AU595662B2

AU595662B2 - Method and apparatus for continuous synthesis of aqueous aluminum sulfate solution from aluminum hydroxide sludge

Info

Publication number: AU595662B2
Application number: AU81232/87A
Authority: AU
Inventors: Hideo Yoshizaki
Original assignee: Yoshida Kogyo KK
Current assignee: YKK Corp
Priority date: 1986-05-16
Filing date: 1987-11-16
Publication date: 1990-04-05
Anticipated expiration: 2007-11-16
Also published as: DE3738888A1; DE3738888C2; GB8726775D0; FR2623489B1; JPH0336768B2; HK102094A; CA1328980C; FR2623489A1; GB2212486B; JPS62270416A; GB2212486A; AU8123287A

Description

COMMONWEALTH OF AUSTRALI 9 6 2 Patents Act 1952 C O M P L E T E S P E. C I F.I-'C-A TI ON

(ORIGINAL)

Application Number Lodged Complete Specification Lodged Accepted Published Priority t R Related Art This document contains he amendments made under Section 49 and is correct for printing.

t Name of Applicant Address of Applicant Actual Inventor/i YOSHIDA KOGYO K.K.

No. 1, Kanda Izumi-cho, Chiyoda-ku, Tokyo, Japan Hideo Yoshizaki F.B. RICE CO., Patent Attorneys, 28A Montague Street, Balmain N.S.W. 2041 Address for Service Complete Specification for the invention entitled: METHOD AND APPARATUS FOR CONTINUOUS SYNTHESIS OF AQUEOUS ALUMINUM SULFATE SOLUTION FROM ALUMINUM HYDROXIDE SLUDGE The following statement is a full description of this invention including the best method of performing it known to us:rrrnrr-.- L BACKGROUND OF THE INVENTION 1. Field of the Invention: This invention relates to a method for continuous synthesis of an aqueous aluminum sulfate solution from the aluminum sludge separating, in a state containing aluminum hydroxide as a main component thereof, from the effluent originating in the step of '10 treatment of alunminum for the production of Alumite (a product of aluminum having a corrosionproof coating formed thereon by o *o anodization) operated by an aluminum-related enterprise and to an apparatus for effecting the method.

2. Description of the Prior Art: In the work of Alumite treatment, as a pretreatment therefor, I C an aluminum blank is etched with a caustic soda solution to remove scratches or other similar flaws inflicted on the surface thereof and smoothen the surface. Otherwise in the work of reprocessing t C c' (recoating) a rejectable aluminum product, this product is treated with a caustic soda solution for the removal of an oxide coat therefrom. Further, the aluminum black is subejcted to a treatment for anodic oxidation and a treatment for electrolytic coloration in a sulfuric acid solution. During these treatments, Al ions are dissolved out of the aluminum blankinto the aforementioned solution and entrained thereby into an adjoining washing tank. The spent washing water is forwarded to the step for disposal of waste water. While the waste water is undergoing a neutralizing treatment, there occurs a white precipitate of aluminum hydroxide.

law 'P"L-_w The waste liquid (slurry) consequently produced is generally caused by addition of an acrylamide type macromolecular flccculant to form a floc -of high solidity. .This floc is dehydrated to form an aluminum sludge (hereinafter referred to simply as "sludge") and the sludge is subjected to the subsequent treatment (to be discarded or recovered). This sludge is generally composed of 83 to 87% of water, 8 to 12% of Al(OH) 3 oo a and 6 to 3% of impurities (such as SiO 2 and organic substances).

o 02 In the surface treatment of aluminum,an aqueous sulfuric 6 0 10 acid solution is used for the removal .of the coat from aluminum o 0 6 oeo surface and the coat adhering to the surface of a jig. These works for the removal of coats give rise to waste sulfuric acid as a waste of aging. This waste sulfuric acid is generally i composed of 75 to 90% of free H 2 0 and 2 to 0.5% of A1 2

SO

4 3 t rt From the standpoint of prevention of environmental pollution and preservation of resources, the sludge and the waste sulfuric acid rising from the works of aluminum surface treatment are S generally utilized for the synthesis of aqueous aluminum sulfate 6t t solution. The reaction involved in the synthesis of aluminum sulfate, however, proceeds so quickly as to render control of reaction velocity difficult. Further, since the aluminum sulfate as a final product has the pH value thereof regulated to suit the purpose for which the product is put to use, the final pH value of the liquid for the synthesis must be adjusted so as to equal the pH value of the aluminum sulfate. Since this adjustment of the pH value is difficult, it has been held difficult to produce the aqueous aluminum sulfate solution by the method of continuous synthesis.

