Fire test method analysis test

Fire assay method (The fire assay method) is the metallurgical principles and techniques applied to a classical analytical chemistry methods analysis, it is one of the oldest methods of chemical analysis.

The fire test method is a method of smelting ore and metallurgical products with a flux to quantitatively determine the content of precious metals therein. The method has good representative sampling, wide applicability of the method, the advantages of good stacking effect, silver and gold is an important means of chemical analysis of precious metals.

5.1 Features of The Fire Assay Method

The fire test is not only an ancient means of enriching gold and silver, but also an important means of gold and silver analysis. Geological, mining, and gold and silver smelters at home and abroad use it as the most reliable analytical method for production. Some countries have this method as a standard method, of the gold concentrate, concentrate and measure the copper gold jewelry, gold crude gold, but also as a national standard methods. With the development of science and technology, more and more new technologies for analyzing gold and silver, and analytical instruments are becoming more and more advanced. Compared with other methods, the fire test method has longer operating procedures and requires certain skills. There are many analysts. Try to use other analysis methods instead of fire test methods. However, the fire test method is irreplaceable. For the determination of gold content in high-content gold raw materials or pure gold, the accuracy and accuracy are not as good as other direct determination methods. In the arbitration analysis of gold and silver content, fire The analysis of the test can give results that are convincing to the parties to the dispute. This is due to the unique advantages of the Fire Test Method that many other analytical tools do not have:

(1) The sampling is good. Gold and silver are often present in the sample in the order of <g/t in a non-uniform manner. The fire test method has a large sample size, generally 20 to 40 g, and even samples up to 100 g or more. Therefore, the sample is representative. The sampling error can be reduced to a minimum.

(2) Wide adaptability. Can adapt to almost all of the samples from the ore concentrate, crude gold into gold, fire assay gold and silver can be accurately measured, including those currently used wet analysis also can not be resolved, including stibnite. For the analysis of the main components of pure gold, the analysis of the fire test can also obtain satisfactory results, except for a very small number of samples, this method can be adapted to almost all minerals.

(3) The enrichment efficiency is high, more than 10,000 times, and a small amount of gold and silver can be quantitatively enriched into the test deduction from dozens of samples containing a large amount of matrix elements, even if the micro-gram of gold and silver is enriched, the loss is also Very small, usually only a few percent. Since the composition of the granulated (or enriched slag) is simple, it is advantageous for subsequent measurement by various test means.

(4) The analysis results are reliable and accurate. Routine analysis of pure gold (>99.9%) by RAND Corporation of South Africa, 74 results of the same sample, standard deviation (S) 0.0058%. The S of 10 domestic analysis results of similar products is also about 0.005%. Over the years, some scholars at home and abroad have attempted to completely replace the fire test method with new wet chemical analysis or instrumental analysis, but have not succeeded so far. Werbicki et al. compared the three analytical methods of Au in solution—AAS, ICP-AES, and gold assays—and gave the standard deviation S for each of the 18 laboratory analyses. The results were ICP-AES and AAS. Basically consistent, but they are slightly worse than the test method. Wall pointed out that the fire test method is applicable to samples with a quantity of gold <1μg~1g, and the accuracy and precision are better than other instruments.

5.2 Principle of the Fire Test Method (Principle of Method)

Fire test analysis is actually a test method of enamel or ash dish as a container, a wide variety, different operating procedures, lead test, 铋 test gold, tin test, 锑 test gold, nickel sulphide test, Assaying copper sulfide, copper-nickel assaying iron, copper assay, like iron assay. However, the melting principle of various new test methods and the reaction in the gold test process still have many similarities with the lead test method. Among all the fire test methods, the most common and most important application is the lead test method. The advantage is that the obtained lead buckle can be ash blown. The lead gold method combined with the ash blowing technique allows the precious metals in dozens of grams of sample to be concentrated in a few milligrams of compositing. In the lead test method, the capture rate of Au is >99%, and the recovery rate of Au as low as 0.2-0.3 g/t is still high. The accuracy of lead test for constant and trace precious metals is very high. The following is a brief introduction to the principle of fire test by taking the lead test method as an example.

