I. Preparation of samples
In the sampling of concentrator, the raw ore is generally coarse-grained material and needs further processing. The specific compilation process is shown in figure 10- 1- 1. Mineral processing products are generally wet pulp, and its sample preparation process should be different from that of raw ore. The sample preparation process should be filtered and dried first, and the subsequent treatment process is the same as that of the original ore sample. Before the sample is submitted for inspection, a copy of the deputy sample must be left, and the sample submitted for inspection is the positive sample.
Figure 10- 1- 1 Raw ore sample preparation process
The particle size (thickness) of chemical analysis samples varies with different ore samples. For example, silicate requires 160 ~ 200 mesh, pyrite only requires 100 ~ 120 mesh, and spectral analysis samples all require 200 mesh (0.075mm). See table 10- 1- 1 for the detailed requirements of other ore sample granularity. Samples used for ferrous determination are usually broken to 100 mesh. Generally, the screened samples are mixed evenly by rolling on the sample cloth or moving the cone on the grinding disk. Generally, the quartering method is adopted, and two diagonal samples are taken as positive samples and the other two are taken as sub-samples; You can also use the grid method to continuously separate multiple small samples in a batch. Before the sample is bagged, the sample name, serial number, date, type of element to be analyzed, etc. It should be marked on the sample bag, and the sample preparer should sign the sample bag.
Table 10- 1- 1 sample processing fineness and sample drying temperature of ordinary raw ore
sequential
The quality of samples for chemical analysis is generally 10 ~ 200 g, and the amount of samples required for analysis usually depends on the number of analysis items. The sample required for single element analysis is 15 ~ 20g, and the sample required for two or more elements analysis is 25 ~ 40g. The sample size for donor phase analysis is 50g. For samples requiring multi-element analysis, 100 ~ 200 g is generally required.
When the particle size of the sample is required to be below 150 mesh, it needs to be ground with a grinder. Generally, there are disc refiner, Star Mill, Rod Mill, Vibration Mill and Ball Mill. Grinding methods are divided into dry grinding and wet grinding.
(1) dry grinding: for the samples below 60 mesh, select a suitable grinder to grind them to below 150 mesh at one time. If the sample grinding cannot be done at one time, it should be done in several times. After the fineness of the sample meets the requirements, it should be fully stirred in a suitable mixer. For samples that are easy to be oxidized, such as pyrite, it is necessary to avoid grinding for too long, and the samples will be oxidized when the temperature is too high.
(2) Wet grinding: When chemical analysis samples are bonded in the mill, in order to avoid high-temperature oxidation of the samples, the grinding time should be shortened as much as possible, and hexane is allowed to be used as the chemical medium for wet grinding in the mill. The principle of sample preparation is that its chemical composition and occurrence state must be completely consistent with the original sample. Therefore, the oxidative deterioration of the sample itself and the introduction of foreign impurities should be avoided in the process of sample preparation. For example, when iron is not the main element, grinding tanks made of iron should be avoided as far as possible.
Second, the pretreatment of chemical analysis samples
The pretreatment of the sample is to change the components to be measured in the sample into a state suitable for determination. Usually, after the sample is decomposed, the elements of the component to be detected exist in the solution in the form of soluble salts, or separate out in the form of precipitation (such as gravimetric determination of silicon), thus being completely separated from other components. Some components to be detected are volatilized and separated from the sample in the form of gas, and then absorbed by a suitable reagent or directly lost according to the original amount, and the content of the components to be detected is calculated.
In practical application, according to the properties of iron ore, the requirements of analysis items and the separation of interfering elements, acid dissolution method and alkali melting method can usually be selected.
Commonly used acid dissolution methods are as follows:
(1) hydrochloric acid decomposition: iron ore can generally be decomposed by heating with hydrochloric acid, iron-containing silicate is insoluble in hydrochloric acid, and a little hydrofluoric acid or ammonium fluoride can be added to completely decompose the sample. Magnetite dissolves slowly, and a few drops of stannous chloride solution can be added to accelerate the decomposition.
(2) Sulfuric acid-hydrofluoric acid decomposition: add 1∶ 1 drops of sulfuric acid and 4 ~ 5ml of hydrofluoric acid to the sample in platinum crucible or polytetrafluoroethylene crucible, heat at low temperature until white smoke of sulfur trioxide appears, and then extract with hydrochloric acid.
(3) Decomposition of phosphoric acid or mixed acid of sulfur and phosphorus (1: 2): When dissolving ore, it needs to be heated until the water completely evaporates until white smoke of sulfur trioxide appears, and then heated for several minutes. However, it should be noted that the heating time should not be too long to prevent the formation of pyrophosphate.
At present, alkali melting method is commonly used to decompose samples. Commonly used fluxes are sodium carbonate, sodium hydroxide, potassium hydroxide, sodium peroxide and the mixed flux of sodium peroxide and sodium carbonate (2∶ 1). Smelting can be carried out in silver crucible, nickel crucible, high aluminum crucible or graphite crucible. Sodium peroxide is also semi-melted in nickel crucible.
Because iron ore contains a lot of iron, melting sodium carbonate directly in platinum crucible will damage the crucible, and the dissolved platinum will also affect the determination of iron, so it is rarely used. For iron ore containing sulfide and organic matter, the sample should be burned at 500 ~ 600℃ for 2 hours in advance to remove sulfur and organic matter, then the sample should be decomposed with hydrochloric acid and a small amount of nitric acid should be added to completely decompose the sample. The presence of nitric acid affects the determination of iron, which can be removed by evaporation with hydrochloric acid.