LIME
The lime is cementitious material received by roasting and the subsequent processing of limestone, chalk and other lime- magnesian rocks. Pure lime is the colorless product, badly dissolved in water (about 0,1 % at 20С), density about 3,4 g/cm3.
Depending on a chemical compound and curing conditions lime can be subdivided on air, curing in air-dry conditions, and hydraulic which gets cure on air and in a water
Air lime is received mainly by roasting of a limestone with the small maintenance of a clay (up to 8 %) at 1100-1300 C in mine or rotating roasting furnaces. Thus the carbonates that make up breed decay, for example as: СаСО3 СаО + СО2. Depending on the content of MgO in breed can be distinguished following kinds of lime: calcic (contains up to 5 % by weight of MgO), magnesian (5-20 %) and dolomitic (20-40 %). Depending on a way of processing of the burnt product the one receive solid not slaked (quicklime), not slaked ground and slaked (hydrated) lime, or hydralime, and also the limy dough. The first represents a mix of pieces of the various sizes formed after a rough grinding of a roasting product. By a chemical compound it consists of СаО and MgO with a small impurity of a not decayed at roasting СаСО3, and also of silicates, aluminates and ferrates of a calcium. Not slaked ground lime - a product of a grinding of solid lime. Slaked lime - finely dispersed dry powder received by interaction of a solid or a ground not slaked lime with a small amount of water or vapour (by extinction), consists mainly from Са(OH) 2 and Mg (OН) 2 with impurity of СаСО3. At lime extinction by a considerable quantity of water the yielding dough, so-called lime paste, is formed.
Activity of an air lime as a cementitious material is defined by the general content of a calcium and magnesium oxides. The greatest activity possesses calcium lime, containing 93-97 % of oxides. High-quality grades of a lime ("fat lime") are characterized by the big outturn of the limy dough (more than 3,5 L for 1 kg of quicklime), the more is the outturn of a dough, the more it yielding and can take in a considerable quantity of a sand at a preparation of building mortars.
By the speed of extinction one can distinguish quick-slaking lime (duration of a process no more than 8 minutes), mean-slaking (no more than 25 minutes) and slow-slaking lime (more than 25 minutes). The time from the moment of mixing of a powder is accepted as a speed of extinction. The lime with water till the moment of attaining of the maximum temperature of a mix.
Offered lime of the "Global Chemicals Company" organization with content of a СаО - 99,82 %, and MgO - no more than 5 % belongs to air not slaked and calcium lime with high activity. This lime is available in the given organization in the form of pieces and in the milled kind.
Usually in enrichment it is used hydraulic lime of a low activity, i.e. the content of a basic components СаО and MgO is less than 50 %, it contains much silica, silicates, alumina, etc. These impurities are ballast forming additional sludge, stirring to process of enrichment of sulphide ores. Therefore use of high activity lime considerably reduces the price of enrichment process of a technogenic complex ores of color noble and rare metals. Now we will consider the role of a lime in the process of enrichment.
If to use pure СаО to which belongs the lime of the "Global Chemicals Company" organization with content of СаО - 99,82 % then a copper extraction in copper, lead in lead, zinc in zinc and of a pyrite in a pyrite concentrates at the average increases by 8-10 %, in comparison with the use of a 70 % activity lime. Thus the expense of a high activity lime essentially decreases. The received concentrates for removal of an organic floatation agents should be processed by a sodium sulphide (Na2S) with the expense of 6-8 kg/t. Sodium sulphide is a toxic substance, besides, has an unpleasant sharp smell. Recently instead of this toxic reagent ecologically safe product representing a mix of a slaked lime Ca (OH) 2 with sulphur in certain parities is being used. Quality of this mix strongly depends on activity of a lime, the more is activity of a used lime, the more fully and better it is cleared from organic floatation agents.
The lime is widely applied in flotation of floatation agents. From the chemical point of view it is the strong basis. In this connection it is applied as a regulator of environment. Along with it the lime is a specific depressor (suppresses flotation) of a pyrite, that is connected with the formation on its surface of almost insoluble hydroxides - Fe (OH) 2 and Fe (OH) 3. Usually oxidation in the alkaline environment goes more intensively at the expense of aeration, thus on a surface of a pyrite except ferric hydroxide also can be formed FeSО4. Ions of Са2 + can co-operate with ions on the SО42 surface forming almost insoluble connection plaster, i.e. СаSО4 2Н2О, that can promote also to suppression of a pyrite flotation. This process is necessary for division of a pyrite from other sulphidic minerals. In most cases lime is used as a mix with other substances.
Joint application of lime with sodium sulphite is connected with interaction of a calcium ion with a sulphite or hydrosulphite ion. It is difficult to assume, that "cementation" of a sulphidic surface can be caused by formation on a surface or in a solution volume of a valent saturated straight-chain compounds of calcium of a CaSхOу (CaSO3, CaS2O3, CaSO4, etc.) type, though such compounds also can be kept, basically, at the expense of physical power of sorption. Position according to which the influence of salts of calcium on flotation of sulphides is caused by "cementation" of a sulphidic surface by intermediate connections of calcium with components of the surface seem to be more true. This process, thus, can be considered as chemical interaction of products of oxidation of a mineral sulphur on a surface with СаОН +. Univalency of a sulfo oxide anion can be used for connection with ferrum cation on a surface; another one is interconnected with Са (IT) + ion. These ions appear as follows: СаО + Н2О = Са (IT) 2, Са (IT) 2 СаОН + + It - СаОН + Са2 + + It-.
