GB1575857A - Process for producing gypsum and magnetite from ferrous sulphate and separating the same - Google Patents

Description

(54) PROCESS FOR PRODUCING GYPSUM AND MAGNETITE FROM FERROUS SULPHATE AND SEPARATING THE SAME (71) We, ISHIHARA SANGYO KAISHA, LTD., a Corporation organised under the laws of Japan, of 3-11, Edobori1-chome, Nishi-ku, Osaka, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a process for producing gypsum and magnetite by neutralizing ferrous sulfate or waste sulfuric acid containing ferrous sulfate with a calcium reagent and then separating them as products of good quality.

Ferrous sulfate or waste sulfuric acid containing ferrous sulfate is produced as a byproduct in large amounts in the production of titanium dioxide by the sulfate process and as a by product of pickling in the iron and steel industry.

Various processes for treating them to make them harmless or to recover iron or sulfuric acid from them have heretofore been proposed. Also many processes for precipitating gypsum and iron oxide by neutralizing ferrous sulfate with a calcium regaent have been proposed. For example, U.S. Patent Specification No. 3,375,066 discloses that a waste sulfuric acid containing iron is first neutralized with a calcium reagent to a pH of 2.5 or less to produce gypsum not contaminated with iron and the separated liquor is then neutralized to a pH of 6-10 to produce low grade gypsum and iron oxide. Also, U.S.Patent Specification No. 3,361,665 discloses that waste sulfuric acid pickle liquor is neutralized with lime at a temperature of from 82"C to the boiling point of the reaction mixture and at a pH of 7-S while carrying out an oxidizing operation by bubbling air therethrough to produce gypsum and magnetite which are both filterable. In these prior art processes, however, the problem arises that iron contaminates at least part of gypsum and it is thus difficult to obtain gypsum of good quality in a high yield.

Therefore, an object of the present invention is to provide a process for producing gypsum and magnetite which are both of good quality, simultaneously from ferrous sulfate and then separating and recovering them efficiently and separately. Another object of the invention is to provide a commercial process for converting ferrous sulfate or waste sulfuric acid containing ferrous sulfate to valuable substances. Another object of the invention is to provide an improved process for producing gypsum and magnetite, which are both of good quality, at a low cost.

According to the present invention, there is provided a process for producing gypsum and magnetite in which calcium carbonate is introduced into an aqueous solution containing ferrous sulfate while an oxidizing gas is blown thereinto, the process being carried out at a pH of 5-6 and a temperature of 60 80"C, the resulting gypsum and magnetite being separated and recovered by magnetic separation.

The advantages of the process of the present invention are as follows: (1) the process of the present invention is suited to practice on a commercial scale and is economically advantageous in that gypsum and magnetite both of good quality can be separated and recovered without requiring two-stage neutralization, addition of gypsum seeds, or supply of a large amount of heat.

(2) the product gypsum is substantially not contaminated with iron in spite of the presence of a large amount of iron in the starting material.

(3) The amount of middlings formed in magnetic separation, that is, the amount of fine gypsum particles containing a large amount of iron produced is very small, and the yields of product gypsum and product magnetite based on the sulfuric acid and iron contained in the ferrous sulfate in the feedstock, respectively, are high.

(4) Since substantially all of the iron becomes a readily filterable precipitate, and the filtrate can be discharged into a river or the ocean as such or after very simple aftertreatment. Even if dissolved heavy metals are present, the heavy metals are captured by the magnetite and water pollution can be avoided.

The aqueous ferrous sulfate solution used as the starting material in the process of the present invention may be exemplified by ferrous sulfate or waste sulfuric acids containing a large amount of ferrous sulphate discharged from the production of titanium dioxide pigment by the sulphate process, or waste sulphuric acid from pickling operations in the iron and steel industry, as well as aqueous solutions prepared therefrom. If the waste sulphuric acids are high in free sulphuric acid content, they may be used in leaching of iron ores or may be reacted with a calcium reagent, while avoiding the precipitation of iron, to form gypsum followed by separation of a solid content by filtration before the process of the present invention is applied to them. In any case, the concentration of iron in the starting material is preferably 10-100 g/l, and preferably 2060 g/l.Of course, a small amount of ferric ion may be present, but it is desirable that the ferrous ion is at least 90 mole% of the iron ions.

