is reduced to metal.
4. Desulfurizer
Desulfurizer is a substance with a strong affinity for sulfur. It can extract sulfur from its original compound and combine with sulfur.
(1) Metal iron (iron nails) is a reducing agent and a desulfurizing agent. It can decompose many metal oxides and sulfides and reduce them to metal. Generally, 8# iron wire is used to cut 5 inches long, and 2 to 4 pieces are added depending on the sulfur content of the test material.
(2) Sodium carbonate (Na2CO3):
The generated MeO is combined with SiO2 to form silicate slag. Na2S is dissolved in alkaline slag. The slag containing sulfide will dissolve precious metals to varying degrees, causing loss of precious metals during the smelting process.
5. Vulcanizing agent
At high temperatures, metals such as Cu, Ni and their oxides can be converted into corresponding sulfides, called vulcanizing agents. There are currently two commonly used:
(1) Sulfur is a strong vulcanizing agent, which can react with metal copper, nickel, iron or CuO, NiO to produce CuS, Ni3S2 and FeS.
(2) Iron sulfide (FeS) can react with Cu and Ni oxides to generate Cu and Ni sulfides.
6. Collecting agent
Substances that have the ability to extract precious metals at high temperatures are called traps, and they are generally metals, alloys or matte. These substances have a high specific gravity and finally settle at the bottom of the gold assay crucible. After cooling, the shape looks like a button, which is called a buckle or a trial button. When lead is used as a collector, the metallic lead that traps precious metals is called a lead button, and when matte is used as a collector, it is called a matte button.
(1) Lead (density 11.34g/cm3, atomic radius 0.175nm, melting point 327.4°C) is the most commonly used and one of the most useful collectors. It has a large specific gravity and is easy to separate from the slag. The metal lead after the collection of precious metals can be separated from the precious metals by a simple ash blowing method to obtain a simple precious metal composite particle, which provides convenient conditions for the next step of determination. . The trapping effect of lead on Ag, An, Pd, Pt, Rh, Ir, Ru, Os is good, most of which are above 98%, and some are slightly lower.
(2) Bismuth (density 9.75g/cm3, atomic radius 0.155nm, melting point 271.3℃) and precious metals can form a series of intermetallic compounds or alloys under high temperature conditions, which can quantitatively capture precious metals with good effect. The capture rates of precious metals are respectively: Au 99%, Ag 98%, Pt 98%, Pd 98%, Rh 99%, Ir 98% Ru 97%.
When the bismuth button is blown, the loss of Os is serious. Bismuth and its compounds are very toxic, which is superior to the lead assay.
(3) Tin (density 7.3g/cm3, atomic radius 0.158nm, melting point 231.9°C) can trap 8 kinds of precious metals. Tin forms intermetallic compounds with Au, Pt, Pd, Rh, Ir, Ru and Os, such as AuSn4, PtSn4, PdSn4, RhSn4, IrSn7, Ru2Sn7, OsSn3, etc. These inter-compounds are enriched in the tin button along with the molten tin.
(4) Nickel matte (density 4.6~5.3g/cm3, melting point Ni3S2 790°C, FeS 1150°C, Cu2S 1120°C, melting point below 800°C when the three are mixed) Nickel matte is also called nickel matte. The main component is nickel sulfide, and also includes sulfides such as copper and iron from the sample (or added). Nickel sulfide has a much stronger ability to capture precious metals than copper sulfide.
Nickel sulfide or nickel matte captures precious metals (except palladium) with an efficiency of more than 96%, and the loss in slag is less than 4%.
(5) Antimony (density 6.68g/cm3, atomic radius 0.161nm, melting point 630.5℃) Antimony has good performance in capturing Au, Pd, Pt, Rh, Ir, Ru, Os, and the recovery rate is over 97%, in the slag The loss is less than 3%. Antimony can blow ash, and Os will not be lost during ash blow. This is its unique advantage, and it is also inferior to lead and bismuth test gold. While antimony traps precious metals, it also traps heavy metals such as Cu, Co, Ni, Bi, and Pb. They cannot be removed during ash blowing. Therefore, the antimony assay can only capture precious metals in simple samples.
(6) Copper-iron-nickel alloy (density 8-9g/cm3, atomic radius: Cu 0.127nm, 0.Ni 125nm, Fe 0.126nm) copper-iron-nickel alloy can simultaneously capture Pd, Pt, Rh, Ir, Ru and 6 kinds of platinum group metals including Os. The trapping effect is very good, the recovery rate is above 98%, and Ir is slightly worse, about 95%. However, it is difficult to separate platinum group metals from a large amount of Cu, Fe, and Ni in the next step. The operation process is lengthy, and the copper-iron-nickel gold test requires a high temperature of 1450℃, which is difficult to reach in the general test furnace.
(7) Copper (density 8.89g/cm3, atomic radius 0.127nm, melting point 1083°C) Using copper as a trapping agent, the recovery rate of trapping Pd, Pt, Rh, and Ir are all above 95%.
7. Covering agent
The covering agent covers the material in the crucible to insulate the air and avoid undesired reactions between the air in the furnace and the material. At the same time, it also plays a role in preventing the splashing of the molten material and reducing the loss during smelting. There are three commonly used covering agents:
(1) Borax is melted earlier than other materials in the crucible. When initially melted, borax is very viscous, which can prevent the loss of mineral-like powder. After borax is combined with the melt, it will change the acidity of the slag. Therefore, you should pay attention to this when using borax as a covering agent.
(2) Table salt is a commonly used and inexpensive covering agent. The chlorides of Pb, As, Sb and Au and Ag are volatile at high temperature, and a large amount of toxic PbCl2 white smoke will be emitted when it is out of the furnace, which pollutes the environment. This is one reason why people do not like to use it.
(3) Borax-sodium carbonate. The performance of this covering agent is the same as that of borax, but by adjusting the ratio of the two, it can be formulated to have the same silicic acidity as the material in the crucible, without changing the silicicity of the slag due to the covering agent entering the melt.
(6) Iron nails
It is a reducing agent and a desulfurizing agent. It can decompose many metal oxides and sulfides and reduce them to metal, RO+Fe=R+FeO, RS+Fe=FeS+R. Generally, 8# iron wire is used to cut 5 inches long, and 2 to 4 wires are added depending on the sulfur content of the test material.

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