CA1156427A - Method of purifying phosphoric acid of heavy metals - Google Patents

Abstract

A B S T R A C T

A method of removing heavy metals, particularly cadmium, from wet-process phosphoric acid produced by digesting raw phosphoric mate-rial with sulphuric acid, by precipitating out the heavy metals in sulphide form. The residual sulphuric acid is removed first by neu-tralization with alkali or as a not-readily dissolved sulphate precipitate. Alkali is added to obtain an Me/P ratio within the range of about 0.05-0.3, where Me is the number of cation equivalents in the alkali and P is the number of phosphorous atoms in the phos-phoric acid , whereafter the heavy metals are precipitated out by adding a sulphide compound which is soluble in the phosphoric acid, and the sulphide precipitate removed.

Description

1 ~ 7 A METHOD OF PURIFYING PHOSPHORIC ACID OF HEAVY METALS

The present invention relates to a process for removing heavy metals7 particularly cadmium, from differing qualities of wet-process phos-phoric acids. The invention is based on the concept of removing the heavy metals in sulfide form.

Phosphate raw materials contain varying quantities of heavy metals which, when digesting the phosphate raw material in acid, such as sulphuric acid, are dissolved and remain in the resultant phosphoric acid solution. When using the phosphoric acid in, for example, the production of fertilizers, animal foods and other more qualified end products, the heavy metals will be present in the end product.
Studies made in the agriculture industry on the problem of heavy metals over recent years have shown that more attention should be paid to the cadmium situation. The bonds by which cadmium ions are bound to the ground are rela~ively weak and consequently cadmium can be readily taken up by plant growing in the ground and, in this way, passed to the nutrient chains via the ground, the animal food and the vegetables intended for human consumption. The element cadmium is considered particularly dangerous, due to its long bio-logical half life. It has been found that the long term exposure ofcadmium to animals has, for example, resulted in proteinuri and kidney damage. Another known sickness promoted by cadmium is the so-called Itai-Itai-sickness in Japan, caused by the release of industrial cadmium and resulting in kidney damage and serious changes in bone structure. Thus, there are strong reasons to prevent, as far as possible, the entrance of cadmium into the various steps of the nutrient chain.

As before mentioned, cadmium and other heavy metals are present in the majori~y of phosphate raw materials and the heavy metal composi-tion of some of such raw materials is listed below in table 1.

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Element Kola Khouribga Taiba Florida Flor;da apatite phosphate phosphate phosphate BPLX 70/72 72 % BPL~ 75 % BPL~
P205, % 39.1 31.6 3~.9 33.3 34.5 As, ~g/g 0O7 12.5 2.2 3.6 5.3 ~ Cu, " 29 39 51 7.0 7.0 Cd, " c 0.2 14 66 8.0 9.0 Pb, " 2.0 3.0 4.0 15 15 Hg, "C 0.01 0.04 0.5 0.12 0.10 _, _ . . .. _ .
~ BPL - Bone Phosphate Lime (% Ca3(P04)2) Cadmium and other heavy metals, such as arsenic, copper, lead and mercury, remain, to a large extent, in the phosphoric acid prepared ~rom the phosphate raw materials. Table 2 below illustrates a heavy-metal content of phosphoric acid, which content is depending upon the raw material.

Element Khour;bga Kola Khouribga phosphoric acid phosphoric acid phosphoric acid 1) 2) . .
P205, % 53.0 53.5 31.4 As,~ug/g 14 1.2 9.2 Cu, " 45 14 18 Cd, " 14 0.5 9.2 Pb, " 0.4 2.2 0.2 Hg~ " 0.002 0.005 0.001 N.b. 1) Evaporated phosphoric acid, hemihydrate process

- 2) Non-evaporated phosphoric acid, dihydrate process.

Various methods of purifying phosphoric acid with respect to its heavy metal content are known to the art. Normally the various heavy metals3 w;th the exception of cadmium, can be readily precipitated out and removed from the phosphoric acid, by, for example, adding hydrogen sulphide, sodium sulphide solution, potassium sulphide solution or an ammonium sulphide solution, or by adding calcium and barium sulphide. Cadmium is much more difficult to precipitate out than the other heavy metals mentioned.

