GB2068348A

GB2068348A - A process for the separation of ferrous, aluminous and manganous contaminations from hydrochloric magnesium chloride solutions

Info

Publication number: GB2068348A
Application number: GB8039072A
Authority: GB
Original assignee: Veitscher Magnesitwerke AG
Current assignee: Veitscher Magnesitwerke AG
Priority date: 1979-12-06
Filing date: 1980-12-05
Publication date: 1981-08-12
Also published as: ES497485A0; FR2471351B1; CS232711B2; AT367376B; GB2068348B; ZA807443B; IT1141118B; GR72252B; CA1144341A; FR2471351A1; ES8200846A1; BR8007986A; DE3045796A1; DE3045796C2; IT8026464A0; ATA772979A

Abstract

For separating ferrous, aluminous and manganous contaminations from hydrochloric magnesium chloride solutions the ferrous and aluminous contaminations are precipitated together with optionally present silicic acid and boron compounds at a temperature of 60 to 110 DEG C and a pH-value of 3 to 5.2 by gassing the solution with air and/or oxygen and subsequently the precipitation of manganese is carried out at a pH-value of /4,5, bivalent manganese being oxidised, too. After the precipitation step, the deposit is separated.

Description

SPECIFICATION A process for the separation of ferrous, aluminous and manganous contaminations from hydrochloric magnesium chloride solutions The invention relates to a process for the separation of ferrous, aluminous and manganous contaminations from hydrochloric magnesium chloride solutions.

Processes for the preparation of pure magnesia from impure, magnesium containing minerals, e.g. magnesite, dolomite, serpentine, chlorite, talcum or intermediate products from industries concerned with the processing of these minerals, e.g. flue dusts, are known. These processes are based substantially on the fact that the mentioned materials are decomposed with hydrochloric acid, resulting first of all in the formation of an impure, frequently slightly hydrochloric liquor. Said liquor contains frequently amounts of undissolved starting material and dissolved substances, namely mainly MgCI2 and also FeCI2, AICI3, MnCl2, dissolved silicic acid and the like.

The hydrous oxides of the trivalent iron and the tetravalent manganese resp. as well as of the aluminum are precipitated by a neutralization of said liquor (which may be preceded optionally by a separation of the undissolved substance) with oxidation of the FeCI2 and the MnCI2 by means of air, chlorine, hydrogen peroxide or other oxidants. The silicic acid being still in dissolution and other contaminations such as boron and the like, are dragged along. After filtration a neutral magnesium chloride brine is obtained which contains substantially only calcium chloride as contamination. After precipitation of the CaCI2 (which is not comprised by the present invention and the methodology of which is not illustrated here in detail) a pure magnesium chloride brine is obtained.The pyrolytic cleavage according to the equation MgCI2 + H20oMgO + 2HCI in suitable furnaces (spray roasting furnaces, fluid bed furnaces, rotary furnaces) converts the magnesium chloride brine into pure MgO and a flue gas containing the hydrogen chloride formed in the above reaction. The flue gas is purified from flue dust (MgO), e.g. by means of magnesium chloride brine, and subsequently separated by means of water, e.g. in a socalled adiabatic column, from its HCI-contents. If an adiabatic column is used at the base thereof hydrochloric acid flows off, which may achieve maximally azeotropic composition.

The precipitation of hydroxides is not reported in detail in the literature describing such processes. The literature is restricted to the statement that the contaminated brine is neutralized together with aeration or introduction of chlorine or the like with the end product of the process (MgO) or flue dusts of the magnesite industry, CaO or the like, the precipitation occuring then in a manner not described in detail.

The problems of the precipitation of the hydroxides which have not been reported and not recognized up to now, especially the problems occuring in the oxidation with air oxygen, are complex. Up to now frequently inexplainable losses of magnesium resulted, if the used substances and the obtained products were balanced exactly. It is clear that such losses are of great disadvantage. Additionally, it has been found that the precipitation of the manganese frequently took place only incompletely or that this precipitation involved long residence periods (or aeration periods). An additional unpleasant phenomenon could be observed with respect to the sedimentability and filtrability of the deposits. Significant variations could be observed, which led frequently to operational difficulties.

It is an object of the invention to provide a substantially improved process for the oxidation of the iron and manganese components and for the precipitation of the hydrous oxides from hydrochloric magnesium chloride solutions.

