GB2147284A - Process for obtaining a high surface area magnetite concentrate - Google Patents

Abstract

A mixture of iron oxides obtained by pyrite roasting is subjected to magnetic separation. The magnetic fraction so obtained is a magnetite concentrate which is highly efficient in absorbing hydrogen sulfide evolved from oil drilling wells. It can be incorporated into drilling muds even in non-alkaline medium.

Description

SPECIFICATION Process for obtaining a high surface area magnetite concentrate This invention relates to obtaining magnetite (Fe3O4) having a high surface area, and the ability to absorb hydrogen sulfide efficiently.

Often, during drilling operations in oil or gas wells, exhalations of hydrogen sulfide occur, which can cause damage to the personnel involved in the drilling operation. Concentrations as low as 1 ppm may irritate mucous membranes, and cause headaches, and nausea. Short exposure to high concentrations can even lead to death. Furthermore, hydrogen sulfide is a drawback to the environment, and corrodes equipment, chiefly linings, which may be weakened and leak. When drilling columns are broken, drilling must be stopped, causing big losses to the oil company.

In order to eliminate or reduce to reasonable levels the hydrogen sulfide present in drilling sites, various materials have been used. The alkalinity of the drilling mud itself serves in part to control the sulfides.

Copper salts, such as carbonates (preferably, cupric carbonate) are commoniy used as well as zinc salts, such as the basic carbonate of zinc and, more recently, zinc chelates. These compounds are efficient scavengers of soluble sulfides such as H2S, HS- and S2- up to certain concentrations above which the reaction products may change the rheology of the drilling mud. In such case, the use of these scavengers is rendered inadequate. The use of certain synthetic iron oxides with special characteristics is then indicated, especially hematites and magnetites of high surface area. The term "magnetite" means any mixture in which Fe3O4 predominates, and "hematite" any mixture in which Fe2O3 predominates.

The reaction between Fe3O4 and H2S, in acidic or neutral medium, forms pyrite, whose insolubility in hydrogen chloride is greatly advantageous, in case of acidification of the oil well.

In US patent 4008775 there is described an iron oxide product made essentially of Fe304, whose particles are sponge-like. This product is highly effective for absorbing large amounts of hydrogen sulfide. This synthetic product is obtained by the oxidation of carbon-bearing iron or iron wastes, under strictly controlled oxidation conditions, the particles being further de-agglomerated through a special crushing technique. This product is marketed under the trademark "lronite Sponge".

US patent 4324298 discloses a compound made essentially of high surface area Fe2O3, the compound being prepared through conventional oxidation of ferrous sulfate at high temperature, followed by quenching. Nevertheless, no proof of the efficiency of such a product is known when used in connection with drilling rigs.

The present invention provides a simple method of preparing magnetite with a high content of magnetic iron oxide, high porosity, and a high capacityfor quick absorption of hydrogen sulfide in drilling muds.

This invention provides a process for producing a high surface area magnetite concentrate which comprises subjecting a residue obtained by roasting pyrite to magnetic separation to produce a magnetic fraction of concentrated magnetite, and a non-magnetic fraction.

In the accompanying drawing, Figure lisa flowsheet of a pyrite roasting process with the process of the present invention in its preferred form.

By "pyrite" is meant a pyritous material i.e., a mineral made basically of iron sulfides of the type FeS2 (pyrite) and FeS (pyrrotite), besides the arsenopyrites (FeAsS). Typically, the iron content of pyrites is from 40 to 44%. The carbonaceous pyrites are those made by the physical treatment of coals and which retain some carbon not removable by this treatment.

Pyrite roasting aims at producing sulfuric acid or elemental sulfur. Iron oxides for ironworks are also produced. With the arsenopyrites, the main product is often valuable metals such as gold, which may be included in the. Pyrite roasting is a high temperature oxidation of the sulfides, typically at 850"C, conducted in the presence of air and in the absence of liquid phase, i.e., without melting. The gas produced contains 13.5% of sulfur dioxide and a solid, roasted product. It is believed that the following series of reactions is indicative of the order in which the oxidation reactions occur:

the formula FeO.Fe2O3 being equivalent to Fe3O4.

Depending on the operation conditions, the behavior of the substances involved in the roasting can vary widely. At present, a fluidized bed reactor is the most economical and competitive equipment in which to conduct roasting. The partial pressures of the gases, oxygen and sulfur dioxide, temperature and residence time are of utmost importance and control of them provides oxidation to various degrees.