Ir 4 The synthesis of the aqueous aluminum sulfate solution from the sludge has been heretofore carried out exclusively by the so-called batchwise process, which generally comprises charging a vessel for the synthesis with a fixed amount of waste sulfuric acid and heating the contents of the vessel with a fixed amount of the sludge supplied gradually thereto.

The aforementioned conventional methodrequires the vessel ,O for synthesis to be made of a material excellent in corrosion- I proofness because the contents of the vessel are composed mainly of highly concentrated waste sulfuric acid during the initial o stage of the synthesis. For exaiple, vessels of the type provided with a glass lining are now in popular use for the synthesis. The vessel of this material has the disadvantage that it is expensive, entails difficult maintenance works of repair and inspection, and tends to break easily. Moreover, since the synthesis is made batchwise, the vessel used therefor is inevitably large and the driving power is proportionately large and the equipment as a whole is voluminous and occupies a large floor area. Thus, the synthesis entails a high cost of equipment and the vessel used therefor cannot be easily expanded for increase of capacity. Further, owing to the batchwise production, the synthesis has the disadvantage that the operation to be involved is complicated in procedure and inferior in efficiency and the vessel for the synthesis which is heated by means of a jacket is destined to suffer from gradual loss of thermal efficiency due to the phenomenon of scaling and entail a high running cost.

'3 i _i c As one of the various kinds of waste occurring in the works of aluminum surface treatment, a dilute aqueous sulfuric acid solution arising, in a state containing aluminum sulfate (hereinafter referred to briefly as "sulfuric alum"), during the recovery of sulfuric acid from the electrolyte constitutes itself a problematic by-product besides the aforementioned sludge and waste sulfuric acid. The aqueous sulfuric acid solution S containing this sulfuric alum has not occurred to any appreciable O extent to date and, therefore, has been sufficiently coped with o %o0 by The treatment of waste water. In recent years, various So methods for the treatment of electrolytes such as the method using ion-exchange resins for separation and the method using diffuse transmission membranes for separation have advanced.

44 Owing to the advance of these methods, the aqueous sulfuric acid solution containing the sulfuric alum is rapidly gaining in volume. In the operation of the method using an ion-exchange resin for separation, for example, part of the sulfuric acidcontaining electrolyte in the electrolytic cell for the treatment of anodic oxidation or the treatment of AC coloration is subjected to an ion-exchange treatment. While the sulfuric acid recovered by this ion-exchange treatment is returned to the electrolytic cell, this treatment gives rise to a treated water containing part of the sulfuric, acid and sulfuric alum. Also in the operation of the method using a diffuse transmission membrane, part of the electrolyte in the electrolytic cell is treated with the diffuse transmission membrane. While the Ssulfuric acid recovered by this treatment is returned to the 4 i electrolytic cell, the treatment produces a treated water containing part of the sulfuric acid and sulfuric alum. The dilute aqueous sulfuric acid solution containing the sulfuric alum generally contains 3 to 6% of H 2 S0 4 and 7 to 9% of A1 2 (S0 4 3 In recent years, as the installation of facilities for the treatment of electrolytes has increased in ratio to keep place with the imporovement of quality of the aluminum surface

S

1 treatment, this dilute aqueoussulfuric acid solution has been growing in volume. In the circumstances, the question as to how .o 21b the dilute aqueous sulfuric acid solution ought to be safely o disposed of has been driving the industry concerned to its wit's end. An idea may possibly be conceived of utilizing this solution for the aforementioned batchwise synthesis of an aqueous aluminum sulfate solution. The present applicant for patent c 15 himself is partially utilizing the solution for the synthesis.

Since the aforementioned sulfuric alum-containing aqueous sulfuric acid solution is a dilute liquid, the use of this solution in an I'oa increased volume inevitably results in a decrease in the reaction .it* velocity. Further, since the synthesis is made batchwise, it does not suit commercialization.