Calcium carbonate is used as the calcium reagent. If calcium hydroxide (slaked lime) is used as the calcium reagent, it is difficult to obtain coarse gypsum crystals and the resulting magnetite is in the form of fine crystals. As a result, it is difficult to separate gypsum and magnetite sufficiently and it is impossible to obtain gypsum of good quality in a high yield. Finely ground lime stone is generally used as the calcium carbonate.

It is also possible to use dolomite, the calcium content of which is substantially calcium carbonate. In this case, however, light burned dolomite obtained by burning dolomite at S00-9000C so that only the magnesium carbonate contained therein is converted into magnesium oxide is desirable. The calcium reagent is conveniently used as a slurry having a calcium content of about 70 to about 120 g/l measured as CaO.

Air is generally used as the oxidizing gas.

Oxygen containing waste gases or oxygen gas may also be used.

The oxidation and neutralization reaction is preferably carried out by introducing the calcium carbonate into the aqueous ferrous sulfate solution while the oxidizing gas is blown thereinto. This operation may be either a batch type or a continuous process type process. The oxidizing gas is blown into the solution so that the gas may be finely dispersed in the solution. When the conversion of the reaction mixture, that is the Fe3+/total Fe ratio in the resulting slurry reaches 65 to 73 /o and substantially all of the iron has been precipitated, the reaction is completed.

The reaction time is generally 2-6 hours.

In the process of the present invention, the pH of the reaction liquid is maintained at 56, and preferably 5.4-3.6, and the temperature of the reaction liquid is maintained at 60--80"C, and preferably 65-750C during the reaction. Since calcium carbonate is used as a neutralizing agent, there is little possibility of the pH of the reaction liquid rising above 6. If the amount of calcium carbonate supplied is small or the temperature of the reaction liquid is too low however, the pH of the reaction liquid may drop below 5. The temperature of the reaction liquid is elevated to the neighbourhood of the above mentioned temperature range by the heat of reaction. In a hot season, therefore, it is often unnecessary to use supplementary heat.The temperature of the reaction liquid can easily be controlled by the use of a small amount of steam or a high temperature waste gas. Therefore, control of the pH and temperature of the reaction liquid is far easier than in prior art processes. If the pH or temperature is lower than the above mentioned respective ranges, however, the resulting iron oxide precipitate consists mainly of a-FeOOH rather than the desired magnetite and magnetic separation becomes impossible. Also if the pH is too high, feebly magnetic iron oxide is formed and the yield in magnetic separation is remarkably reduced.

The amount of oxidizing gas introduced during the reaction is preferably about 1 to about 8 I/min per litre of the reaction liquid in the case of air.

Also seeds for growing crystals conventionally used in prior art production of gypsum may be used without trouble, but the use of such seeds is not essential in the process of the present invention. In this regard, the process of the present invention is simpler than prior art processes.

The slurry formed in me reaction contains magnetite of 3-2QM in particle size and gypsum in the form of coarse tabular column crystals. Here, the term "magnetite" means ferrosoferric oxide (FeO . mFe2Q, nH2O) or a magnetic iron oxide consisting mainly of ferrosoferric oxide. Thus, the term is a generic term for strongly magnetic crystalline iron oxides. A major part of the magnetic agglomerates into large particles. The coarse particle portion of the magnetite has good magnetic attractability while the fine particle portion or low specific susceptibility portion thereof becomes coarse by magnetic agglo meration in a magnetic field and is attracted to a magnet.

On the other hand, the gypsum is in the form of coarse tabular column crystals. Also, the surface of the crystals not substantially contaminated with iron under the above mentioned reaction conditions and gypsum is not affected by magnetic forces at all.

Such a product slurry thus can easily be separated by magnetic separation into gypsum and magnetite, which can be separately recovered. Usually, the solid content of the slurry is adjusted to 20-200 g/l and then subjected to magnetic separation by a wet magnetic separator.

