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Determination of precious metals in ores by nickel matte gold assay
Source: Mine Road Network 217-7-242784
Nickel matte gold assay (also known as nickel sulfide gold assay) uses matte composed of nickel sulfide, iron sulfide (and copper sulfide) to capture
precious metals. It can also be used for other high-sulfur and high-nickel samples that are difficult to smelt by gold assay. Nickel sulfide has enough density (5.3g/cm3) to be easily separated from slag and easily crushed. Nickel matte buckle has a strong ability to capture precious metals, and the weight of gold test buckle above 12g can completely capture precious metals in 5g samples. The silicic acid degree of slag in nickel matte gold assay is preferably between 1.5 and 2.. However, it should be noted that the acidity of the slag should not be too large, otherwise the slag is very sticky, which is not conducive to the separation of the matte and the slag. Generally, the matte is dissolved by hydrochloric acid, and sulfur escapes into hydrogen sulfide. Copper, iron and nickel (including silver) enter the solution in the form of chlorine complexes, and gold and platinum group metals remain in the residue. When the matte button is dissolved by concentrated hydrochloric acid, the loss of osmium is relatively large, and platinum and palladium will also lose to some extent. The loss of these elements can be reduced by dissolving with dilute hydrochloric acid or by closed dissolution. The reproducibility of gold determination by nickel matte assay is not very good, which includes both the collection efficiency of gold and the loss of gold when it is dissolved in hydrochloric acid. The recovery rate and reproducibility of gold can be improved by tellurium precipitation method. Nickel matte gold assay needs to add sulfur as reducing agent and vulcanizing agent, and the amount of sulfur should be appropriate. If less is added, the loss of osmium and ruthenium will increase when the buckle is dissolved with hydrochloric acid. Too much brings difficulties to the dissolution of the gold test buckle. In order to avoid excessive sulfur, iron sulfide can be used to replace part of sulfur, and such a test gold buckle can be pulverized by itself in water.
1. Characteristics of matte nickel assay
1. Advantages of matte nickel assay
(1) It can capture all platinum group elements.
(2) The composition of different types of samples has relatively little change.
(3) The samples containing sulfur and nickel do not need to be removed in advance.
(4) The ratio of flux to sample is small, so a large number of samples can be processed.
(5) Nickel sulfide button can be directly used for laser ablation.
2. Limitations of matte nickel assay
(1) The blank is high. Sometimes the blank value of some elements can reach hundreds of pg/g levels. Especially, the blank of nickel reagent is high, so it is suggested to use nickel carbonyl powder with high purity.
(2)Os volatilizes in the form of OsO4.
(3) When nickel sulfide button is dissolved with hydrochloric acid, some precious metal elements such as Ru and Pd will be lost by volatilization in the form of chloride or chlorine-containing complex.
(4) The recovery of individual elements will be poor due to the low collection efficiency of nickel sulfide or the incomplete separation of tellurium precipitation (<: 9%）。
(5) When the nickel sulfide button is dissolved by hydrochloric acid, a large amount of hydrogen sulfide gas will be produced, which requires effective flue gas exhaust equipment.
the reagent blank can be reduced by reducing the gold test button of nickel sulfide. The previous nickel sulfide gold assay method generally needed to add more than p>1g of nickel, but now it is about a few grams (depending on the content range of platinum group elements in the sample and the properties of the sample matrix).
ii. batching by nickel matte assay
batching is a key step in nickel matte assay analysis. First of all, we should know the types of samples to determine the formula of the melt. According to the composition of the material in the sample, according to the expected silicic acid degree of the slag, the addition amount of various reagents in the ingredients can be obtained by calculating the reaction formula.
1. The general range of ingredients for general geological samples is
(1) Rock, sediment and soil, such as Shi Ying, pyroxene, olivine, calcite, sedimentary rock, soil, river sediments, marine sediments, etc., and the formula is generally (2g samples): Na2CO3: 2-25g. Flour: .5-1g.