SUMMARY OF THE INVENTION This invention, therefore, has been produced for the purpose of overcoming the drawbacks suffered by the prior art as described above. An object of the inventio is to provide a method for permitting continuous synthesis of an aqueous aluminum sulfate solution by effective use of the sludge, waste sulfuric acid, and sulfuric alum-containing aqueous aluminum 5 sulfate solution arising in the works of aluminum surface treatment and causing serious anxiety to the industry concerned regarding safe disposal thereof and an apparatus to be used for effecting the method.

Another object of this invention is to provide a method for continuous synthesis of an aqueous aluminum sulfate solution, which method. excels in productivity, workability, safety, etc., °o operates with relatively inexpensive equipment, and affords the S product in a high yield, and an apparatus for effecting the o 0 O "J0 method.

Yet another object of this invention is to provide a method S for effecting continuous synthesis of an aqueous aluminum :'ulfate solution with high energy efficiency at a low running cost and an apparatus for working the method.

t Il "15 To accomplish the objects described above according to the present invention, there is provided a method for continuous S production of an aqueous aluminum sulfate solution from by-products occurring in the work of aluminum surface treatment, which method is characterized by the steps of feeding aluminum sludge arising in the work of aluminum surface treatment and consisting mainly of aluminum hydroxide, waste sulfuric acid arising during the removal ofeea39 in the aforementioned work of surface treatment, and an aluminum sulfate-containing aqueous sulfuric acid solution arising during the recovery of sulfuric acid in the aforementioned work of surface treatment to a reaction tank consisting of a plurality oftanks, i~e. first through n'th tanks, causing the fed substances to flow sequentially through 1, d CI 6 to the first tank through the n'th tank (last tank) to undergo reactions under temperature conditions in the range of 80°C to the boiling point of the reaction solution and, at the same time, feeding part of the aforementioned aluminum sulfate-containing aqueous sulfuric acid solution to at least one of the\second tank through the last tank, and adjusting the pH value of the reaction solution in the -eat tank at a level in the range of o 1.6 to 00 2 a° In the method described above, the adjustment of the pH value 0 o0 of the reaction solution constitutes itself one of the important 0 factors for efficient continuous synthesis aimed at. In accordance with the present invention, therefore, there is provided an apparatus for continuous production of the aqueous aluminum S" sulfate solution, which apparatus is designed in order for the t fA adjustment of the pH value to be effected properly. This apparatus is characterized by comprising: a reaction tank consisting of first through n'thAtanks ada~p ted-fce=otmi=continuous reactions respectively of aluminum sludge arising in the work of aluminum surface treatment and consisting mainly of aluminum hydroxide, waste sulfuric acid arising during the removal of coat in the aforementioned work of surface treatment, and an aluminum sulfate-containing aqueous sulfuric acid solution arising during the recovery of sulfuric acid in the aforementioned work of surface treatment, a first feed pipe f£i feeding the aforementioned aluminum sulfate-containing aqueous sulfuric acid solution to the -aEeme t-f-m first tank, 7 o a' 0 0r 0 0 009 I C an .n'th feed pipe (n for an integer of the value of at least 2) E feeding the aforementioned aluminum sulfate-containing aqueous sulfuric acid solution to at least one of the second and subsequent tanks, a first pH indicator-controller-recorder disposed inside the aforementioned first or second tank, a first control value interlocked with the aforementioned o 0 first pH indicator-controller-recorder and 'aig ed=a==f-i o9 the flow volume within the aforementioned first feed pipe, *i,0 an n'th indicator-controller-recorder (n for an integer of o0 S the value of at least 2) disposed in at least one of the aforementioned second and subsequent tanks, and an n'th control valve (n for an integer having the value of l at least 2) interlocked with the aforementioned n'th indicatorcontroller-recorder and 4 e ad-ed e-t eoo:ntse the flow volume within the aforementioned n'th feed pipe.

SFurther in the aforementioned method, the control of the i temperture of the reaction solution also constitutes itself t.t another important factor efficient continuous synthesis aimed at.

