PROCESS FOR TREATING IRON-CONTAINING WASTE STREAMS

This appiication claims priority to U.S. Patent Application Serial No. 11/586,888 filed October 26, 2006, the contents of which are hereby incorporated by reference In their entirety.

FIELD OF THE INVENTION

This invention relates to a process for treating iron-containing waste streams produced in the chlorination of titaniferous raw materials.

BACKGROUND OF THE INVENTION The manufacture of titanium dioxide pigment is commercially performed by either the sulfate process or the chloride process. The chloride process first converts titania-containing feedstocks to titanium tetrachloride via a high temperature (800- 1200 ⁰ C) carbochlorination reaction that is performed in a chlorinator in the presence of chlorine gas and petroleum coke added as a reductant. Since titania-containing feedstocks, such as ores and slags, typically contain many elements in addition to titanium (e.g., Fe, Mn, Cr, V, Al, Nb, Mg, Si, Zr, and Ca), the chlorination reaction produces other volatile and non-volatile- metal chlorides, or oxychlorides, in addition to titanium tetrachloride. The titanium tetrachloride product is purified by separation from the other metal chlorides or oxychlorides prior to oxidizing the titanium tetrachloride to form titanium dioxide pigment.

Historically, the chlorinated impurities have been separated from the titanium tetrachloride and disposed as waste. However, with increasing environmental regulation and decreasing availability of landfills, there has been a movement to find uses for the impurities, as well as to develop methods to render them useful.

Since iron is a major impurity in titania-containing feedstocks, many methods have been proposed to utilize the iron by-products. U.S. Pat. No. 4,994,255, for instance, teaches a process wherein the ferrous chloride byproduct of chlorination is oxidized to produce iron oxide and chlorine gas which can be recycled back to the chlorinator reactors. U.S. Pat. No. 5,282,977 teaches a neutralization and precipitation process for separating chromium, vanadium, and titanium from waste water generated by the sulfate or chloride process. The process is taught as having a lesser effect on iron.

U.S. Pat. No. 6,800,260 discloses a process for producing iron oxides

^' from the treatment of iron-containing waste streams by a neutralization and oxidation procedure. One of its disclosed embodiments comprises first dividing a liquid slurry stream into a first and second slurry stream, then adding a calcium-containing neutralization agent to the first slurry stream to form a metal hydroxide-containing precipitate and a calcium chloride-containing liquid phase.

A majority of the calcium chloride-containing liquid phase is separated from the metal hydroxide-containing precipitate, and the metal hydroxide-containing precipitate is then added to the second slurry stream to form a first precipitate and a first liquid phase, which are then separated. Lastly, the first liquid phase is lastly subjected to an oxidation, neutralization and precipitation process to form an iron-containing compound.

A process for purifying an acidic technical-grade iron chloride solution formed in the chloride process is taught In U.S. Pat. No. 5,407,650. The process teaches first adjusting the pH with a first neutralizing agent (e.g., CaCOs) and thereafter introducing the pH adjusted solution in a controlled manner into a solution containing a second neutralizing agent. The undesired ions (such as Cr, V, Zr ₁ and/or Nb) precipitate in the form of hydroxides which can be filtered.

U.S. Pat No. 5,935,545 discloses a process for preparing an aqueous FeCIe solution, comprising the steps of: (a) reacting a titaniferous ore with chlorine and coke to form a metal chloride vapor stream comprising titanium tetrachloride, ferrous chloride, ferric chloride and unreacted coke and ore solids; (b) cooling the metal chloride vapor stream, to a temperature in the range of 350 to 500 ⁰ C to condense at least some of thό ferrous chloride; (c) separating the condensed ferrous chloride and the unreacted coke and ore solids from the metal chloride vapor stream; (d) cooling the metal chloride vapor stream to a temperature in the range of 180 to 24O ⁰ C to form a precipitate comprising ferric chloride; and (e) adding the precipitate to water to form an aqueous solution comprising ferric chloride. In sum, there remains a need to develop cost-effective processes for treating chlorination streams that contain iron impurities and to retrieve useable iron containing products from these streams.