For the magnetic separation, there can be used general high intensity wet magnetic separators such as a rotary filter-type (manufactured by Nippon Magnetic Dressing Co.), a Jones-type (manufactured by Klockner Humboldt dutz A.G.), an HGMS-type (manufactured by Sala International AB), or an HIW-type (manufactured by Eriez Magnetics Co.). The magnetic flux density in the magnetic field is set to a suitable value within the range of 1,000-15,000 gauss.

Usually, a neutralized slurry is first subjected to magnetic separation at 3,000- 10,000 gauss to separate the slurry into concentrates consisting mainly of magnetite and tailings consisting mainly of gypsum. The concentrates are then subjected to magnetic separation at 1,000-6,000 gauss to separate them into magnetite as the second concentrates and the second tailings (middlings).

Only the first tailings or a mixture of the first tailings and the second tailings is subjected to magnetic separation at 4,000- 10,000 gauss to separate them into gypsum as the third tailings and the third concentrates (middlings). If a combination of several such magnetic separation treatments is carried out, the amount of middlings can be minimized. Further treatments by hydrocyclones, thickeners or filters can be carried out before or after the magnetic separation.

The process of the present invention has been explained above with regard to the utilization of ferrous sulfate or a waste sulfuric acid containing a large amount of ferrous sulfate, but ferrous sulfate can be added to an industrial waste water containing heavy metals such as Cr, Cd, Ni, or Mn, and the resulting mixture can be treated according to the process of the present invention to recover iron as magnetite containing the heavy metals thereby purifying the waste water.

The invention can be put into practice in various ways and a number of specific embodiments will be described to illustrate the invention with reference to the accompanying examples.

Example 1.

A 20 I-volume long vertical reactor of 25 cm internal diameter equipped with a stirrer, an inlet tube for introducing steam and an inlet tube for introducing air near the bottom of the reactor beneath the stirring blade is used. 101 of an aqueous ferrous sulfate solution having an iron content of 55 g/l is charged into the reactor. A limestone slurry containing 135 g/l of CaCO, is added in an amount equivalent to the sulfuric acid content of the aqueous ferrous sulfate solution.

The resulting mixture is stirred to effect a reaction, whilst 50 I/min. of air is blown thereinto and a small amount of a limestone slurry and steam are introduced to maintain the pH in the range 5-6 and temperature of the reaction mixture at 600 to 800C.

When substantially all of the iron has precipitated,. the reaction is completed and a slurry containing gypsum and hydrated iron oxide is obtained. Water is added to this slurry to adjust its solids content to 50 g/l.

Magnetic separation is then carried out by the use of a HIW L-4 type magnetic separator (manufactured by Eriez Magnetics Co.), at a slurry supply rate of 15 I/min and a magnetic flux density of 5,000--10,000 gauss.

Examples 2 to 5.

For comparison, experiments using pH and temperature values for the reaction liquid outside the ranges specified in the present invention and experiments using slaked lime as the calcium reagent were carried out. The slaked lime was used in the form of a slurry containing 100 g/l of Ca(OH)2 and an appropriate amount of conventional gypsum seeds were added in this case.

The conditions and results obtained for the neutralization and oxidation reaction are shown in Table 1 and the conditions and results obtained of magnetic separation are shown in Table 2. Example No. 1 is an example of the present invention while the other experiments are comparative examples.

TABLE 1

Reaction conditions Hydrated iron oxide Calcium Temp. Time Conver- Main Example reagent pH ( C) (hrs) sion (%) composition Shape 1 Lime 5.5 + 0.1 70 # 1 4.5 69::4 FeO.Fe203 Particulate stone 2 " 5.5 * 0.1 55# 1 , 70.0 a-FeOOH Needle 3 " 4.7 # 0.1 70 # 1 " 70.0 " " 4 Slaked 8.1 # 0.1 " 4 67.2 FeO.Fe2O3 Particulate lime 5 " 5.9 # 0.1 " " 69.5 " " TABLE 1 (continued)

Gypsum Particle size Thickness Width Length Example ( ) Shape ( ) ( ) ( ) 1. 3-15 Tabular 50-80 50-80 100-300 column 2. 0.03 x 0.3 " " " " 3.