In accordance with the present invention, therefore, there is provided an apparatus for reaction temperature control designed to permit continuous production of the aqueous aluminum sulfate solution from by-products occurring in the work of aluminum surface.treatment, which apparatus is characterized by comprising: the aforementioned reaction tank consisting of first through n'th tanks, a first heat exchanger ar effecting exchange of heat between the final aqueous aluminum sulfate solution emanating from the aforementioned reaction tank and possessing a boiling point approximating the boiling point of the reaction solution and the aforementioned aluminum sulfate-containing aqueous sulfuric acid solution, a first, steam pipe disposed inside the aforementioned first a %g tank, Sa first temperature indicator-controller-recorder disposed 40 inside the I--e first tank, S° a first temperature control valve interlocked with the aforementioned first temperature indicator-controller-recorder and 4 i -i e -E the flow volume of steam within the aforementioned first steam pipe, f a an n'th steam pipe (n for an integer of the value of at 0 ooei' least 2) and an n'th temperature indicator-recorder (n for an o integer of the value of at least disposed inside at least one oo of the aforementioned second and subsequent tanks, and an n'th temperature control valve (n for an integer of the value of at least 2) interlocked with the aforementioned n'th temperature indicator-recorder and4 1 manually or automatically the flow volume of steam within the aforementioned n'th temperature indicator-recorder.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flow sheet schematically illustrating a typical apparatus adapted to continuous synthesis of an aqueous aluminum sulfate solution according with the present invention.

S Fig. 2 is a partial flow sheet schematically illustrating 9 L i i -LL i-li- ;lil~L 7 another embodiment of this invention, and Fig. 3 is a partial longitudinal cross section another reaction tank usable in the apparatus according with the present invention.

DETAILED DESCRIPTION OF THE INVENTION In the present invention, an aqueous aluminum sulfate solution is continuously synthesized by causing aluminum sludge o. occurring in the work of aluminum surface treatment, waste o o sulfuric acid occurring during the removal of a coat in the "o 0 aforementioned work of surface treatment, and an aluminum o 0 sulfate-containing aqueous sulfuric acid solution occurring during o o 0 0 the.recovery of sulfuric acid in the aforementioned work of surface treatment to be reacted sequentially in a plurality of to reaction tanks. The synthetic aluminum sulfate solution obtained at the end of these reactions is subjected to filtration and concentration to give rise to aluminum sulfate as a finished o 4 product. The attributes of the finished product depend.on the final pH value of the synthetic aluminum sulfate solution and are I. regulated in a fixed range suitable for a particular purpose for which the finished product is used. In the present invention, the sulfuric alum-containing dilute aqueous sulfiric acid solution is utilized for the adjustment of the pH value of the reaction solution.

The reaction for the synthesis of aluminum sulfate is represented by-the following formula and the reaction formula for the sake of calculation is represented by the following formula 10 .1 2AL(OH) p 3HzSO4 A.z 2 (SO4r) 3 6Hz AL0 3 3H 2 SO6 AZ 2 (SO4) 3+ 31-H20 The reaction velocity constant in the reaction mentioned above and the effect of pH on the reaction were studied using a sludge, a waste sulfuric acid, and a sulfuric alum-containing dilute aqueous sulfuric acid solution possessing respective compositions indicated in Table 1 below.

Table 1 4 0 #4 4 4 04 e tttt t tr F i4~Ct' Waste Sulfuric alum-containing Composition Sludge sulfuric aqueous sulfuric acid acid solution Al(OH) 3 9.5% H20 86.8% 9.4% 85.8§ Other substances (such 3.7% as micro-molecular flocculant)

H

2

SO

4 90.0% 5.3% Al 2

(SO

4 3 0.6% 8.7% Since the reaction of the aforementioned three materials was experimentally demonstrated to be regarded as a homogenous second order reaction, thevelocity constant of the reaction was determine by a batch rest. From the results of the test, it is found that the velocity constant at 90° to 100 0 C was 0.12- j.

6 71 I/Al 2 0 3 -mol.sec. The results of various runs of the test conducted using the three materials in varied mixing ratios indicate that the proper value of the velocity constant ought to 11 exceed at le t 0.01 2/Al 2 0 3 -mol'sec. In view of commercial operation of the synthesis, the highest permissible limit of the velocity constant is 2.0 I/Al 2 0 3 .molsec.