The lead fire test method is mainly divided into three stages:

(1) Melting. It is mixed with rock, ore or smelting products by means of solid reagents, heated and melted in a crucible, and trapped gold, silver and precious metals in a molten state by lead to form a lead alloy (generally called lead buckle, also called noble lead). Due to the large proportion of lead alloy, it sinks to the bottom of the crucible. At the same time, the sample and base metal oxide and silica gangue, borax, sodium carbonate, etc. The reaction flux occurring compound, a silicate or a borate generated slag, because of its small specific gravity and floating on top Thereby, gold and silver are separated from the sample. Therefore, in the fire test process, two functions of decomposing the sample and enriching the precious metal are simultaneously performed.

(2) Ash blowing. The obtained lead alloy is placed in a ash dish and ash is blown off at a suitable temperature. When ash is blown, the lead is oxidized to lead oxide and penetrates into the porous ash dish, thereby removing lead in the lead buckle and a small amount of bismuth metal, gold. Silver and precious metals are not oxidized and remain in the ash dish to form gold and silver granules.

(3) Dividends. The gold and silver particles are dissolved in nitric acid to dissolve the silver, and the gold remains solid. The obtained gold particles are weighed and quenched, and the gold content can be calculated. The silver content can be determined according to the difference between the gold and silver particles and the gold mass. .

After the fire test method completes the separation and enrichment of gold, silver and precious metals, in addition to the above-mentioned method for determining the weight of gold and silver, after the gold and silver granules are dissolved by aqua regia, gold, silver and other precious metals can be determined by various chemical analysis methods. .

The theoretical basis of the fire test can be summarized into five aspects.

First, the correct use of chemical reagents to reduce the melting point, to ensure that the minerals with good fluidity can be obtained at the temperature reached by the gold furnace.

Second, the high-temperature molten metal lead has a great ability to capture gold, silver and precious metals, and can completely melt the gold and silver exposed in the molten state in lead.

Third, the metal lead and the slag have different specific gravity, and the lead in the molten metal sinks to the bottom to form a lead buckle, and the slag floats thereon, thereby achieving good separation of lead and slag.

Fourth, lead is easily oxidized at a certain temperature, and lead oxide can be absorbed by the fine porous ash dish, and gold and silver cannot be oxidized to form a granule-retained ash dish.

Fifth, by the difference in solubility of gold and silver in nitric acid, gold and silver are separated, silver forms silver nitrate into the solution, and gold is weighed to calculate the grade of gold.

5.3 Espressors and equipment commonly used in fire test methods (Equipments) 5.3.1 Vessels (1) Test 坩埚

The crucible used for the test smelting is generally called the test gold crucible, and the material is refractory clay. The general requirement for the test enamel is: it has sufficient refractory degree, that is, it does not become soft or collapse when heated at high temperature; it can maintain sufficient pressure while heating, and will not break when clamped or forked. It can resist the chemical action of the melt and is not subject to corrosion by various melts including strong acid, strong alkali or a large amount of lead oxide.

(2) ash dish

The ash dish is a porous refractory vessel for absorbing lead oxide (or yttrium oxide) when the ash is blown with a lead (or buckle). There are three commonly used cupel: cupel cement, ashes - magnesium cement and sand cupel cupel.

1 Cement ash dish with 400,500 Portland cement, add 8-12% water, mix and press on the ash machine. The composition of the Portland cement contains 60 to 70% of CaO, 4 to 7% of A12O3, 9 to 24% of SiO2, and 2 to 6% of Fe2O3. Cement is a cheap and common material. The cement ash dish is hard and not easy to crack, but the loss of precious metal in ash blowing is larger than the latter two.

2 Ashes and ashes-cement grayware Ashes are obtained by burning, grinding, and burning with the bones of cattle and sheep, in which the organic matter must be completely removed. Its composition is 90% calcium phosphate, 5.65% calcium oxide, 1% magnesium oxide, 3.1% calcium fluoride. The fineness of the ashes is less than 0.147 mm, of which 0.088 mm should account for more than 50%. The ash dish made of pure ash is looser and can be used for ash blowing of crude gold and merging gold. For the gold analysis, the mixed ash dish of ashes and cement is generally used. The ashes and cement are mixed in different proportions, and 8-12% of water is added and pressed on the ash machine. The results of different people doing experiments are different. Some think that 3:7 is good, and some think it is 4:6 or 5:5. The ashes-cement ash dish is harder than a pure ash ash dish, but softer than a cement ash dish. Use the ashes-cement ash dish to ash, the loss of gold and silver is smaller than that of cement ash. The preparation of the ashes is cumbersome, and it can be made after several steps of burning and grinding.