In the known technique there are only three methods for removing cadmium from phosphoric acid which, according to results shown, provide acceptable residual contents of cadmium in the purified phosphoric acid. One method (disclosed in Japanese patent specifica-tion No. 7575-115) is based on the concept of adding a large quan-tity of sodium sulphide to the phosphoric acid, cadmium sulphidebeing precipitated out in an autoclave at overpressure. The over-pressure is probably necessary in order to reduce the solubility of the resultant cadmium sulphides in the phosphoric acid. The method requires a large surplus of sulphide in relation to the heavy metals, and, subsequent to filtering off the sulphides, results in the presence of hydrogen sulphide in the acid, which hydrogen sul-phide must be removed. The hydrogen sulphide can, for example, be driven off with air or removed by oxidation. This procedure is par-~icularly disadvantageous in practice. From the aspect of working hygiene and from the point of view of the care and protection of the environment, the handling of hydrogen sulphide is a serious matter.

Another Japanese patent specification, No. 53075-196, proposes a method in which water-soluble metal sulphides are first pre-mixed with a calcium hydroxide suspension. This mixture is then added to the phosphoric acid, whereby cadmium sulphide, together with other heavy metals, are precipitated out and then separated. The examples given illustrate that acceptable purification with regard to cadmium is obtained at molar ratios of Ca/S04 of 0.5; 1.0, and 1.5 and a molar ratio of S /heavy metals greater than 10.

S64~7 This latter methodology, has been found applicable only to acids o~
par~icular composition. Thus, experiments show ~hat the method cannot be used on wet-process phosphoric aoid produced from the raw materials specified in Table l above. Since these these raw materials are the most common when seen commercially, at least in Europe, the Japanese method has a very limited field of use. The method was applied ex-perimentally on differing qualities of wet-process phosphoric acid, obtained from the raw materials recited in Table 1. The results of these experiments are shown in Example lO below, from wh~ch it can be seen that no satisfactory removal of cadmium could be obtained.

The third method (German Offenlegungsschrift 2422902) is based on the concept of passing sulphuric acid under pressure (1-50 atm) down into the phosphoric acid and then removing the cadmium sulphide ~ormed. This method has the same disadvantage as the method dis-closed in the aforementioned Japanese Patent Spec1fication 7575~115.
The phosphoric acid was not satisfactorily purified of cadmium.

An object of the present invention is to provide a method by which wet-process phosphoric acid can be freed of heavy metals, partic-ularly cadmium, irrespective of the phosphate raw material used in producing the phosphoric acid, and without needing to apply the difficult process steps associated with known techniques.

As is generally known, wet-process phosphoric acid produced by di-gesting raw phosphates with sulphuric acid always contains sulPhUric acid residues. In accordance with the method of the present inven-tion, the residual sulphuric acid present in the phosphoric acid is first removed, which can be effected, in principle, in two different ways, namely by neutralization with alkali or by precipitation as not-readily dissolved sulphate. Subsequent to removing the sulphuric acid, the phosphoric acid is admixed with alkali to an Me/P ratio within the range of about 0.05-0.3, preferably about 0.1-0.2; in which Me represents the number of cation equivalents in the alkaline solution used, while P represents the number of phosphorus atoms in the phosporic acid. After adding the alkali, the heavy metals are ~ 2 7 precipitated out in the phosphor;c acid by adding thereto a sulphide compound which is soluble in the phosphoric acld and by then removing the resultant sulphide precipitant in a suitable manner. In practice, the sulphide is suitably supplied in an excess quantity, the amount added pre~erably being selected so as to correspond to about 0.2-lg of s2 per litre of phosphoric acid.

When the method selected for removing residual sulphuric acid from the phosphoric acid is by precipitating out not-readily dissolved sulphate, the alkali addition may conveniently be effected in t~o stages, whereat in the first stage a stoichiometric guantity of salt-forming cations corresponding to the residual sulphuric acid is added and ~he resultant sulphate separated out~ while in the second stage further alkali is added to produce an Me/P ratio within said interval, or, alternatively, the alkali additon can be made in a single step while neutralizing the residual sulphuric acid to ob-tain the Me/P ratio.

A multiplicity of alkaline compounds, such as ammonium hydroxide, sodium hydroxide or potassium hydroxide or mixtures thereof, can be used for neutralizing the sulphuric acid or for the addition of alkali to obtain the desired Me/P ratio. Alternatively, calcium hydroxide, calcium carbonate or calcium oxide, or a mixture thereof, can be used. The kind of alkaline compound added is suitably se-lected with respect to the future use of the phosphoric acid. Thus,for example, if the phosphoric acid is to be used for the prepara-tion of calcium phosphate) the alkali added is suitably a calcium compound, for example one of those beforementioned.