If the pH value is unduly high, namely the amount of free H2So4 is undully small, the reaction velocity is too low for the synthesis to be commercially feasible. In consideration of the fact that the final p! value of the synthetic solution is subject to control, therefore, it is most desirable in the case of continuous reaction to control this continuous reaction in sucha 0 manner that the pH value of the reaction solution in the first tank in the compound of reaction tanks will be lowered to heighten the

+F

reaction velocity and the pH values of the reaction solutions in the subsequent tanks will be gradually increased sequentially to adjust the final pH value of the synthetic solution eventually to the prescribed level. As the result of an experiment conducted by the inventors, it has been found that when the sludge, the waste sulfuric acid, and the sulfuric alum-containing dilute aqueous sulfuric acid solution are continuously fed in a fixed gravimetric ratio to a multi-stage (3 to 6 stages) reaction tank having the component stages provided with respectively fixed volumes commensurate with the intended volumes of treatment at respecitvely fixed temperatures (in the range of 80 0 C to the boiling point of the reaction solution, preferably 90 0 C to the boiling point), the synthetic aluminum sulfate solution possessing a pH value (in the range of 1.6 to 2.5, preferably 1.8 to resulting from completion of the reaction is continuously obtained from the final stage of the reaction tank. The sulfuric alumocontaining dilute sulfuric acid solution is fed in such a manner 12 that the pH values of the reaction solutions in the component tanks will be retained at respectively prescribed levels. For Sthe purpose of increasing the reaction velocity thereby producing the aqueous.aluminum sulfate solution in an increased yield, it is desirable to control.the pH value in the first reaction tank in the range of 0.1 to 1.0. Fo.r commercial operation of the synthesis, it suffices to control this pH value in the range of 0.1 to e o o* Now, the present invention will be described in detail below o l with reference to working examples of the present invention depicted in the accompanying drawings.

Fig.'l is a flow sheet schematically illustrating a typical apparatus adapted to effect continuous synthesis of an aqueous aluminum sulfate solution by the multi-stage tank type parallel- 0 *I -"oI flow reaction of the present invention. In the diagram, 1 stands S for a belt conveyor for forwarding sludge having aluminum hydroxide ao as a main component thereof, 2 fbr a mixing tank, 3 for a reaction tank consisting of first., through n'th tanks, 4 for a tank for holding a sulfuric alum-containing dilute aqueous sulfuric. acid solution, and 5 for a waste sulfuric acid tank.

The reaction tank illustrated in Fig. 1 represents a case satisfying n 5, i.e. using/f=e component tanks. The first tank R1 and the second tank' R 2 are partitioned with a weir-like bulkhead 8a raised upright so as to permit overflow. The second tank R 2 and the third tank R 3 the third tank R 3 and the fourth tank R4, and the fourth tank R 4 and the fifth tank R 5 are severally partitioned with respective bulkheads 8b suspended from above so 13 13- 1'*

;-I

as to form openings in the bottom parts thereof. These component tanks are each provided with a stirrer 9 adapted to be rotated by an electric motor M.

The sludge is fed as carried on the beltofey r 1 to the mixing tank 2. In the meantime, the sulfuric alum-containing dilute aqueous sulfuric acid solution kept at normal room temperature in the dilute sulfuric acid solution tank 4 is forwarded by a pump 10 via a feed line 11 to a first heat exchanger 6, t wherein the solution is heated to a temperature of about 650 to 70 0 C through exchange of heat with the produced synthetic aluminum o sulfate solution possessing a temperature of about 100*C, and then forwarded to a second heat exchanger 7, wherein the solution is heated to a still higher temperature of about 950 to 100 0 C through S exchagne of heat with steam possessing a temperature of boiling point, and thereafter fed to the mixing tank 2 through a first feed pipe 12. Part of the heated solution is fed to the fourth S tank R 4 preceding the last tank through a second feed pipe 13.

The mixture formed of the sludge with the sulfuric alum-containing S aqueous sulfuric acid solution inside the mixing tank 2 is now in a state kept at an elevated temperature of about 700 to 80 0

C,

mixed with the stirrer 9, and then forwarded to the first reaction tank R

I

Meanwhile, the waste sulfuric acid held inside the waste sulfuric acid tank 5 is forwarded by a pump 14 to a waste sulfuric acid receiving tank 15, deprived of floating substances with a screen 16 disposed:inside the receiving tank 15, and then forwarded to the first reaction tank R 1 by a pump 17.