SUMMARY OF THE INVENTION

The invention is a process for treating an iron-containing effluent produced by the chlorinatioπ of a titaniferous feedstock. The process comprises dividing the effluent into a first and a second stream, then adding a neutralization agent (e.g., calcium hydroxide) to the first stream to form a partially neutralized slurry, having a pH of 4.7 or greater. The partially neutralized first stream is then combined with the second stream to form a combined stream comprising ferrous chloride and metal hydroxide precipitates, and having a pH of 3.2 to 4. Lastly, the ferrous chloride is separated from the metal hydroxide precipitates. Surprisingly, the partial neutralization of a portion of the waste stream and recombination with the remainder of the waste stream results In avoiding the formation of a gelatinous mass of precipitates encountered if the process is performed in a single stage.

DETAILED DESCRIPTION OF THE INVENTION

The invention is a process for treating an iron-containing effluent produced by the chlorination of a titaniferous feedstock. Titaniferous feedstocks are raw materials that contain significant amounts of titanium dioxide and iron oxides, in addition to other impurities. Particularly preferred titaniferous feedstocks include anatase ores, ilmenite deposits, slags, and tar sands. Preferred titaniferous feedstocks contain about 40 to 80 weight percent titanium dioxide and about 20 to 50 weight percent iron oxide. They typically also contain from about 0.04 to 2 weight percent MnO, Cr ₂ O ₃ , and V ₂ O ₆ .

The iron-containing effluent of the invention is typically produced in the chloride ^" process. The chloride process is well known in the art. See, for example, U.S. Pat Nos. 2,486,912 and 2,701,179. In the chloride process, the titaniferous feedstock is chlorinated at high temperptures (800-1200 ⁰ C) in the presence of chlorine gas and petroleum coke added as a reductant. The reaction is typically performed in a fluid-bed reactor, although static bed reactors may also be used. The chlorination reaction produces a mixed chloride stream that comprises titanium tetrachloride and ferrous chloride, In addition to other volatile and non-volatile metal chlorides.

In order to use the titanium tetrachloride in the production of titanium dioxide pigment, it is necessary to separate the titanium tetrachloride from the

other metal chlorides. A crude titanium tetrachloride stream is separated from the mixed chloride stream to leave the iron-containing effluent. Typically, the mixed chloride stream is cooled (preferably to about 150-450°) in a cooling vessel, such as a cyclone. Low-volatile metal chloride impurities (e.g., iron, manganese, magnesium, and chromium) are condensed in the cooling vessel to produce the iron-containing effluent while the crude titanium tetrachloride is separated as a vapor stream.

Preferably, substantially ail of the iron in the iron-containing effluent will be ferrous chloride (iron (II) chloride). The concentration of ferrous chloride in the iron-containing effluent is not critical, however, preferably the iron-containing effluent will be as concentrated as possible in terms of the ferrous chloride. Typically, the iron-containing effluent will also contain the chlorides and oxychlorides of other metals. Examples of these chlorides and oxychlorides include, but are not limited to, the chlorides and oxychlorides of titanium, manganese, chromium, vanadium, aluminum, niobium, magnesium, calcium, silicon and zirconium. The titanium in the iron-containing effluent will typically be residual titanium. As previously discussed, most if not all of the titanium will preferably already have been removed so that it may be processed separately.

The iron-containing effluent is processed according to the process of the invention. The process for treating the iron-containing effluent first comprises dividing the iron-containing effluent into two streams: a first stream and a second stream.. The first stream preferably comprises 25 to 45 weight percent of the entire iron-containing effluent (by weight or volume), most preferably 30 to 40 weight percent. The second stream comprises the remainder of the iron- containing effluent.

The first stream is treated with a neutralizing agent. The neutralizing agent is added to the first stream to form a partially neutralized first stream. Preferably, the step of adding the neutralizing agent is accompanied by.stim'ng or otherwise mixing the neutralizing agent with the first stream. The pH of the first stream (and therefore the iron-containing effluent) prior to the addition of the neutralization agent is typically less than pH 2.5, preferably from pH 1.5 to 2.5. Following addition of the neutralizing agent, the partially neutralized first stream has a pH of 4.7 or greater.

Neutralizing agents are any basic substances that are capable of raising the pH of the first stream to a pH of 4.7 or greater. Preferably, the neutralizing agent is a calcium-containing substance. Calcium-containing substances tend to be relatively inexpensive, though relatively pure, and the cakes that they form, when the precipitates are filtered, are relatively easily retrieved. The phrase "calcium-containing substance" refers to a substance that contains calcium and that is useful for neutralizing solutions that contain metal chlorides. The neutralizing agent may also be a sodiύm-containing substance such as sodium carbonate. Preferred neutralizing agents include calcium hydroxide, calcium oxide, and mixtures thereof. Calcium hydroxide is most preferred. The amount of neutralization agent that one uses will easily be determined by persons skilled in the art. In part, the amount is dependent on the amount and character of the first stream and the neutralizing agent itself.