4. 0.1-1 Needle 3-5 3-5 20-40 5. 0.1 - 0.5 ,, 1-3 1-3 10-30 TABLE 2

Concentrates Magnetic flux Separation % Fe SO3 density content content Example (gauss) Concentrates Tailings (%) (%) 1 5,000 30.5 68.5 67.8 0.4 4 10,000 30.0 60.5 64.0 1.2 5 10,000 25.0 57.5 65.5 û.9 TABLE 2 (continued)

Tailing Fe Fe SO, SO, recovery content content recovery Middlings Example (%) (%) (%) (No) (fiXo) 1. 98.0 0.2 45.7 98.0 1.5 4. 92.0 2.2 44.8 85.5 9.5 5. 78.0 1.8 44.5 80.5 17.5 Note: Magnetic separation is impossible in the case of Examples Nos. 2 and 3.

Example 6.

A 50 m3-volume (effective volume 35 m3) long vertical reactor of 4 m in diameter equipped with a stirrer, an inlet tube for introducing steam and an inlet tube for introducing air was used. 20 m3 of an aqueous ferrous sulfate solution having an iron content of 50 g/l prepared from ferrous sulfate produced as a by-product in the production of titanium dioxide and 9 m3 of a limestone slurry containing 200 g/l of CaCO,s were charged into the reactor. The resulting mixture is stirred and air is blown thereinto at a rate of 6 Nm3/min to effect reaction. During the reaction, a small amount of steam and a supplemental limestone slurry are introduced to maintain the pH and temperature of the reaction liquid at 5.5 t 0.1 and 70 t 20C, respectively.When the conversion of the reaction mixture had reached 70.6% after 4 hours, the reaction is completed. The resulting gypsum is in the form of coarse tabular column crystals having a length of 60-140, a width of 20-40 and a thickness of 20-40 while the resulting magnetite is in the form of large particles of 3-15a in size formed by agglomeration of many particulate crystals.

Water is added to the resulting slurry to adjust its solids content to 50 g/l. A first magnetic separation is carried out by the use of a HIW CF5-type magnetic separator (manufactured by Eriez Magnetics Co.) at 5,000 gauss. The concentrates obtained in the first magnetic separation are again subjected to magnetic separation at 5,000 gauss (the second magnetic separation). The concentrates obtained in the second magnetic separation are again subjected to magnetic separation (the third magnetic separation).

Thus, magnetite as concentrates and middlings as tailings are obtained. The tailings obtained in the first magnetic separation is combined with the tailings obtained in the second magnetic separation, and the combined tailings are then subjected to magnetic separation at 10,000 gauss (the fourth magnetic separation). Thus, the combined tailings are separated into gypsum as tailings and middlings as concentrates.

The resulting products have the following contents: Gypsum: CaSO4. 2H20 97.5%, Fe 0.15%, SO, yield 98.0%.

Magnetite: Fe 67.8%, CaSO4 0.5%, Fe yield 97.4%.

Middlings: Fe 25.0%, CaSO4 49.3%.

Yield 1.8% (based on the dry total weight of solids).

Example 7.

Into the reactor as used in Example 6 is charged 20 m3 of water, and steam is blown thereinto to warm the water to 700 C.

Through an inlet tube which opens at the bottom of the reactor, 6-7 m3/hr of an aqueous ferrous sulfate solution having an iron content of 35 g/l and 1.4-1.6 m3/hr of a limestone slurry containing 300 g/l of CaCQc are introduced intermittently and in parallel with each other. Simultaneously, 6 Nm3/min of air and a small amount of steam are blown into the reactor to maintain the pH and temperature of the reaction liquid at 5.5 + 0.2 and 70 t 20C, respectively. The resulting slurry is continuously run off through an overflow exit at the top of the reactor. The conversion of the slurry is 66.472.5% and the content of iron dissolved in the slurry is 0.1-0.5 g/l.