~hen the residual sulphuric acid present in the phosphoric acid is removed by precipitation of the not-readily dissolved sulphate, there can be suitably used calcium or barium compounds, such as hydroxides, carbonates or oxides thereof. The precipitated sulphate can be removed in a suitable manner, for example by filtration.
Any suitable soluble sulphide compound can be used to precipitate ~ 4 ~'~

out the heavy metals, among them cadmium, present in phosphoric acid prepared in the afore described manner. As examples of suitable sul-phide compounds can be mentioned sodium sulphide, potassium sulphide, ammonium sulphide, calcium sulphide or hydrogen sulphide.

In conjunction with the advent of the present invention ik was dis-covered that if the phosphoric acid contains minor quantities of cadmium, e.g. about 10 ~9 per gram phosphoric acid and if the weight ratio of Cd:(As~Cu) is less than about 0.5 the precipitation of cad-can be ~acilitated by adding large excess quantities of the sulphidecompound in relation to the heavy metals present. This is partic-ularly the case when the Me/P molar ratio is less than about 0.1.
In pract;ce, it ;s su;table to add about twice the amount of sulphide compared with the afore mentioned normal quantity of about 0.2-lg of s2 per litre of acid.

The invention will now be described in more detail with reference to a number of examples, which should not be considered limitive of the invention.
Example 10 is intended to illustrate the unsatisfactory purification of phosphoric acid with respect to cadmium obtained when applying the afore described known techniques.

1000 ml of wet-process phosphoric acid, containing 53.5 % P205,

3.0 % H2S04 and 16 mg As, 17 mg Cu, 25 mg Cd, calculated per litre of acid,were admixed with lime milk to a molar ratio of Ca/S0~ of 1Ø The gypsum precipitate was removed by ~iltering after a reaction time of 1 hour at 50C. The phosphoric acid purified of sulphate was admixed with sodium hydroxide to obtain an Na:P molar ratio of 0.10, whereafter 1 ~ram of commercially available sodium sulphide (as a 5 % Na2S solution) was added. This solution was introduced close to the bottom of the reaction vessel. After a reaction time of 2 hours at 50C, the heavy metal sulphides precipitated out were removed by filtration. The result is illustrated in Table 3 below.

~ ~B~27 1000 ml of wet-process phosphoric acid haYing the same composition as that of Example 1, with the exception that the cadmiu~ content was 12 mg/l, were treated in the manner described in Example 1.
The results are given in Table 3 below.

From 100 ml of phosphoric acid, which had been pur;fied of sulphaté
and which had the same composition as that in Example 2, arsenic and copper were precipitated in d first stage and the heavy metal sul-phides removed by filtration. The filtered phosphoric acid was then neutralized to an Na:P molar ratio o~ 0.10 and admixed with 0.5 gram of commercially available Na2S and cadmium sulphide was filtered off after 2 hours as in Example 1. The results are given in Table 3 below.

. . ~ .
Element Example No. 1 Example No. 2 Example No. 3 content 25 mg Cd/l 12 mg Cd/l 12 mg Cd/1 one stage one stage two stages - _ P205, % 45.0 45.0 45.0 As, mg/l 0.2 0.2 0.3 Cu, mg/l 0.4 0.5 0.4 Cd, mg/l 1.4 12.0 1.4 Sulphate-purified phosphoric acid, containing 45 % P205 and, calcu-~ated per litre of acid, 16 mg As, 17 mg Cu and the following cadmium contents; 10, 20, 40 and 80 mg respectively, were neutralized to an Na:P ratio of 0.10 and admixed with 1 gram of commercially available Na2S per litre o~ phosphoric acid. The heavy metal sulphides were filtered off after 2 hours. The results are given in Table 4.