In Figure 1, the pyrite fluidized bed reactor is fed with the pyrites at a rate of about 19 tons/day and per m2 of feeder. Also, the reactor is blown with air, at a rate of about 1900 Nm3/h and per m2 of feeder. Typically, the pyrites size range is: 3% from 4 to 6 mm, 15% from 2 to 4 mm, 25% from 1 to 2 mm, 30% from 0.5 to 1 mm and 27% below 0.5 mm. Part of the contents of the reactor evolve as effluent gases 3, which contain entrained solids and are directed to a heat recovering boiler 4. Nearly 30% of the roasted product exists as a reactor residue 2 - i.e., the reactor overflow. Water enters and vapor leaves the heat recovery boiler. Also, about 40% of the roasted product is discharged as boiler residue 5.Solid particles of smaller size than those of the boiler residue are entrained by the gaseous flow 6which exits the boiler and is directed to the cyclone 7. The top flow of the cyclone contains a small percentage of solids fines and is directed to an electrostatic settler 13. The cyclone residue 8 is discharged through the bottom and makes up about 27% of the roasted product. From the electrostatic settler 13 exit the gases which are washed in a gas washer. About 3% of the roasted gases is discharged into the electrostatic settler.

The solid roasted residue is a mixture of purple iron oxides (purple ore). The boiler takes up the larger entrained particles, the cyclones take up chiefly the particles of next reduced size, and the settler collects the fines.

The boiler residue Sand the cyclone residue 8 are useful in the practice of the present invention.

Preferably, it is the cyclone residue 8which is subjected, after cooling in air at about 50"C, to magnetic separation, since, besides being richer in magnetic substance, unlike the boiler residue it does not require grinding and this serves better the objects of the present invention, when the magnetic concentrate is to be used in the absorption of hydrogen sulfide in drilling muds. Also, the surface area of the magnetic fraction from the cyclone residue is nearly always larger than that of the magnetic fraction from the boiler residue.

This preferred form of the practice of the present invention is shown in Figure 1, where the cyclone residue 8 is led to a magnetic separator 10 where the separation into a magnetic fraction 11 (about 57 weight %, in a typical operation condition) and a non-magnetic fraction 12 (about 43 weight %) occurs.

Part of the cyclone residue can be separated by particle size to enhance the magnetic substance content of the concentrate, since the larger size fractions have a higher content of magnetic material. For example, the 40 weight % of larger size (above 73 microus) contain, on the average, 68% of magnetic substance, whereas all the cyclone residue has on the average 57.3% of magnetic material.

Any adequate magnetic separator (wet or dry) can be used in the present invention, for example one of low intensity of the type David tube model E.D.T.

The magnetic separation produces the magnetite concentration of high surface area which is the product of the present process.

In order to evaluate the performance of the magnetite concentrate of the present invention, the following Examples describe tests of reactivity and consumption, in comparison with commercial products.

Example 1- Reactivity 1.40 g of Na2S.9H2O were dissolved in 47.5 ml of water and adjusted to pH 8.5. The content A of sulfide ion was determined in an aliquot of 5 ml. To the remaining solution was added excess of test product (2.5 g), allowing the reaction to proceed for one hour at room temperature. An aliquot of 5 ml was collected in 4 ml of Na2CO3 solution of pH of about 12. After addition of 2 g of NaCI and centrifugation, the content B of S2- ion was determined in the supernatant layer.

For each product the reactivity index AA B was calculated and the results are listed in Table 1. In the table, the meaning of each symbol is: - COAT 1131 refers to a commercial salt mixture containing chromium and zinc; - cyclone means the cyclone residue; - boiler means the boiler residue; 270 after "cyclone" or "boiler" refers to the size fraction from 53 to 74 microns (270 to 200 mesh).

TABLE 1 product Reactivityindex, % COAT1131 100 "IroniteSponge" 100 Overall cyclone magnetic 100 Cyclone 270 magnetic 99.5 Overall boiler magnetic 91 Boiler 270 magnetic 82 Overall cyclone non-magnetic 68 Cyclone 270 non-magnetic 90 Overall boiler non-magnetic 63 Boiler 270 non-magnetic 67 Example II - Kinetics The procedure of Example 1 was repeated with the exception that instead of taking only one aliquot after one hour reaction, four aliquots were taken, after 15 minutes, 30 minutes, 45 minutes and 60 minutes of reaction. The treatment of each aliquot was identical.