The crystals of the resulting gypsum grow with the lapse of time. The size of the crystals becomes almost constant in about 30 hours. The gypsum particles consist of a mitxure of coarse tabular column crystals (200 - 1400u X 80 - 340 X 80 -340 ) and particulate crystals. Magnetite is in the form of agglomerate of particulate crystals which has a particulate size of 3 15u.

The resulting slurry is first passed through a hydrocyclone and the resulting fine particle portion is then subjected to a first magnetic separation at 2,000 gauss. The coarse particle portion from the hydrocyclone is mixed with the tailings obtained in the first magnetic separation. The resulting mixture is subjected to a second magnetic separation at 10,000 gauss to separate the mixture into gypsum as tailings and middlings as concentrates. The concentrates obtained in the first magnetic separation are subjected to a third magnetic separation at 2,000 gauss to separate them into magnetite as concentrates and middlings as tailings. Thus, the whole solid content is separated into 61.3% of gypsum, 31.5% of magnetite and 7.2% of middlings.The thus obtained gypsum shows a grade of CaSO4 . 2H2O 97.5% and an SO yield of 92.4%. Also, the thus obtained magnetite shows a grade of Fe2Q, 97.0% after drying at 700C and a Fe yield of 93.5%.

When the physical properties of the gypsum are measured, it is found that it has a normal consistency of 79.5% and a wet strength of 10.25 kg/cm2. Thus, the gypsum is quite comparable to commercial gypsum for gypsum wallboard. Also, as for the magnetite, when it is washed with dilute hydrochloric acid containing 30 g/l of HCI, burned at 700 8000 C, and pulverized and the color and gloss as a pigment of the product is measured, it is found that it has properties similar to those of commercial iron oxide as a red pigment.

Example 8.

Dolomite is pulverized and then burned at 850"C for one hour, and a slurry having a solid content of 100 g/l is formed therefrom.

Into a 300 I-volume reactor having an almost similar shape to that of the reactor used in Example 1 is charged 200 1 of an aqueous ferrous sulfate solution having an iron content of 40 g/l prepared from ferrous sulfate produced as a by-product in the production of titanium dioxide. The aqueous ferrous sulfate solution is oxidized and neutralized with the above-mentioned dolomite slurry at a pH of 5.6 + 0.1 and a temperature of 70 + 20C.

When the conversion of the reaction mixture reaches 67.3%, the reaction is completed.

Magnetic separation is carried out in the same manner as in Example 6. As a result, gypsum having a purity of CaSO4.2H2O 97.4%, magnetite having an iron content of 67.5% and an SO:, content of 0.4%, and middlings having an iron content of 6.2% and an SOn content of 41.9% are obtained in a ratio of 55.6:42.2:2.2.

WHAT WE CLAIM IS: 1. A process for producing gypsum and magnetite in which calcium carbonate is introduced into an aqueous solution containing ferrous sulfate while an oxidizing gas is blown thereinto, the process being carried out at a pH of 5-6 and a temperature of 60 800 C, the resulting gypsum and magnetite being separated and recovered by magnetic separation.

2. A process as claimed in Claim 1 in which the calcium carbonate is ground limestone.

3. A process as claimed in Claim 1, in which the calcium carbonate is ground light burned dolomite.

4. A process as claimed in Claim 1, 2 or 3 in which the oxidizing and neutralizing operation is carried out at a temperature of 65-750C.

5. A process as claimed in any one of Claims 1 to 4 in which the separation of gypsum and magnetite is carried out by a wet magnetic separator at a magnetic flux density of 1,000 to 15,000 gauss.

6. A process as claimed in Claim 1 substantially as specifically described herein with reference to any one of Examples 1, 6, 7 or 8.

7. A method of purifying aqueous liquids containing heavy metals which comprises adding ferrous sulphate to the said liquid, then adding calcium carbonate while an oxidizing gas is blown thereinto, the oxidizing and neutralizing operation being carried out at a pH of 5-6 and a temperature of 60 to 800 C, the resulting gypsum and magnetic being separated and recovered by magnetic separation, the heavy metals being concentrated in the magnetite.