-14

'NT

In the reaction tank 3, the reaction solution formed inside the first tank R 1 of the mixture of sludge, waste sulfuric acid, and sulfuric alum-containing aqueous sulfuric acid solution is caused to flow from the first tank R1 to the final fifth tank R as kept stirred by the stirrer 9 and undergo reactions represented by the formula mentioned above. As the result, the synthetic aluminum sulfate solution is obtained at about 100 0 C from the a fifth tank R 5 This solution is forwarded by a pump 18 through a produced solution line 19 and the first heat exchanger 6 to a o 1°D filtration-purification unit (not shown).

Now, the control system will be described below. First, as c regards the control of the pH value of the reaction solution, the pH value of the reaction solution in the first tank R 1 and the it S*t' second tank R 2 is fixed at a level of about 0.8. When the actual 15 feed amount of the sludge is changed so as to fall far below the t theoretical feed amount, the feed amount of the waste sulfuric i 't acid is proportionately increased, with the result that the pH S. value shifts from the fixed level, pH 0.8, toward the acidic side (to pH 0.2, for example). In this case, therefore, the pH value must be controlled. This adjustment of the pH value is effected by a first pH indicator-controller-recorder (PHICR) 20 which is disposed inside the second tank R 2 When the reading of pH value on the first pH indicator-controller-recorder is below 0.8 (strongly acidic side), a first control valve 21 disposed in the first feed pipe 12 for the sulfuric alum-containing dilute aqueous sulfuric acid solution and interlocked with the..aforementioned recorder 20 is closed. Conversely when the reading of pH value 15 I __t is above 0.8, the first control valve 21 is opened. Thus, the pH value of the initial reaction solution is automatically controlled to 0.8 by suitable control of the feed amount of the sulfuric alum-containing dilute aqueous sulfuric acid solution.

The adjustment of the pH value in the first tank is normally effected as described above. If the actual feed amount of the sludge increases beyond the prescribed level as when the operation a of the apparatus is resumed after a stop or when the pH value in a 04 the first tank is appreciably varied from the prescribed level by Sl'O a certain other factor, it naturally follows that the pH value 0 a of the first tank R 1 deviates from the prescribed level. For the a, a V a purpose of returning the pH value to within the fixed range, the waste sulfuric acid having a lower pH value (strongly acidic side) than the aforementioned sulfuric alum-containing dilute aqueous -113 sulfuric acid solution may be used in the place of the dilute o aqueous solution just mentioned. When the reading of pH value on the first pH indicator-controller-recorder 20 is below 0.8, the supply of the waste sulfuric acid is discontinued by turning off the pump 17 of the waste sulfuric acid receiving tank 15. When i the reading of pH is above 0.8, the pump 17 is opened and the capacity thereof is increased (by opening the valve) to start feeding the waste sulfuric acid to the first tank R 1 Thus, the pH value is automatically controlled to 0.8. The suitable increase or decrease of the feed amount of the waste sulfuric acid possessing a low pH value is advantageous for pH adjustment particularly where the pH value heavily deviates because the time required for the pH value to return to within the fixed range is 16 1~ smaller than when the sulfuric alum-containing dilute aqueous sulfuric acid solution is used. It will be readily understood by persons skilled.in the art that this pH adjustment can be accomplished by using both the sulfuric alum-containing dilute aqueous sulfuric acidsolution and the waste sulfuric acid.

Optionally, the first pH indicator-controller-recorder may be disposed inside the first tank R 1 and adapted to detect o the pH value on the outlet side of the first tank R As the So° reaction proceeds, the pH value of the reaction solution increases 3O and shifts toward the weakly acidic side. When the pH value of o a 0 the synthetic aluminum sulfate solution within the fifth tank 0 0 0 (last tank) R 5 falls on the strongly acidic side below the prescribed level (1.6 to 2.5, preferably 1.8 to the detector Sof a second pH indicator-controller-recorder (RPHCR) 22 disposed inside the fifth tank R 5 detects this deviation of the pH value and closes a second control valve 23 disposed in the second feed Spipe 13 for the sulfuric alum-containing dilute aqueous sulfuric S acid solution. When the pH value conversely rises above the prescribed level, the aforementioned detector automatically opens the second control valve 23. It is permissible to connect feed pipes for the sulfuric alum-containing dilute aqueous sulfuric acid solution one each to the component tanks, disposing pH indicator-cntroller-recorders one each in the component tanks, and effect fine adjustment of the pH values of the reaction solution in the component tanks by the procedure described above.

