Process for Producing Pure Magnesium Oxide

The invention relates to a process for the preparation of pure magnesium oxide, especially suitable for the preparation of refractory products, from magnesium silicate and magnesium hydrosilicate materials such as olivine, serpentine, garnierite, or the like, the starting material being decomposed with hydrochloric acid, the residue being separated from the decomposition slurry and the sesquioxides to be regarded as contaminants, and other contaminants, being precipitated from the crude brine thus obtained by the addition of a pH-increasing substance, the precipitates being separated and the magnesium chloride solution thus obtained being subjected to thermal decomposition, particularly by spray-calcining, whereby magnesium oxide is obtained and hydrogen chloride is recovered.

The following are suitable as starting materials for this process: so-called ultra-mafitic rocks of the peridotite, dunite, harzburgite group and the like, the transformation products thereof, usually known as serpentine or serpentinite, weathered products of said rocks, halfproducts and byproducts arising during the processing thereof, and industrial byproducts and waste products of similar composition, as long as they have an MgO content of at least 15% by weight. These materials are called hereinafter with the term "serpentine" or "crude serpentine".

In known processes of this kind the pH-increasing substance used to precipitate sesquioxides is generally magnesium oxide, mostly in the form of caustic calcined magnesia. However, the use of such a precipitant has disadvantages. The serpentines, and similar starting materials, usually have a high content of iron and other sesquioxide contaminants and large additions of precipitant are needed. Since all kinds of caustic magnesia are costly, for the reason that they can be prepared only with a considerable expenditure especially of thermal energy, the use of caustic magnesia as a precipitating agent imposes a heavy economic burden. If pure magnesium oxide obtained as the end product of the process is used as precipitating agent, depending upon the amount of iron and other sesquioxide contaminants contained in the crude serpentine or the like starting material, up to about 30% by weight of the obtained magnesium oxide must be circulated internally, and this is economically unacceptable.

It is known from U.S. Pat. 2,398,493 to add calcined serpentine to a magnesium chloride slurry produced by decomposition of serpentine with hydrochloric acid as precipitating agent for contaminants. However, this method has the disadvantage that the silicic acid portion of the serpentine is converted, by the calcination, into a form which is more easily dissolved or suspended in the magnesium chloride slurry. The result thereof is that during the subsequent separation of the residues or precipitates a larger proportion of silicic acid remains in the magnesium chloride brine and the SiO.sub.2 content in the end product is therefore higher. In addition this increased amount of amorphous or dissolved silicic acid in the magnesium chloride slurry means that the Fe(OH).sub.3 and other hydroxide precipitates obtained during subsequent precipitation are slimy and therefore difficult to filter. Finally, a considerable amount of energy is also required to calcine serpentine.

It is the object of the invention to provide a process of the type mentioned at the beginning hereof in which the disadvantages mentioned are eliminated, which process permits economical operation, and which makes it possible to obtain easily filtered hydroxy precipitates.

The process according to the invention is characterized in that finely ground crude serpentine having a grain size of between 0 and 1mm, preferably between 0 and 0, 1mm, is used as the pH-increasing substance for precipitating oxides of iron and of aluminum.

Said precipitating agent ensures complete precipitation of aluminum and extensive precipitation of trivalent iron in the pH range below 5.

If bivalent iron is present in the starting material and if it also is to be removed, according to a special examide of an embodiment of the process according to the invention for transforming the bivalent iron into precipitable trivalent iron, it is possible to introduce air and/or some other oxidizing agent, such as chlorine or hydrogen peroxide H.sub.2 O.sub.2, into the crude brine under intensive stirring of the latter. If bivalent manganese is present in the starting material and should be removed, it may be transformed into quadrivalent manganese by oxidation. This may be carried out in the course of an oxidizing treatment carried out for the purpose of transforming bivalent iron into trivalent iron; it is also possible to carry out an oxidizing treatment additionally to the oxidizing treatment of iron in order to oxidize manganese.

The precipitation occuring in the process according to the invention may be carried out continuously or intermittently.

One of the advantages of the process according to the invention is that the starting material need merely be broken up; no other pre-treatment, especially calcining is needed.

Details of the precipitation procedure are as follows: after the crude serpentine has been added to the crude brine obtained by separating the undissolved decomposition residue, the pH value of the crude brine initially increases rapidly to about 4, whereby oxidizing of the still bivalent iron in the acid crude brine into the trivalent form of iron is encouraged; this, in turn, brings about relatively rapid precipitation of contaminants at the beginning of the whole procedure. Subsequently, as a result of oxidizing Fe.sup.++ to Fe.sup.+++, the pH value falls slightly and then rises again. This increase in pH value is steady and slow, the pH value remaining below 5.