8. A method as claimed in Claim 7 in which the heavy metals are chromium, cadmium, nickel or manganese.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. The resulting slurry is first passed through a hydrocyclone and the resulting fine particle portion is then subjected to a first magnetic separation at 2,000 gauss. The coarse particle portion from the hydrocyclone is mixed with the tailings obtained in the first magnetic separation. The resulting mixture is subjected to a second magnetic separation at 10,000 gauss to separate the mixture into gypsum as tailings and middlings as concentrates. The concentrates obtained in the first magnetic separation are subjected to a third magnetic separation at 2,000 gauss to separate them into magnetite as concentrates and middlings as tailings. Thus, the whole solid content is separated into 61.3% of gypsum, 31.5% of magnetite and 7.2% of middlings.The thus obtained gypsum shows a grade of CaSO4 . 2H2O 97.5% and an SO yield of 92.4%. Also, the thus obtained magnetite shows a grade of Fe2Q, 97.0% after drying at 700C and a Fe yield of 93.5%. When the physical properties of the gypsum are measured, it is found that it has a normal consistency of 79.5% and a wet strength of 10.25 kg/cm2. Thus, the gypsum is quite comparable to commercial gypsum for gypsum wallboard. Also, as for the magnetite, when it is washed with dilute hydrochloric acid containing 30 g/l of HCI, burned at 700 8000 C, and pulverized and the color and gloss as a pigment of the product is measured, it is found that it has properties similar to those of commercial iron oxide as a red pigment. Example 8. Dolomite is pulverized and then burned at 850"C for one hour, and a slurry having a solid content of 100 g/l is formed therefrom. Into a 300 I-volume reactor having an almost similar shape to that of the reactor used in Example 1 is charged 200 1 of an aqueous ferrous sulfate solution having an iron content of 40 g/l prepared from ferrous sulfate produced as a by-product in the production of titanium dioxide. The aqueous ferrous sulfate solution is oxidized and neutralized with the above-mentioned dolomite slurry at a pH of 5.6 + 0.1 and a temperature of 70 + 20C. When the conversion of the reaction mixture reaches 67.3%, the reaction is completed. Magnetic separation is carried out in the same manner as in Example 6. As a result, gypsum having a purity of CaSO4.2H2O 97.4%, magnetite having an iron content of 67.5% and an SO:, content of 0.4%, and middlings having an iron content of 6.2% and an SOn content of 41.9% are obtained in a ratio of 55.6:42.2:2.2. WHAT WE CLAIM IS:

1. A process for producing gypsum and magnetite in which calcium carbonate is introduced into an aqueous solution containing ferrous sulfate while an oxidizing gas is blown thereinto, the process being carried out at a pH of 5-6 and a temperature of 60 800 C, the resulting gypsum and magnetite being separated and recovered by magnetic separation.

2. A process as claimed in Claim 1 in which the calcium carbonate is ground limestone.

3. A process as claimed in Claim 1, in which the calcium carbonate is ground light burned dolomite.

4. A process as claimed in Claim 1, 2 or 3 in which the oxidizing and neutralizing operation is carried out at a temperature of 65-750C.

5. A process as claimed in any one of Claims 1 to 4 in which the separation of gypsum and magnetite is carried out by a wet magnetic separator at a magnetic flux density of 1,000 to 15,000 gauss.

6. A process as claimed in Claim 1 substantially as specifically described herein with reference to any one of Examples 1, 6, 7 or 8.

7. A method of purifying aqueous liquids containing heavy metals which comprises adding ferrous sulphate to the said liquid, then adding calcium carbonate while an oxidizing gas is blown thereinto, the oxidizing and neutralizing operation being carried out at a pH of 5-6 and a temperature of 60 to 800 C, the resulting gypsum and magnetic being separated and recovered by magnetic separation, the heavy metals being concentrated in the magnetite.

8. A method as claimed in Claim 7 in which the heavy metals are chromium, cadmium, nickel or manganese.