3 Magnesium sand ash dish will grind the calcined magnesia, and it is required that 63% or more pass 0.074mm sieve, and the granules should be 0.2~0.1mm not more than 20%. The ground magnesia should be pressed within a few days, otherwise it will be agglomerated after being left for a long time. Take 85 parts of ground magnesia and 15 parts of No. 500 cement, mix well, add 8-12% water pressure to make the dish. When the ash dish made of magnesia is blown, the loss of precious metal is smaller than the first two.

The main component of magnesia is magnesia, which is a good refractory material and resistant to alkaline fluxes. Lead oxide produced by the lead buckle ash is a very strong alkaline flux. At high temperatures, lead oxide has a strong affinity for silica and can invade the silicate in the ash dish. There are many silicates in the ashes-cement ash dish. After the ash is blown with this ash dish, small pits will appear on the table, and the precious metal will suffer. The use of magnesia ash dish, this phenomenon does not occur after ash blowing, the surface is very smooth.

Gold and silver were ash-baked in three kinds of ash dishes. The gravimetric method was compared in the literature [23], which proved that the loss of the magnesia ash dish was the smallest, the pure ash ash dish and the ashes-cement (1+1) ash dish were the second, and the cement ash dish lost the most. . In recent years, more intuitive experiments have been conducted with Ag110 and Au198 isotopes. Literature [24] reported that Ag110 was measured in the ashes and ash by ash (895 °C) in the ashes and magnesia ash with Ag10 isotope and 5 mg of non-radioactive silver. The results are shown in Table 5-1. The loss of silver in the ash dish is better than that of magnesium. The sand ash dish is 25% larger.

The literature [25] reported using the Au198 isotope as a test to compare the loss of gold in magnesia and ash. The ash was blown at 960 ° C and the results obtained showed that the loss of gold in the ashes was much greater than in the magnesia. The results are shown in Table 5-2.

Table 5-1 Loss of silver in various ash dishes

Table 5-2 Loss of gold in various ash dishes

(3) baking dish

Rectangular porcelain dish for sample roasting to remove S, As, length 120mm, width 65mm, height 20mm, generally put 20 ~ 40g sample, up to 50g.

5.3.2 Equipment (1) Trial furnace and ash blowing furnace

The high-temperature ash blowing furnace used for the test is generally called the muffle furnace. The national materials have been introduced to certain extents and have certain technical requirements. The literature [22] pointed out that "ash blower - a muffle type furnace, which should have air inlets and outlets for air circulation, preferably to preheat the air and allow it to pass stably The furnace temperature can be uniformly heated from room temperature to 1100 ° C. According to the South African data, the test furnace used in the laboratory can be completed at one time when placing the ash dish, and the lead buckle is placed in the ash dish as well. After the ash is blown, all the ash can be Take it out at once.

(2) Balance and weight

The fire test analysis method is a quality analysis method, and the requirements for the test gold balance are relatively strict. The early Japanese double-arm swinging gold balance, the maximum weighing is 1-2g, has stricter requirements on the weight, and is made of platinum- rhodium alloy. Most of the test analysis rooms in China use a precision analytical balance weighing 20g and a sensitivity of 0.01mg. Many units have used precision analytical balances with a sensitivity of 0.001mg. The balance and weight requirements are often corrected. Depending on the size of the workload, the calibration period is preferably one month or one quarter.

(3) Golden basket

Countries have specific regulations for the special gold baskets for the analysis of test funds. Japan is made of platinum or porcelain plates; the former Soviet Union is made of platinum; India's platinum or quartz frame is made up of many small sleeves, which are based on a platinum frame or a fused silica frame. Porous fused silica cup; China's gold analysis chamber is made of platinum or stainless steel.

(4) Grey dish machine and rolling machine

At present, there is no clear requirement for the ash dish machine and the milling machine at home and abroad. It is only required to make the forming pressure of the ash dish consistent when making the ash dish, and the gold and silver alloy pieces should be formed in the same way during the rolling to avoid the increase. Analysis error.