According to the invention this object is achieved if iron is precipitated at a temperature of 60 to 1 0 C, preferably of 80 to 90"C, by addition of a substance increasing the pH-value, such as MgO, CaO, Mg(OH)2, Ca(OH)2 or a mixture thereof, with intensive gassing with air and/or oxygen at a pH-value of 3 to 5,2, preferably 4 to 5, in form of substantially Mg-free hydrous oxide, e.g. mainly having the X-ray structure of the S-FeO(OH), together with aluminum hydroxide, silicic acid and boron compounds, if present, and subsequently at a pH of not less than 4,5 for the precipitation of the manganese, bivalent manganese is oxidized and finally the precipitated deposit is separated together with solids present from the magnesium chloride solution, e.g., by sedimentation and/or filtration.

The two following procedures may be used for the precipitation of the manganese: The pHvalue may be increased to 6,0 and bivalent manganese may be oxidized by the introduction of air and/or oxygen offering the advantage of a problem-free oxidant. However, it is also possible to maintain the pH-value at a value of 4,5 and to oxidize bivalent manganese by the introduction of gaseous chlorine allowing a more rapid run of the oxidation.

The reaction may be carried out discontinuously in a stirring apparatus or continuously in a stirring vessel cascade. In the latter case the precipitation of the iron takes place in one or several vessels in the pH-range of from 3 to 5,2, preferably 4 to 5, and the precipitation of the manganese is effected in one or several subsequent vessels at the pH-value of 4,5.

The process of invention is based substantially on a suitable adjustment of the pH-value in the course of the precipitation. As mentioned already an impure magnesium chloride brine is used as starting material; said brine is hydrochloric and the contents of solids thereof are optionally separated. The latter fact depends on factors, such as e.g. the composition of the raw material.

This brine is subjected to an increase of the pH-value with intense introduction of air or oxygen; generally MgO or Mg(OH)2 is used. Of course, also other compounds such as e.g. CaO, Ca(OH)2, NaOH, KOH, NH4OH and the like may be used, although such substances may cause an undesired contamination of the brine. If MgO is used such a contamination does not occur.

Therefore, finely divided, optionally hydrated MgO in solid form or in form of an aqueous suspension is especially suitable for the neutralization. A reasonable precipitant is flue dust containing MgO and/or CaO.

It should be mentioned that if the oxidation is effected with air the precipitation of the iron takes place before the manganese is precipitated, which means, the precipitation of the manganese will start only then if the iron is present already in the form of hydrous oxide.

Now it has been found that two types of hydrates may be obtained in the precipitation of the iron which depends on the adjustment of the pH-value. Hydrate type 1 will be formed if the pHvalue of the brine is increased rapidly to > 5,2. Hydrate type 2 will be formed if the precipitation of the iron is effected in the pH-range of from 3 to 5,2, preferably from 4 to 5, with intensive gassing with air or oxygen.

At the end of the precipitation the iron is present in both types of hydrates in trivalent form.

However, they differ in their X-ray structure. The X-ray spectrum of hydrate type 1 corresponds to a large extent to that of the pyroaurite Mg6Fe2CO3(0H),6.4H20 or that of the brugnatellite Mg6FeCO3(0H),3.4H20. Probably they are double hydroxides of the iron and magnesium related to the pyroaurite and brugnatellite resp. or even identical therewith. In contrast thereto mainly the X-ray reflexes of 8-FeO(OH) may be proved in the hydrate type 2. All X-ray lines of the hydrates are strongly broadened.

A precipitation of the hydrate type 2 has been shown to give a minimum of magnesium losses, whereas in the case of the precipitation of the hydrate type 1, greater amounts of Mg are coprecipitated, which are coseparated in the subsequent separation of the Fe-hydroxide and discharged together with the contaminations.

Additionally it should be remarked that the hydrate types 1 and 2 if formed do not change in case of an additional increase of the pH-value, which means that neither magnesium is removed from hydrate type 1 nor hydrate type 2 incorporates subsequently magnesium.

As to the subsequent precipitation of the manganese it is known that it is favored by high pHvalues. If the iron is precipitated in form of hydrate type 2 a substantially lower addition of MgO will be required for the adjustment of a definite end pH-value than in case of precipitation of hydrate type 1. Thus, by the procedure leading to the formation of the hydrate type 2 also conditions are created, which favor the precipitation of the manganese.