I ~ S ~ ~ 2 7 TA~LE 4 . _. ~ . ........................... _ _.
content Phosphoric acids purified of heavy metals 5Example 4, one stage Example 5, two stages 10 mg 20 mg 40 mg 80 mg 10 mg 20 mg 40 mg 80 mg Cd/l Cd/l Cl/l C1/1 Cd/l Cd/l Cd/l Cdll P205, %44.8 45.0 44.6 44.7 44.8 44.9 45.0 ~4.6 As- mg/l 0.2 0.4 0.3 0.3 0.2 0.2 0.3 0.2 Cu, mg/l0.4 0.4 0.5 0.5 0.3 0.4 0.5 0.5 Cd, mg/l10 2.5 1.1 0.7 205 2.2 1.3 0.6 A phosphoric acid which had been purified of sulphates and which had the same composition as that of Example 4 was purified of heavy metals ;n two stages, whereat arsenic sulphide and copper sulphide were precipitated out and filtered of~ in a first stage and cadmium sulphide was precipitated out in a second stage. It will be seen from Table 5 that very small quantities of cadmium were also precipitated out in phosphoric acid in the absence of As3~ and Cu2~.

Phosphoric acids which had been purified of sulphate and which had varying P20~ concentrations were purified of heavy metals in a si:ng~e stage in which the Na:P molar ratio was 0.10 and in which the tempe~a-ture was 25 and 50C respectively. In all cases the cadmium content of the phosphoric acids was 15 mg per 100 grams of P205 or ~J 60 mg/l.
In all cases the phosphoric acid was admixed with I gram of commer-cially available Na2S per litre, and after 2 hours the heavy metal sulphides were ~iltered of~. The results are shown in Table 5 below.

1~5B'12 ~

Temp. Phosphoric ac;d present Puri~ied o ' phosphoric C PzO Cd/As~Cu Na:p Cd Cd Weight Molar ratio mg/l , mg/
% ratio , ~
42 5 0.10 65 0.2 37 5 0.10 55 0.2 31 5 0.10 45 0.2 42 5 0.10 65 0.2 37 5 0.10 55 0.2 31 S 0.10 45 0.2 -Phosphoric acid which had been purified of sulphates and which con-tained 40 mg of Cd per litre was admixed with lime milk to a CaO:P205 molar ratio of 0.20, whereafter the heavy metal sulphides were precipitated out in a single stage by adding 1 gram of commer-cially available Na2S per 1itre of acid. The purified phosphoric acid contained 0.9 mg of Cd per litre.

In the following example an evaporated wet phosphoric acid produced from a different phosphate raw material than the phosphoric acid treated in Examples 1-7 was purified of heavy metals.

The phosphoric acid had the following composition:
53.0 % P205 2.65 % S0 10 mg As/l 45 mg Cu/l 14 mg Cd/l 1 ~ S ~
lo The sulphate content of the phosphoric acid was precipitated out with stoichiometric quantities of Ca(OH)2, which was added in the form of lime milk containing ~ 150 grams of CaO per litre. Subse quent to the precipitation of gypsum, 1 gram o~ commerclal1y avail-able Na2S was added as a 5 % solution to 1 litre of suspension.
After 1 hour, gypsum and heavy metal sulphides were filtered off, whereafter the phosphoric acid had the following heavy metal composi-tionO
46.2 % P205 10003 mg As/l 10 mg Cu/l 12 mg Cd/l The phosphoric acid, purified of arsenic and partially purified of copper, was admixed with lime milk to obtain CaO:P205 molar ratios of 0.10 0.20 and 0.30, and was admixed with varying quantities of sodium sulphide, whereat the following results were obtained.

-Na~S CaO - o 10 - ~ 5 ~ = 0.30 9/1 . .. _._ ..
As Cu Cd As Cu Cd As Cu Cd mg/l mg/lmg/l mg/l mg mg/l mg/l mg/l mg/l _ _ _ _ 0.5 0.06 0.1 8.5 0.06 0~4 5.8 0.05 0.2 1.3 1.5 0.0~ 0.1 4.4 0.05 0.1 l.O 0.05 0.4 0.9 3.0 0.05 0.1 1.4 0.05 0.1 0.5 0.05 0.~ 0.3 _ In this example the non-evaporated phosphoric acid having the follow ing composition was purifled of heavy metals:

4 ~ ~

31.4 % P205 9.2 mg As/l 18 mg Cu/l 9.2 mg Cd/l 35.5 9 So2 /1 9 A.
Ammonium hydroxide was added to 1000 ml of phosphoric acid in sto-ichiometric quantities relative to the content of sulphiric acid, wherea~ter ~urther ammonium hydroxide was added to obtain an NH4/P205 molar ratio of 0.25, wherea~ter 1.5 gram of Na2S was added as a 5 % solution. After 2 hours at ~ 60C, the heavy-metal sulphide precipitate was filtered off.
The results were: 0.19 mg of As/l, 0.2 mg of Cu/l and 0.9 mg of Cd/1.