5.4 Main reagents & functions used in fire test

In the fire test method, various reagents are added, and the noble metal to be measured is separated from the matrix component in the sample by melting at a high temperature. The various reagents added have different effects. Some can capture the precious metal in the sample after being chemically treated at high temperature, and it is called a collector; some can melt the sample and combine with the matrix components to form slag such as silicate or borate. It is called flux or flux, slag agent. According to the role of the reagents in the smelting process, the reagents for the test are divided into seven categories: flux, reducing agent, oxidizing agent, desulfurizing agent, vulcanizing agent, collector and covering agent. Some reagents have only one application. For example, SiO2 is only used as an acidic flux, but other reagents have several different uses. For example, PbO is an alkaline flux, a collector and a desulfurizer.

5.4.1 Flux

The role of the flux is to decompose the sample by melting the matrix component such as refractory Al2O3, CaO or silicate in the sample to form a good slag. Flux is classified into three types: acidic, basic and neutral according to chemical properties.

(1) Silica (SiO2), that is, quartz powder, is a strong acid flux.

(2) Glass powder (mainly composed of xNa2O·yCaO·zSiO2) is a commonly used acidic flux that can be used in place of silica powder. In addition to SiO2 containing an acidic component, the glass powder has an alkaline component such as CaO or Na2O, so its acidity is weaker than that of quartz powder, and generally 2 to 3 g of glass powder is equivalent to 1 g of SiO2. Usually, the flat glass is used as a raw material, washed with water and then pulverized in a grinder to 0.246 mm to 0.175 mm.

(3) Borax (Na2B4O7·10H2O) is an active and fusible acidic flux which begins to lose its crystal water at 350 ° C during smelting and rapidly expands. Therefore, the use of excess borax in the furnishing material tends to cause material spillage during smelting, resulting in loss of the sample in the crucible. Borax can form borate with many metal oxides, and their melting point is lower than the corresponding silicate. For example, the melting point of CaSiO2 is 1540 ° C, the melting point of Ca 2 SiO 4 is 2130 ° C, and the melting point of CaO · B 2 O 3 is only 1154 ° C. After adding borax to the batch, the melting point of the slag can be effectively reduced.

(4) Boric acid (H3BO3) is an acidic flux that can replace borax. Boric acid loses moisture after heating, and produces B2O3 with strong slagging ability.

(5) Sodium carbonate (Na2CO3) is a cheap, commonly used alkaline flux that easily forms a sulfate with an alkali metal sulfide during melting, sometimes acting as a desulfurization or oxidation. Anhydrous sodium carbonate starts at 852 °C. Melting, when heated to 950 ° C, began to release a small amount of carbon dioxide and slightly decomposed.

Na2CO3 → △Na2O+CO2

The generated sodium oxide combines with an acidic substance to form a salt.

Na2O+SiO2→△Na2SiO3

(6) Potassium carbonate (K2CO3) Its properties are similar to those of sodium carbonate, and it is also an alkaline flux. Its price is more expensive than sodium carbonate.

(7) Lead oxide (PbO), also known as Huangdan powder, is a strong alkaline flux and is also a sulphurizer, desulfurizer and precious metal collector, so it is widely used in lead test. Lead oxide has a strong affinity with silica and combines with silica at a lower temperature to produce a highly fluid lead silicate. The purpose of applying lead oxide in the fire test method is to capture gold and silver, and the added lead oxide is quantitatively reduced to lead. The gold and silver content must be checked before the lead oxide is used. The gold content should be less than 20×10-6%, and the silver should be less than 2×10-5%. Otherwise you will not be able to use it.

(8) Pb3O4 (Pb3O4), also known as red dan powder, has the same properties, use and quality as lead oxide, but its oxidizing power is much stronger than that of lead oxide.

(9) calcium oxide (CaO) is one kind of basic flux infrequently used, inexpensive, can reduce the proportion of slag, increasing the fluidity of slag, and some workers claimed in assaying chromite, copper-nickel ore sample Add a certain amount of calcium oxide when gold is used.

(10) Calcium fluoride (CaF2) is a kind of neutral flux that is not commonly used. It can increase the fluidity of slag. Calcium fluoride should be added to the ingredients of some chromite or copper-nickel ore.