After the hydroxides have been precipitated the formed liquor being approximately neutral is to be separated from the solids. This separation step has been proved to be important for the performance of the process. Surprisingly, it has been found that more favorable conditions for the separation are existing if the iron has been precipitated in form of hydrate type 2. This deposit gives a substantially denser (containing more solids) slurry in sedimentation devices and also a denser filter cake in case of the subsequent filtration. In case of both hydrate types the sedimentation rate is about the same.

Furthermore it has been found surprisingly that precipitations leading to hydrate type 2 give end products of the process (spray-roasted MgO) having significantly lower amounts of SiO2 and Awl203. E.g. a magnesium oxide prepared from a brine after precipitation of the iron in form of hydrate type 1 contains on average 0,1 1% of SiO2 and 0,1 2% of Awl203, whereas MgO obtained after precipitation of the iron in form of hydrate type 2 contains 0,02% of SiO2 and 0,01% Awl203.

It can be summarized that hydrate type 2 leads to a reduction of the losses of magnesium, an increase of the oxidation rate and of the yield of the manganese, an improvement of the sedimentation and filtration properties of the total deposit and to a reduction of the amount of SiO2 and Al203 in the end product of the process.

If the process of the invention is carried out discontinuously, first of all the pH-value will be increased to about 4,5 as quickly as possible and maintained at this value with simultaneous oxidation with air or oxygen until a complete precipitation of the iron will have taken place.

Subsequently the pH-value may be increased (preferably to a value not less than 6) and the precipitation of the manganese may be carried out under otherwise the same conditions. In case of a continuous precipitation in a cascade of stirring vessels the conditions of the first one or more vessels (in flowing direction) is adjusted e.g. such that a pH-value of about 4,5 is existing there, and the residence time is selected such that in these vessels the entire precipitation of the iron occurs. In the other one or more subsequent vessels the pH-value may be increased gradually to such an extent that indeed a precipitation of the manganese as completely as possible occurs, however, without formation of magnesium hydroxide. A pH-value of about 6 or higher will be desirable.

Chlorine may be used for the oxidation of iron, however, it is not necessary, as said oxidation takes place relatively rapidly with air or oxygen in a similar manner such as with chlorine.

However, a residence time of 2 to 3 hours is necessary for the precipitation of the manganese if it is oxidized with air. This residence time may be reduced to 10 to 1 5 minutes if the oxidation of the bivalent manganese is carried out with chlorine. Furthermore, a smaller amount of MgO will be consumed, as a more rapid oxidation of Mn" with Cl2 is possible already at a lower pHvalue (4,5) than it would be necessary for the air oxidation. If the starting solution does not contain too much manganese the costs of the chlorine are not so important compared with the mentioned advantages.

The following examples illustrate the present invention.

The starting solutions used in the examples and comparative tests have been prepared by dissolving natural magnesite in 19% hydrochloric acid and separating the residue by filtration.

They had the following analytical composition: MgCI2 260 g/l (110g MgO/l) Fe 2,8 g/l Mn 0,2 g/l Si 0,04 g/l Al 0,03 g/l free HCI 4,1 g/l.

The pH-value, measured at room temperature, was 1,2.

The aqueous suspension of a technically prepared ground MgO was used for the neutralization, concentration thereof about 100 g/MgO/l.

Comparative Test A: 1 I of a magnesium chloride solution was heated in a glass flask provided with a high-speed stirrer and a reflux condenser to a temperature of 85"C. Air was supplied in an amount of 1 6,7 I/hr through a gas inlet frit and 3,38 g of MgO were added in one portion.

First a black deposit precipitated which turned in the course of the reaction to red-brown. The pH-value increased to a maximum of 6. However, up to the end of the precipitation is dropped again to 5,4. The precipitation of the iron was completed after 1 5 minutes. After a reaction time of 2 hours at a temperature of 85"C serving for the precipitation of the manganese the precipitate was filtered off, washed and dried at 120"C. Table IV shows the measurements carried out on the precipitation suspension, the filter cake and the drying residue.