It is desirable for the crude serpentine used as precipitating agent to be added to the crude brine in an amount between 1 1/2 and 21/2 times the stoichiometrical requirement. In this connection, the stoichiometrical requirement is regarded as the amount of crude serpentine the MgO content of which is sufficient to neutralize the acidity of the crude brine; the acidity of the crude brine is determined by the amount of free HC1 present in the acid crude brine and by the amount of crude brine in the oxidic contaminants accessible to precipitation. In this connection, both the oxidic contaminants which were present in the crude brine before the addition of the crude serpentine, acting as precipitating agent and the oxidic contaminants which were introduced into the crude brine with the crude serpentine acting as the precipitating agent, must be taken into account.

It is also advantageous to maintain the temperature of the crude brine at above 80.degree. C for the precipitation of contaminants effected by the addition of crude serpentine to the crude brine.

Another embodiment of the process according to the invention is characterized in that, upon completion of the precipitation of contaminants effected by the addition of crude serpentine to the crude brine, any iron, manganese, nickel still present in the brine, and any other sesquioxide contaminants remaining in the brine, are precipitated in known manner by the addition of pH-increasing substances known for that purpose, preferably caustic magnesia or flue dust arising during the calcination of magnesite.

A variant of this embodiment is characterized in that the precipitation carried out be adding crude serpentine to the crude brine at a pH value of below 5 is continued only until most of the iron and aluminum hydroxide have been precipitated. Thereafter, the precipitation procedure is rapidly completed by increasing the pH value to between 6 and 7 means of a small amount of caustic magnesia. In this connection, the precipitation effected, for example, by the addition of crude serpentine, may last for about four hours, while the following precipitation, in which the pH value is increased by the addition of caustic magnesia, may last for another two hours, for example.

The invention will now be explained in greater detail in the following examples.

1730kg of crude serpentine containing 728kg of MgO, 122kg of Fe.sub.2 O.sub.3, 4.66kg of NiO and 5kg of CaO were decomposed with 7330kg of hydrochloric acid containing 1330kg of Cl-. About 710kg of undissolved residue was separated from the decomposition slurry and 8350kg of acid crude brine was thus obtained. This crude brine containing 713kg of MgO, 113kg of Fe.sub.2 O.sub.3, 4.6kg of NiO, 1330kg of Cl- and 1.8kg of CaO, was mixed with 647kg of crude serpentine with a grain size of 0 to 0.1mm as the precipitating agent and air was introduced, under intensive stirring, into the crude brine, the temperature of which was maintained at above 80.degree. C. The pH value increased to about 4 and precipitation was completed in a few hours. The precipitated material together with the undissolved portion of the crude serpentine added as the precipitating agent was separated resulting in a filter cake weighing 823kg. The filter cake contained 225kg of MgO, 157kg of Fe.sub.2 O.sub.3, 1kg of NiO and 1.4kg of CaO. The remaining pure brine contained 748kg of MgO, 0.08kg of Fe.sub.2 O.sub.3 , 5.15kg of NiO, 1324kg of Cl and 3kg of CaO.

1585kg of crude serpentine containing 667kg of MgO, 112kg of Fe.sub.2 O.sub.3, 4.27kg of NiO and 4.59kg of CaO was decomposed with 6746kg of hydrochloric acid containing 1220kg of Cl-. About 625kg of undissolved residue was separated from the decomposition slurry and 7706kg of acid crude brine was thus obtained. This crude brine containing 643kg of MgO, 104kg of Fe.sub.2 O.sub.3, 4.20kg of NiO, 1220kg of Cl- and 1.8kg of CaO was mixed with 597kg of crude serpentine with a grain size of 0 to 0.1mm as the precipitating agent and air was introduced, under intensive stirring, into the crude brine, the temperature of which was maintained at above 80.degree.C. The pH value increased to about 4 and after 4 hours of reaction was quickly increased to pH 6.8 by the addition of 98kg of flue dust. The flue dust contained 82kg of MgO, 5.20kg of Fe.sub.2 O.sub.3 and 2.60kg of CaO. This allowed the precipitation to be completed within 1 hour. The precipitated material was separated together with the undissolved portions of crude serpentine and caustic magnesia, resulting in a filter cake weighing 830kg and containing 255kg of MgO, 145kg of Fe.sub.2 O.sub.3, 1.85kg of CaO and 4.27kg of NiO. The remaining pure brine (7571kg) contained 721kg of MgO, <0.01kg of Fe.sub.2 O.sub.3, 4.25kg of CaO, <0.01kg of NiO and 1210kg of Cl-.