The addition of the neutralization agent to the first stream will yield a liquid phase and precipitates of metal hydroxides. The precipitates are metal hydroxides that are formed in the first stream that are capable of precipitating when the pH is changed to a pH of 4.7 or greater, preferably to a pH of 4.7 to 5.3. For example, if the iron-containing effluent contains the chlorides of AJ, V ₁ Cr, arid/or Nb, the precipitate may contain the hydroxides of these metals. Following the treatment of the first stream, the partially neutralized first stream is combined with the second stream to form a combined stream. The terms "combined" and "combining" refer to any methods that are either now known or come to be known to persons skilled in the art for introducing substances to be combined with each other. Combining may be accompanied by stirring or otherwise mixing the substances to be combined. The combined ^■ stream comprises ferrous chloride and metal hydroxide precipitates. Following combination of the partially neutralized first stream with the second stream, the combined stream will have a pH of 3.2 to 4, preferably a pH of 3.2 to 3.7.

Lastly, the ferrous chloride is separated from the metal hydroxide precipitates. Methods for separating a precipitate from the liquid phase out of which it has been precipitated are well known to persons skilled in the art and by way of example include, but are not limited to, filtration, decantation, and centrifugation. Filtration is particularly preferred.

The following examples merely illustrate the invention. Those skilled in the art will recognize many variations that are within the spirit of the invention and scope of the claims.

EXAMPLE 1 : SINGLE STAGE PARTIAL NEUTRALIZATION

Exampje 1A - Neutralization Runs #1: Samples of a slurry waste stream are taken from a chloride TiO _∑ process. The stream is the feed to the effluent treatment plant. The slurry waste stream is the overflow from the hydrocyclone that recovers coarse ore and coke from the sluice slurry. The coarse ore and coke are recovered as the underflow product In order to recycle back to the chlorinator. The slurry waste stream samples contain 10-12 weight percent fine solids in aqueous suspension with the remainder a solution of specific gravity 1.3, containing 32 weight percent metal chlorides. The solution contains 263-293 g/L ferrous chloride and other soluble metal chlorides in amounts proportional to' that found in the starting slag. The pH of the sample is 2.2.

Portions of the waste stream sample are partially neutralized to pH 2.7 - 4.5 using calcium hydroxide slurry, in order to determine the required pH value to produce a suitable ferrous chloride liquor assay. In a series of experiments, a waste stream sample is agitated and a pH electrode is placed in the slurry prior to calcium hydroxide addition. Calcium hydroxide slurry is then added and the resulting slurry agitated while the pH is monitored. When the pH stabilizes, the partially neutralized slurry is filtered and the filter cake is washed with water. This combined washings and filtrates are analyzed for metal chlorides concentration. A portion of the waste stream sample used in the experiments is also filtered, and these filtrates are analyzed for metal chlorides concentrations. The percentage of each metal chloride precipitated is then calculated from the analyses of the waste stream sample and the partially neutralized samples.

The results (see Table 1) show that samples taken to a final pH value of < 3 result in formation of gelatinous precipitates during addition of the calcium hydroxide. This is more pronounced at lower pH values. In a specific example, an immobile reacting mass containing gelatinous precipitates is formed at pH 2.7 such that the reacting mass could no longer be agitated, in order to proceed further with the neutralization. The immobile slurry could not be filtered in a

reasonable time, in comparison to filtration times of 9 - 16 minutes for higher pH values.

It is also found from analysis of the liquor obtained from the partially neutralized slurries that the pH required for the partial neutralization process is a minimum of 3.2. This minimum pH ensures that the salts of Al, Cr, Nb, Si, Ti, V, and Zr (i.e., those salts other than Fe, Mg ₁ Mn & Ca) are precipitated, and can therefore be removed by filtration. The analysis shows that the salts of Al, Cr, Nb, Si, Ti, V, and Zr are predominantly precipitated from solution and do not appear in the filtrate (see Table 2).

Example 1 B - Neutralization Runs #2: Slurry waste stream samples are taken from a chloride T1O ₂ process as described above. The samples contain up of 10 weight % fine solids in aqueous suspension with the remainder a solution (specific gravity 1.33) containing 32% weight percent metal chlorides. The samples contain 270 g/L ferrous chloride and other soluble metal chlorides in amounts proportional to that found in the starting slag. The pH of the sample is 2.5.