Example 1: 1 I of the magnesium chloride solution was heated to a temperature of 85"C in a glass flask provided with a high-speed stirrer and a reflux condenser. Air was supplied in an amount of 1 6,7 I/hr through a gas inlet frit and 1,126 g of MgO were added. Thereby the pHvalue increased to 4,6. During the next 30 minutes the pH-value was maintained between 4,4 and 4,6 by the addition of MgO. 1,198 g of MgO were necessary. First of all a red-brown deposit precipitated. After 30 minutes the precipication of the iron was practically complete. At this time additional 1,054 g of MgO were added. Thereby, the pH-value increased to 6,3. Up to the end of the precipitation it dropped to 6,2.After a reaction time of 2 hours at a temperature of 85"C serving for the precipitation of the manganese the deposit was filtered off, washed and dried at a temperature of 120"C. The measurements carried out on the precipitation suspension, the filter cake and the drying residue are shown also in Tabelle IV.

Examples 2 and 3 and Comparative Tests B and C: The precipitation of the iron was carried out in a glass flask provided with a high-speed stirrer, a reflux condenser and an overflow pipe.

The addition of the solution of MgCI2 and of the MgO-suspension was carried out continuously.

A gas inlet frit was used for gasing with air. The reaction mixture was heated to a temperature of 85"C; the reaction volume was 1 I. The suspension flowing off continuously from the glass flask was collected. Then the precipitate was filtered off, washed and dried at a temperature of 120"C. Four tests were carried out at different pH-values, that means the examples 2 and 3 within the scope of invention and the comparative tests B and C outside thereof.

Table I shows that iron(lll)oxide hydrate having mainly the structure of 8-FeOH(OH) and the desired low sedimentation volume is formed only in the pH-range of about 15,0. In case of higher pH-values the MgO-consumption increases very highly.

Table I Example and comparative test resp. 2 3 B C temperature ( C) 85 85 85 85 pH (measured at 85 C) 4,5 5,0 5,5 6,0 MgO (g/l of MgCl2-solution) 2,1 2,6 6,0 12,5 mean residence time (hrs) 0,49 0,43 0,42 0,42 air (l of air/l of MgCl2-solution) 16,7 14,7 14,8 15,9 precipitation of the Fe (%) 97 100 100 100 sedimentation volume of the deposit after#24hrs (cm /l of MgCl2-solution) 29 40 96 172 contaminations in the filtrate: Fe 77 < 5 < 5 < 5 (mg/l) Mn 100 100 100 100 X-ray structure of the dried filter cake: pyroaurite + + + + + + + + + + + + #-FeO(OH) + + + + + + + + Mg(OH)2 + Example 4: The precipitation was carried out continuously in a four-step stirring vessel cascade. The working conditions are shown in the following table II.

Table II gasing (l of air/l temperature Vessel reaction volume (I) mean residence time (hrs) of MgCI2-solution ( C) 1 3,76 0,476 5,19 85 2 5,8 0,718 2,07 85 3 7,9 0,978 0,91 85 4 8,1 1,002 0,91 85 1-4 25,6 3,174 9,08 85 Vessel 1 was provided with a high-speed basket stirrer (2000 rpm) to achieve a better distribution of the air and therefore a lower consumption of air. The other vessels were provided with wing stirrers (700 rpm). Air was supplied to the first vessel by a glass pipe terminating vertically beneath the stirrer and to the other vessels through a gas inlet frit.

The hydrochloric magnesium chloride solution and the MgO-suspension were added simultaneously to the first vessel, the addition of MgO being dosed such that a pH-value of 4,5 resulted. 2,15 g MgO/l of magnesium chloride solution were necessary. In the second vessel a pH-value of 6,1 was adjusted by the addition of further 1,30 g of MgO/l of magnesium chloride solution.

99,6% of the iron was precipitated in the first vessel. The additional residence time in the vessels 2 to 4 served for the precipitation of the manganese. The measurements carried out on the precipitation suspension, the filter cake and the drying residue are also shown in table IV.

Example 5: The initial precipitation (Fe-precipitation) was carried out as described in Example 1. After a precipitation period of 30 minutes the supply of air was discontinued. Subsequently 0,5 g of MgO were added and 0,25 g of chlorine were introduced so as to precipitate manganese. After a precipitation time of totally 45 minutes the reaction mixture was filtered.

The analysis of the filtrate showed: Fe < 0,5 mg/l Mn 22 mg/l.