For each aliquot was calculated the non-consumed fraction B/A, and the results are listed in Table II. The meaning of the symbols is the same as for Table 1.

TABLE II Product % ions S2' non-consumed after t(min) t=O t= 15 t=30 t=45 t=60 COAT1131 100 3 2 1 0 "IroniteSponge" 100 33 12 4 0 Overall cyclone magnetic 100 34 13 4 0 Cyclone 270 magnetic 100 34 14 4 0.5 Overall boiler magnetic 100 71 67 50 37 Boiler 270 magnetic 100 72 4 53 33 Overall cyclone non-magnetic 100 69 53 45 32 Cyclone 270 non-magnetic 100 70 44 23 10 Example Ill- Sulfide Consumption 1.40 g of Na2S.9H2O were dissolved in water, diluted to 55 ml and NaHCO3 was added up to pH 8.5. A 5 ml aliquot was taken and its content of sulfide ion was determined. To the remaining solution was added test product in limited amount - W = 0.100 g allowing the reaction to proceed for one hour at room temperature.

A 5 ml aliquot was taken into 4 ml of Na2CO3 solution of pH of about 12.2 g of NaCI were added, the whole was centrifuged and the content B of S2- ion was determined in the supernatant.

The consumption of S2- ion per gram of test product was calculated: Consumption of S = A - B = g S2/g product, W x 20.000 where A and B are expressed in mg/1 and W is in grams.

The results are listed in Table III.

TABLE III Product Consumption of S2- ions (g S2Vg product) COAT 1131 0.120 "Ironite Sponge" 0.085 overall cyclone magn. 0.090 cyclone 270 magn. 0.105 Example tV-Consumption of H2S in various media, at pH = 7.0 A nitrogen gas stream containing hydrogen sulfide from the acidulation of sulfide solution was bubbled into 100 ml of test liquid or slurry, the test sample containing 0.1 g of the test product. The emerging gas was collected and its hydrogen sulfide was absorbed in 50 ml of a 0.1 N NaOH solution. After one hour the content of sulfide in the NaOH solution was determined.The consumption of H2S was calculated: X consumption = X = Y g H2S/g product, where W X = initial amount of S2- ions, in the sulfide solution to be acidulated, mg; Y = amount of 2. ions, in the NaOH solution, mg; W = weight of test product to be tested, mg.

For each test product, the procedure was carried out in three liquids or slurries: A- 1% NaHCO3solution B - slurry based on sea water, starch treated C - slurry based on KCI.

The results are listed below in Table IV.

TABLE IV Product H2S consumption, g H2Slg product A B C COAT 1131 0.13 - "Ironite Sponge" 0.75 0.42 0.91 *overall cyclone magn. 0.72 0.47 0.65 **overall cyclone, magn. 0.63 0.48 0.50 * 1 sot experiment **2nd experiment From the Examples it can be inferred that the product of the present invention is better than the "Ironite Sponge" in alkaline and neutral media, starch-treated, for slurries based on seawater.

Claims (10)

1. A process for producing a high surface area magnetite concentrate which comprises subjecting a residue obtained by roasting pyrite to magnetic separation to produce a magnetic fraction of concentrated magnetite, and a non-magnetic fraction.

2. A process according to claim 1, in which the residue is a residue of a carbonaceous pyrite.

3. A process according to claim 1 or 2 in which the residue is a residue of a pyrite used as a raw material in the manufacture of elemental sulfur or sulfuric acid.

4. A process according to any of claims 1 to 3 in which the said residue is the solid product of a solid-gas separation of the products of the roasting.

5. A process according to claim 4, in which the products of the roasting subjected to the solid-gas separation are products entrained by a gaseous stream from a heat recovering unit.

6. A process according to claim 4 or 5, in which the said residue is a portion, separated by particle size, of the total solid products of the solid-gas separation.

7. A process according to claim 4, 5 and 6 in which the solid-gas separation is carried out by a cyclone.

8. A process according to claim 1 substantially as hereinbefore described with reference to the accompanying drawing.

9. High surface area magnetite produced by the process of any of the preceding claims.

10. A drilling mud comprising high surface area magnetite as claimed in claim 9.