In any event, since the adjustment of the pH value of the reaction solution is effected by controlling the feed amount of the sulfuric 17

AJ

i_ L ii^ B alum-containing dilute aqueous sulfuric acid solution and/or that of the waste sulfuric acid and further since the pH value of the reaction solution is not less than 0.8 and the conversion in the first tank is about 90% and the pH value in the second tank cannot fall below the aforementioned level, it suffices to use stainless steel of the grade of about SUS 316 JIL as the material for the first tank and stainless steel of the grade of about SUS 316 L 0 0 as the material forthe second and subsequent tanks.

0 0 0 9 Then, as regards the control of the temperature of the 0 reaction solution, the temperature of the mixture of the sludge and the sulfuiic alum-containing dilute aqueous sulfuric acid solution inside the mixing tank 2 is elevated to a level in the oe 0 o 0 S range of 700 to 80 0 C owing to the supply of the sulfuric alumcontaining dilute aqueous sulfuric acid solution which possesses g'4I a temperature in the range of about 950 to 100 0 C. The sulfuric S* alum-containing dilute aqueous sulfuric acid solution kept at room .I temperature inside the dilute sulfuric acid tank 4 is heated, as

L

described above, to a level of about 650 to 70 0 C (as detected by a temperature indicator-recorder (TR) 24) by means of the first heat exchanger and then elevated further to a level of about 950 to 100 0 C by means of the second heat exchanger 7. This temperature is detected by a temperature indicator-controller-recorder (TRC) disposed behind the second heat exchanger 7 in the feed line for the sulfuric alum-containing dilute sulfuric acid solution.

When the temperature.so detected is below the prescribed level, a steam control valve 27 disposed in a steam feed pipe 26and interlocked with the aforementioned recorder 25 is opened.

18- LV r.

I

When the detected temperature is above the prescribed level, the steam control valve 27 is narrowed. By thus controling the amount of steam entering the second heat exchanger 7, the temperature is automatically controlled to the prescribed level. To the reaction tanks are connected steam feed pipes. A first steam pipe 28 is immersed in the form of a coil or a planar plate in o the reaction solution held inside the first tank R and then ed o o to a waste discharge groove 32 so as to effect exchange of heat.

o 0 o. To the second through fourth tanks, a second steam pipe 29, a oi0 third steam pipe 30, and a fourth steam pipe 31 are respectively Sn o connected so as to effect introduction of steam. The temperature of the reaction solution inside the first tank R 1 is controlled by a.first temprature indicator-controller-recorder (TRC) 33 and 0 00 o o a first temperature control valve 34 disposed in the aforementioned first steam pipe 28 and interlocked with the aforementioned o* o* recorder 33. When the temperature of the reaction solution in the s first tank R- is above the prescribed level, the first temperature control valve 34 is narrowed. When this temperature is below the prescribed level, the control valve 34 is opened. Thus, the temperature is automatically controlled to the prescribed level.

To the third and fifth tanks R 3 and R5 are respectively connected S. a third and a fifth temperature indicator-recorder (TR) 35 and 36.

Depending on the temperatures detected by these temperature indicator-recorders 35 and 36, the control of temperature is manually effected by opening or closing second through fourth control valves 37 through 39. Optionally, the temperature of the reaction solution may be automatically effected by disposing 19 temperature indicator-controller-recorders one each in the component tanks and interlocking these recorders to the respective temperature control valves in the steam pipes. Where the system is designed so as to elevate the temperature by introducing.

steam into the component tanks, the reaction solution is diluted and consequently suffered to entail use of extra energy in the S subsequent work of purification. It is, therefore, desirable to S effect the elevation 6f the temperature of the reaction solution in the first tank by the exchange of heat between the steam pipe t 'd and the reaction solution as illsutrated in the diagram.

oo _"o 0 0 Further in Fig. i, the reference numeral 40 stands for a liquid level indicator-controller (LC) in the waste sulfuric acid receiving tank 15 for preventing the motor of the pump 17 from idle rotation. It automatically actuates the pump 14 when the liquid level of waste sulfuric acid in the waste sulfuric acid receiving tank 15 falls below a fixed line mark and it stops the operation of the pump when the liquid level rises above the mark.