Example 6: The precipitation was carried out continuously in a two step stirring vessel cascade. The working conditions are shown in the following table Ill.

Table III gasing l of air/l of g of Cl2/l of Vessel Reaction volume (l) mean residence time (h) MgCl2-solution MgCl2-solution temperature ( C) 1 3,76 0,465 50,5 - 85 2 2,52 0,311 - 0,25 85 1+2 6,28 0,776 5,05 0,25 85 Vessel 1 was provided with a high-speed basket stirrer (2000 rpm) and vessel 2 with a wing stirrer (700 rpm). Air was supplied to the first vessel through a glass pipe terminating vertically beneath the stirrer. Chlorine was supplied to vessel 2 through a gas inlet frit.

The hydrochloric magnesium chloride solution and the MgO-suspension were added simultaneously to the first vessel, the addition of MgO being dosed such that a pH-value of 4,5 resulted. 2,15 g of MgO/l of magnesium chloride solution were necessary. Additional 0,5 g of MgO/g of magnesium chloride solution were added to the second vessel. The suspension flowing off from the second vessel was cooled to a temperature below 40"C and filtered.

The analysis of the filtrate showed: Fe < 0,5 mg/l Mn 25 mg/l.

Table IV Filter cake dried at 120 C Example Consumpt. pH-value Sedimen- Wet MgO removed Contamiof MgO of the tation filter X-ray structure Weight Fe Mg from the nations (g/l of Fe-preci- volume cake (g/l of (%) (%) solution in the MgCl2- pitation of the (g/l Pyroaurite #-Fe(OH) MgCl2- together filtrate solution) deposit MgCl2- solution with the (mg/l) after solution) filter #24hrs cake g/l Fe Mn (cm /l of MgCl2of MgCl2- solution) solution A 3,38 6,0 144 59,4 + + + + + 10,1 27,7 14,3 2,39 0,5 101 1 3,38 4,5 60 16,6 + + + + + 5,29 52,7 5,3 0,46 0,5 66 4 3,45 4,5 58 16,4 + + + + + 5,18 53,5 5,5 0,47 0,5 55

Claims

1. A process for the separation of ferrous, aluminous and manganous contaminations from a hydrochloric magnesium chloride solution which comprises increasing the pH value of the solution to a pH value of 3 to 5.2, gassing the solution whilst at a temperature of from 60 to 110"C with air and/or oxygen to precipitate iron in the form of a substantially-free hydrous oxide together with aluminium hydroxide silicic acid and boron compounds, if present, precipitating manganese with the solution at a pH value of not less than 4.5 by oxidising bivalent manganese and separating the precipitated deposit together with any solids present from the solution.

2. A process according to Claim 1 in which the solution is maintained at a temperature of from 80 to 90"C during the precipitation of the iron.

3. A process according to Claim 1 or 2 in which the pH value of the solution is increased by the addition of one or more of the compounds MgO, CaO, Mg(OH)2 and Ca(OH)2.

4. A process according to Claim 3 in which finely divided optionally hydrated MgO in solid form or in the form of an aqueous suspension is employed.

5. A process according to Claim 3 in which flue dust containing MgO and/or CaO is employed.

6. A process according to any one of the preceding claims in which the iron is precipitated mainly in the form having the X-ray structure of 8-FeO(OH).

7. A process according to any one of the preceding claims in which, for the precipitation of the iron, the pH value of the solution is increased to a pH value of 4 to 5.

8. A process according to any one of the preceding claims in which, for the precipitation of manganese, the pH of the solution is increased to a value not less than 6 and bivalent manganese is oxidized by the introduction of air and/or oxygen.

9. A process according to any one of claims 1 to 7 in which, for the precipitation of manganese, the pH of the solution is maintained at a value not less than 4.5 and bivalent manganese is oxidised by the introduction of gaseous chlorine.

10. A process according to any one of the preceding claims in which the reaction is carried out discontinuously in a stirring vessel.

11. A process according to any one of claims 1 to 7 in which the reaction is carried out continuously in a cascade of stirring vessels, the precipitation of the iron being effected in one or several vessels and the precipitation of the manganese being effected in one or more subsequent vessels.

1 2. A process for the separation of ferrous, aluminous and manganous contaminations from a hydrochloric magnesium chloride solution substantially as herein described with reference to any one of Examples 1 to 6.