At a cyanideless mode of division of a copper leaded concentrate the basic reagents-depressors of a galenite (PbS) are ions of sulphite, calcium and ferric sulfate. At use of lime reagents up to рН=9-9,5 depression of a galenite is insignificant. At increase of рН over 9,5 can be observed sharp deterioration of a galenite flotation, that can be a consequence of increase in concentration in system of ions OH-, as concentration of ions Са2 + in these conditions should decrease. In this case there is an increase of concentration of ions СаОН +, and there is a formation of Са (IT) 2 in strongly basic media, which in reaction with the sulphite-ion adsorbed on a mineral raises hydrophilic property of a surface.
In ferrous metallurgy not slaked lime is used in the form of pieces as a flux in pig-iron manufacture by domain way, i.e. for removal of impurities from a flux, such as, silica, partially sulphur and phosphorus in the form of slag
Formation of slags can be presented as follows: CaO + SiO2 = CaSiO3 - silicate slag; 3CaO + P2O5 = Ca3 (PO4) 2 - phosphoric slag; CaO + SO3 = CaSO4 - sulfuric slag.
A complex polymetallic sulphidic ores is being processed at the concentrating factories of Kazakhstan. Useful components in these ores are following metals: copper, lead, zinc, iron, and insignificant little there are precious metals: silver and gold. The basic metals are in a form of sulphides, for example, technical names and formulas of minerals of copper, lead, zinc and iron: pyritous copper (CuFeS2), chalcocite (Cu2S), covelline (CuS), bornite (Cu4FeS4); galenite (PbS), sphalerite(ZnS) and pyrite (FeS2). The content of these minerals in ore by weight depending on its structure fluctuates within 1-4 %. After enrichment with floatation agents' quantity of useful minerals increases at 10-15 time and turns out, so-called, collective concentrate. Now, by regulating рН Medias and adding suppressors it is possible to divide a collective concentrate into separate concentrates: copper, lead and pyrite. For example, for division of pyrite a lime needs to be added and leaded up рН to 11-11,5 to suppress a lead and zinc concentrate together with lime sodium sulphite (Na2SO3) can be added and рН needs to be reduced to 8-8,5, a copper concentrate should be floated. For flotation of a zinc concentrate one raises рН with lime up to 10-10,5 by simultaneous addition of the activator of a sphalerite CuSO4. Here it is necessary to notice, that for qualitative division of these concentrates it is desirable to use lime with high activity, i.e. with the big content of a CaO. Since the CaO reacting with water creates the alkaline media on reaction CaO + H2O = Ca (OH) 2 and suppresses pyrite. Mix of Ca (OH) 2 + Na2SO3 is a suppressor of galenite and sphalerite at рН 8-8,5.
In ferrous metallurgy as a flux is used limestone (СаСО3), not slaked lime (СаО) of various activities. However limestone is a natural product, that means a lot of impurities, content of СаО no more than 40 %, therefore offered almost pure not slaked lime СаО of the "Global Chemicals Company" organisation leads to getting of a high-quality pig-iron, in this case the content of impurity in pig-iron decreases for 10-12 %.
| Floatation agents laboratory chief, RSE "Center of sciences about the Earth, metallurgy and enrichment " Committee of science of DES RK, Tusupbaev N.K. Cand.Chem.Sci., associate professor. |
2.1. Building lime should be produced according to these standard requirements on the process regulations confirmed in an order established by the ministry-manufacturer.
2.2. The materials applied during manufacture of building lime: carbonate strata, mineral additives (slags granulated domain or phosphoric slag, active mineral additives, quartz sand), should meet requirements of corresponding operating standard documents.
2.2.1. Mineral additives should be added in powdered building lime in the quantities supposed by requirements to the content in it of the active CaO + MgО under p. 2.4.
2.3. Air not slaked lime without additives can be subdivided into three grades 1, 2 and 3, not slaked powdered with additives - on two grades 1 and 2 and hydrated (slaked) without additives and with additives - on two grades 1 and 2
2.4. Air lime should correspond to the requirements specified in tab. 1.
| Index name | Standard for lime, %, by weight | ||||
| Unslaked calcic | hydrated | ||||
| grade | grade | ||||
| 1 | 2 | 3 | 1 | 2 | |
| Active СаО+МgО, not less than:
without additives with additives |
90 65 |
80 55 |
70 - |
67 50 |
60 40 |
| Active MgO, not more than | 5 | 5 | 5 | - | - |
| СО2, not more than:
without additives with additives |
3 4 |
5 6 |
7 - |
3 2 |
5 4 |
| Unhydrated grains, not more than | 7 | 11 | 14 | - | - |
Notes:
1. In brackets content of МgО for dolomitic lime is specified.
2. CO2 in lime with additives can be defined by gas extensional method.
2.4.1. Humidity of a hydrated lime should not be more than 5 %.
2.4.2. If on a separate indicators lime corresponds to different grades a rating can be defined by a size of the indicator corresponding to the lowest grade.
2.5. Air lime without additives with the Quality mark appropriated in a due course, should meet the requirements of 1 grade with following changes:
coefficient of variation of the content of an active CаO and МgO should not be more than 3 %;
content of unhydrated grains for calcic lime should not be more than 5 %, for magnesian and dolomitic - more than 8 %;
content of an active СаО and MgО for hydrated lime should be not less than 70 %, humidity - no more than 4 %.
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