(11) Cryolite (Na3AlF6) is a neutral flux that is rarely used. High alumina-containing sample assay, cryolite is added to reduce the temperature of the slag.

5.4.2 reducing agent

The role of the reducing agent is to reduce the metal oxide added to the furnish to a metal or alloy, thereby trapping the precious metal. Another effect is to reduce high-valent oxides to low-oxides, which is beneficial for slag formation with silica.

The reducing agents commonly used in assay analysis are carbohydrates, carbons and metallic iron. Carbohydrates include wheat flour, rye flour, corn flour, sucrose, starch, etc., the most commonly used of which is wheat flour. Among the carbon type reducing agents, charcoal powder and coke powder are more commonly used. Metal iron is both a reducing agent and a desulfurizing agent.

Flour (C6H10O5) is a commonly used reducing agent in gold analysis. It loses moisture after being heated, and produces fine amorphous carbon. It can be evenly distributed in the bismuth material. The reduction reaction starts at less than 500 °C, when 600 °C. It has the fastest response time. The theoretical value of the reducing power of flour is 15.3, that is, 1 g of flour can reduce 15.3 g of lead, but in fact only 10 to 12 g of lead can be reduced.

5.4.3 oxidant

The purpose of adding an oxidizing agent is to partially or completely oxidize the sulfide in the sample to an oxide, thereby allowing the metal oxide to enter the slag while avoiding the formation of bismuth sulfide (a mutual solution of various metal sulfides). The precious metal was damaged.

(1) Potassium nitrate (KNO3), also known as saltpeter, is a strong oxidant. In high temperature oxygen evolution time division explained, and the sulfide is oxidized to arsenic compounds like oxides, sulfides control the reducing power of lead oxide in order to obtain a suitable mass of lead button. When using potassium nitrate, the sample must be subjected to an oxidative test first, and then the required amount of potassium nitrate is calculated, which is generally calculated by oxidizing 4 g of metallic lead per gram of potassium nitrate.

(2) Sodium nitrate (NaNO3) The properties are similar to those of potassium nitrate, which is cheap and can replace potassium nitrate.

(3) Lead oxide (PbO) When it is co-heated with heavy metal sulfides, it easily releases oxygen, oxidizes sulfides into oxides (except for sulfides of precious metals and lead), and lead oxide itself is reduced to metal.

5.4.4 Desulfurizer

A desulfurizer is a substance that has a strong affinity for sulfur. It can extract sulfur from its original compound and combine it with sulfur.

(1) Metal iron (iron nail) is a reducing agent and a desulfurizing agent. It can decompose and reduce many metal oxides and sulfides into metal. Generally, it is cut by 5 inches with 8# iron wire, and 2 to 4 are added according to the sulfur content of the test material.

(2) Sodium carbonate (Na2CO3) The desulfurization reaction formula is as follows:

MeS+* is *q. *叨10*批十*Q

The resulting MeO combines with SiO2 to form a silicate slag. Na2S is dissolved in the alkaline slag. The slag containing sulfides dissolves the precious metals to varying degrees, causing loss of precious metals during the smelting process.

5.4.5 vulcanizing agent

A substance that converts metals such as Cu and Ni and their oxides into corresponding sulfides at high temperatures is called a vulcanizing agent. Currently there are two commonly used ones:

(1) Sulfur is a strong vulcanizing agent capable of reacting with copper, nickel, iron or CuO, NiO to form CuS, Ni3S2 and FeS.

(2) Iron sulfide (FeS) can react with oxides of Cu and Ni to form sulfides of Cu and Ni.

5.4.6 Collecting agent

Substances that have the ability to extract precious metals at high temperatures, known as collectors, are typically metals, alloys or ruthenium. The ratio of these substances is significant and finally settles at the bottom of the test. The shape after cooling is like a button, which is called a buckle or a gold buckle. When lead is used as a collector, it is said that the lead metal which traps the precious metal is a lead buckle, and when it is used as a collector, it is called a buckle.

(1) Lead (density 11.34 g/cm3, atomic radius 0.175 nm, melting point 327.4 ° C) is one of the most commonly used and most useful collectors. It has a large specific gravity and is easy to separate from the slag. The metal lead after the precious metal is trapped can separate the lead from the precious metal by a simple ash blowing method, and a simple precious metal granule is obtained, which provides convenient conditions for the next step measurement. . The trapping effect of lead on Ag, An, Pd, Pt, Rh, Ir, Ru, Os is good, most of them are above 98%, and some are slightly lower.