(2) Ore samples, such as chromite, ultrabasic rocks, pyrite, chalcopyrite, sphalerite, nickel ore, etc. The formula is generally (2g sample): Na2B4O5(OH)4 or Li2B4O7:25g, Na2CO3:15-2g, Ni:. Sodium metaphosphate can be added to completely melt chromite, and its flux formula is: sample 1g, SiO2 9g, (napo3) x15g, li2b4o73g, Ni7.5g, S4.5g. The melting temperature must reach 12℃. < P > 2. Key points of adjusting the formula according to different materials
(1) Silica in silicate samples accounts for more than half.
(2) Carbonate samples: The main components of this kind of samples are calcium carbonate and magnesium carbonate, and carbon dioxide is decomposed to become calcium oxide and magnesium oxide when melting the samples, so it is necessary to add more acidic flux silica and more borax when melting the samples.
(3) Oxidized ore samples: Oxidized ore samples refer to samples containing more hematite and magnetite, which have certain oxidizing power and can consume part of reducing agent, and should be considered in batching.
(4) Sulfide ore sample: contains more sulfides and has strong reducing power. More sodium carbonate should be added to reduce the amount of sulfur.
Copper concentrate, copper-nickel sulfide ore, stibnite ore, nickel ore, pyrite and other minerals often contain platinum group elements and gold and silver, so it is relatively difficult to melt the ore, so it is necessary to increase the amount of sodium carbonate and silicon dioxide in batching.
iii. application examples
determination of precious metals in ores by matte nickel assaying -ICP-MS
most matte nickel assaying -ICP-MS analysis processes do not include the determination of osmium, because osmium is oxidized into osmium tetroxide, which is lost by volatilization. In the past, osmium was distilled first, and then the residue was dissolved with aqua regia to measure osmium. The process of this method is too long, which is not conducive to large-scale sample analysis. An improved method for determination of platinum group elements by nickel matte gold assay-tellurium * * * precipitation ICP-MS, which combines closed dissolution of noble metal sulfide filter residue with isotope dilution method, solves the determination of all platinum group elements and gold including osmium, avoids distillation separation and/or separate determination of osmium, and simplifies the analysis process. The key points of the method are as follows:
1. Sample treatment steps
(1) Sample 2g into a glass triangular flask, add mixed flux, shake and mix thoroughly, and then transfer to a clay crucible.
(2) add an appropriate amount of osmium diluent accurately, cover a small amount of flux, and put it into a muffle furnace heated to 11℃ for melting for 1.5h
(3) Take out the crucible, pour the melt into the iron mold, cool it, take out the matte nickel button and crush it. Transfer to a beaker, add 6mL concentrated hydrochloric acid, and heat and dissolve until the solution becomes clear and no longer bubbles.
(4) Add 1 mL of tellurium precipitant (containing .5mg of tellurium) and 1 mL of 1mol/L tin dichloride solution, heat it for .5h and leave it for several hours to coagulate tellurium.
(5) negative pressure filtration with .45μm filter membrane and 2mol/L hydrochloric acid washing precipitation for several times.
(6) Transfer the precipitate and the filter membrane into a Teflon sealed sample dissolver with a screw cap, add 1 mL aqua regia, seal it, and dissolve it on an electric heating plate at about 1℃ for 2-3h.
(7) After cooling, transfer it into a 1 mL colorimetric tube, and the water will be measured at constant volume.
2. determination of platinum group elements
the sample solution was directly determined by ICP-ms. RuRhPdIrPt was determined by standard solution. Os was determined by isotope dilution method. The internal standard is 1ng/ mL Cd and Tl standard solution. Although sealed slag melting method is adopted, isotope dilution determination of osmium is still essential. On the one hand, because the sealing dissolution can not ensure that there is no gas leakage; On the other hand, even if there is no leakage loss of osmium in the dissolution process, because of the great difference in sensitivity of osmium with different oxidation degrees in ICP technology, the standardization of standard solution will cause great errors in the analysis results.
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