Portions of the starting material are partially neutralized to pH 3.8 - pH 5.1, in a single stage, using calcium hydroxide slurry according to the procedure of Example 1. The time required to reach a stable pH value is recorded In each case (see Table 3). It is found that the reaction time required for partial neutralization to pH 5 was 10 minutes compared to greater than 120 minutes for pH less than 4. The relatively long neutralization times required to reach pH 3.2 - 4 results in the slurry spending a considerable amount of time passing through a pH of less than 3. This is the pH zone where gelatinous precipitates form.

These results demonstrate the advantage of taking the slurries to a higher pH value, with a quicker neutralization, thus avoiding the pH zone where gelatinous precipitates form. However, this pH is outside the window to produce a suitable ferrous chloride assay due to losses of Fe value by precipitation and dilution associated with alkali slurry added.

EXAMPLE 2: BATCH TWO-STAGE PARTIAL NEUTRALIZATION A portion of the slurry waste stream sample material from Example 1B is taken to pH 5 and then acidified down to pH of 3.5 - 4.0, by addition of further starting slurry waste stream material. The pH is monitored until the pH stabilizes. The results, shown in Table 4, demonstrate that the reaction time for a two stage partial neutralization is comparable to a single stage partial neutralization. However, the two stage partiarneutralization process avoids the pH zone where gelatinous precipitates form.

EXAMPLE 3: CONTINUOUS TWO STAGE NEUTRALIZATION

Samples of a slurry waste stream are taken from a chloride Tiθ2 process over an extended period to provide a feedstock for a continuous two-stage partial neutralization. The slurry waste stream samples contain 10-11 weight percent fine solids in aqueous suspension with the remainder a solution having specific gravity 1.32-1.33, containing 34-35 weight percent metal chlorides. The solution contains 306-320 g/L ferrous chloride and other soluble metal chlorides in amounts proportional to that found in the starting slag. The pH of the sample is 1.8 - 2.5.

A continuous two-stage partial neutralization process is run by utilizing two continuous flow stirred tank neutralization reactors, one reactor being the first stage reactor and the other the second stage reactor. The first stage reactor is fed with waste stream slurry and calcium hydroxide slurry, the flow of calcium hydroxide slurry being controlled to give a first stage partially neutralized slurry pH target of 4.7 to 5.3. The combined flow of waste stream slurry and calcium hydroxide slurry is controlled to ensure sufficient residence time to complete the reaction to pH 4.7 to 5.3. The product, first stage partially neutralized slurry, is then overflowed into the second stage reactor.

The second stage reactor is fed with first stage partially neutralized slurry and waste stream slurry. The waste stream slurry flow is controlled to achieve a product pH of 3.2 to 4 and ensure sufficient residence time is allowed for complete reaction. The product, second stage partially neutralized slurry, from this second stage reactor is then overflowed to a storage tank for later filtration by a filter press unit

Slurry waste stream is continuously fed (17-23 kg/hr) to the first stage neutralization reactor such that 30-40% of the stream is fed to the first stage and the remainder 60-70% is fed to the second stage. In the first stage, the slurry waste' stream is partially neutralized with calcium hydroxide slurry. The resulting partially neutralized slurry product from the first stage is then reacted with the remainder 60-70% slurry waste stream in the second stage. The first stage pH is maintained at a pH of 4.7 - 5.3 and the second pH is maintained at a pH of 3.4 - 3.6. .

The continuous reactor is run for a total of 243 hours in three campaigns of 154, 43 and 46 hours. The process is stable, did not produce any problems with gelatinous precipitates with the whole range of starting material concentrations, and produces suitable ferrous chloride liquor. See Table 5.

Samples of product from the continuous unit are collected and filtered, the resulting liquor is analyzed and found to contain the chlorides of Fe, Mn, Mg, Ca with only traces of Al, Si ₁ Ti and Nb. An assay of a typical ferrous chloride liquid product shows 19.7 wt.% FeCI ₂ , 4.5 wt.% CaCI ₂ , 3.9 wt.% MnCI ₂ , 2.6 wt.% MgCI ₂ , just traces (< 0.1 wt.%) of NbOCl ₃ and the chlorides of Al ₁ Si and Ti.

Table 1 : Results of Single Stage Neutralization

Table 2: Precipitated Metal Results of Single Stage Neutralization

Table 3: Single Stage Partial Neutralization

Table 4: Two Stage Partial Neutralization

Table 5: Continuous Two Stage Partial Neutralization