9 B.
Potassium hydroxide was added to 1000 ml of phosphoric acid in sto-ichiometric quantities relative to the sulphuric acid content, where-further potassium hydroxide was added to obtain a K20/P205 molar - ratio of 0.25, whereafter 1.59 Na2S were added, as a 5 % solution.
After 2 hours at ~J 60C, the heavy-metal sulphides precipitated out were filtered off. The purified acid had the following heavy-metal content.
0.2 mg of As/l, 0.4 mg of Cu/l, 0.9 mg of Cd/l.

. ..
The method disclosed in Japanese Patent Specification 53075-196 for precipitating out heavy metals in Khouribga phosphoric acid wikh varying P205 concentrations was applied in this working example.
A mixture o~ lime milk and a 10 % Na2S-solution was added to the phosphoric acid and the following results illustrate that cadmium sulphide was not precipitated.out.

1 ~ 51~2~

.. .. _ i--Element POS Ca Ca POS Ca present ~ = 1.5 ~ ~ 1.5 ~ = 1.0 S2 /Me=25 s2 /Me=25 S2 /Me=Z5 _ ~
P205, % 53.0 45.4 49.7 31.4 2g.4 As, mg/l 14 0.15 0.23 9.2 0.1 Cu, mg/l 45 3.4 2.2 18 0.3 Cd, mg/l 14 12 13 9.2 9.0 ; It can be noted that, even after increasing the cadmium content of the phosphoric acid to 45 mg/l, no cadmium purification was obtained with the known method.

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of removing heavy metals from wet-process phosphoric acid produced by digesting raw phosphoric material with sulphuric acid, by precipitating out the heavy metals as sulphides, comprising first removing from the phosphoric acid any residual sulphuric acid present therein by neutralization with alkali or by precipitation as a not-readily dissolved sulphate, supplying alkali to the phosphoric acid to obtain an Me/P ratio within the range of about 0.05-0.03, where Me is the number of cation equivalents in the alkali and P is the number of phosphorous atoms in the phosphoric acid; precipitating out the heavy metals by adding a sulphide compound which is soluble in the phosphoric acid; and removing the resultant sulphide precipitate.

2. A method of removing cadmium from wet-process phosphoric acid produced by digesting raw phosphoric material with sulphuric acid, by precipitating out the cadmium as sulphides, comprising first removing from the phosphoric acid any residual sulphuric acid present therein by neutralization with alkali or by precipitation as a not-readily dissolved sulphate, supplying alkali to the phosphoric acid to obtain an Me/P ratio within the range of about 0.05-0.03, where Me is the number of cation equivalents in the alkali and P is the number of phosphorous atoms in the phosphoric acid;
precipitating out the cadmium by adding a sulphide compound which is soluble in the phosphoric acid; and removing the resultant sulphide precipitate.

3. A method according to claims 1 or 2, wherein the residual sulphuric acid is removed by adding a calcium compound to precipitate out gypsum, which is removed.

4. A method according to claims 1 or 2, wherein the residual sulphuric acid is removed by adding an alkali.

5. A method according to claims 1 or 2, wherein the residual sulphuric acid is removed by adding calcium hydroxide or calcium carbonate to precipitate out gypsum, which is removed.

6. A method according to claims 1 or 2, wherein the residual sulphuric acid is removed by adding ammonium hydroxide, potassium hydroxide or sodium hydroxide.

7. A method according to claims 1 or 2, wherein when the Cd:(As+Cu) ratio in the phosphoric acid is less than about 0.5, the sulphide compound is added in excess quantities relative to the heavy metals present.

8. A method according to any one of claims 1 or 2, wherein the sulphide compound is added in an excess quantity relative to the heavy metals to be precipitated out, in a quantity corresponding to about 0.2-1g S2- per litre of phosphoric acid.

9. A method according to claims 1 or 2, wherein when the Cd:(As+Du) ratio in the phosphoric acid is less than about 0.5, the sulphide compound is added in excess quantities relative to the heavy metals present, the sulphide compound being added in the amount which equals up to 2x(0.2-1g S2-) per litre of phosphoric acid.

10. A method according to claims 1 or 2, wherein alkali is added to obtain an Me/P ratio within the range of about 0.1-0.2.