(2) 铋 (density 9.75g/cm3, atomic radius 0.155nm, melting point 271.3°C) and precious metals can form a series of intermetallic compounds or alloys under high temperature conditions, which can quantitatively capture precious metals with good effects. The capture rates of precious metals were: Au 99%, Ag 98%, Pt 98%, Pd 98%, Rh 99%, and Ir 98% Ru 97%. When the squeaking ash is blown, the loss of Os is serious. The toxicity of hydrazine and its compounds is small, which is superior to the lead test method.

(3) Tin (density 7.3 g/cm3, atomic radius 0.158 nm, melting point 231.9 ° C) Eight kinds of precious metals can be trapped. Tin forms intermetallic compounds with Au, Pt, Pd, Rh, Ir, Ru and Os, such as AuSn4, PtSn4, PdSn4, RhSn4, IrSn7, Ru2Sn7, OsSn3 and the like. These interpolymers are enriched in the tin buckle with molten tin.

(4) Nickel niobium (density 4.6 to 5.3 g/cm3, melting point Ni3S2 790 ° C, FeS 1150 ° C, Cu 2 S 1120 ° C, melting point of 800 ° C when three are mixed) Nickel niobium is also called nickel matte. The component which plays a major role is nickel sulfide, and also includes sulfides such as copper iron from the sample (or added). Nickel sulphide is much more capable of trapping precious metals than copper sulphide. Nickel sulphide or nickel ruthenium captures precious metals (except palladium ) with efficiencies above 96% and losses in slag less than 4%.

(5) 锑 (density 6.68g/cm3, atomic radius 0.161nm, melting point 630.5°C) 锑 captures Au, Pd, Pt, Rh, Ir, Ru, Os with good performance, recovery rate of over 97%, in slag The loss is less than 3%.锑 can be ash-blowing, Os does not lose when ash is blown, this is its unique advantage, and it is also beyond the lead and sputum test. While trapping precious metals, cesium and other heavy metals such as Cu, Co, Ni, Bi, and Pb are also trapped, and they cannot be removed by ash blowing. Therefore, the ruthenium test method can only capture the precious metals in a simple sample.

(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 such as Os. The capture effect is very good, the recovery rate is above 98%, and Ir is slightly worse, about 95%. However, the separation of platinum group metals from a large amount of Cu, Fe, and Ni is difficult. The operation process is tedious, and the copper-iron-nickel test requires a high temperature of 1450 ° C, which is difficult to achieve in general test furnaces.

(7) Copper (density 8.89 g/cm3, atomic radius 0.127 nm, melting point 1083 ° C) Using copper as a collector, the recovery of Pd, Pt, Rh, and Ir was 95% or more.

5.4.7 Covering agent

The cover of the covering agent acts as a barrier to air above the material in the crucible to avoid undesired reactions between the air and the material in the furnace. At the same time, it also acts to prevent splashing of the melt and reduce loss during smelting. There are three commonly used covering agents:

(1) Borax It melts first than other materials in the crucible. When it is initially melted, the borax is very viscous and can prevent the loss of the ore sample powder. When borax is combined with the melt, it changes the acidity of the slag. Therefore, this should be noted when using borax as a covering agent.

(2) Salt is a commonly used, inexpensive covering agent. Pb, As, Sb and Au, Ag chlorides are volatile at high temperatures. When a large amount of toxic PbCl2 white smoke emerges at the time of discharge, it pollutes the environment, which is one reason people do not like to use it.

(3) Borax-sodium carbonate. This kind of covering agent has the same performance as borax, but by adjusting the ratio of the two, it can be formulated into the same degree of silicic acid as the material in the crucible, so as not to change the silicic acidity of the slag due to the inclusion of the covering agent into the melt.

(6) nails

It is a reducing agent and a desulfurizing agent. It can decompose many metal oxides and sulfides into metals.

RO + Fe = R + FeO, RS + Fe = FeS + R. Generally, 8# iron wire is used to cut off 5 inches long, and 2 to 4 are added depending on the sulfur content of the test material.

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