The reference numeral 41 stands for a liquid level indicatorcontroller (LICA) provided with an alarm and adapted to control the liquid level of the reaction tank 3. It automatically narrows a control valve 42 when the liquid level of the reaction solution 4 falls below the fixed mark and it opens the control valve 42 when the liquid level rises above the mark. The reference numeral 43 stands for a cumulative flow amount indicator (FIQ) adapted to display the feed amount of the sulfur alum-containing dilute aqueous sulfuric acid solution. Continuous synthesis of an aqueous aluminum sulfate solution was carried out with the apparatus of Fig. 1 under the following conditions.

20 Conditions of synthesis Sludge: e o 00 a a 00 0 c 0o 09 0 0 0 o o a 5 o 009 0 0 Feed.amount 1,429 kg/hour Composition Al(OH) 3 Impurities H.0 Waste sulfuric acid: Feed amount 72.6 liters/hour Composition Free H 2

SO

4 9.5% by weight 3.7% by weight 86.8% by weight 90.0% by weight A1 2

SO

4 3 0.6% by weight Specific gravity 1.804 (25 0

C)

Sulfuric alum-containing aqueous sulfuric acid solution Feed amount 3,112 liters/hour Composition Free H2SO 4 5.3% by weight A1 2 (SO4)3 8.9% by weight Balance 2 0 Specific gravity 1.134 (12 0

C)

Reaction temperature: 990 to 100 0

C

pH value of first tank: About 0.8 pH value of fifth tank: About As the result, the aqueous aluminum sulfate solution could be continuously synthesized at a conversion of about 98%. The aqueous aluminum sulfate solution thus obtained was subjected to a filtration test under the conditions, i.e. 300 ml as the amount of synthetic solution used, 20 0 C as the filtration temperature, filtration under a vacuum (without filtration aid) as the manner of filtration, 95 cm 2 as the available area of 21

I

filtration, Filter Paper, of Toyo Roshi Co., Ltd.~s the filtration pressure (degree of vacuum). The test revealed no problem as to the filtration property of the solution.

Fig. 2 illustrates another typical apparatus for continuous synthesis of an aqueous aluminum sulfate solution of this invention, depicting the only portions with respect to which this r apparatus differs from the apparatus of Fig. i. The devices S (such as, for example, waste sulfuric acid tank and heat exchangers), the measuring instruments (such as, for example, pH o0 indicator-controller-recorders), and the feed lines (such as, for 0 00 0o 0 example, feed line for sulfuric alum-containing aqueous sulfuric acid solution) which.are not Shown in Fig. 2 are identical to those t shown in Fig. i. The apparatus illustrated in Fig. 1 is provided with the mixing tank 2. This mixing tank is disposed as illustrated simply because no reaction tank can be installed below the belt conveyor where the existing apparatus is adopted in its Sunmodified state. This installation of the mixing tank is not always necessary. As illustrated in Fig. 2, the aforementioned three raw materials can be directly fed to the first tank R 1 of the reaction tank 3 instead. In the apparatus of Fig. 2, the sludge is forwarded by a pump through a sludge feed pipe 44. Of course, it can be forwarded by a belt conveyor as in the apparatus of Fig. 1. In the reaction tank 3 illustrated in Fig. 2, the component tanks R 1 through R 5 are partitioned alternately with weir-like bulkheads 8a and bulkheads 8b suspended from above so as to prevent otherwise possible short pass of the reaction solution. For the purpose of more effectively preventing the short l- 22 NT 0'i pass of the reaction solution, the component tanks may be provided 53 therein with a baffle plate Aas illustrated in Fig. 3.

The control of the temperature of the reaction solution in the first tank R 1 is automatically effected, similarly to the aforementioned apparatus of Fig. 1, by causing the first temperature indicator-controller-recorder 33 to control the first temperature control valve 34 disposed in the first steam pipe 28.

e* 0# "By the same token, the temperatures of the reaction solution in the second through 4th tanks R 2 through R 4 are automatically 2 4 controlled by means of second through fourth temperature indicator-controller-recorders (TRC) 45 through 47 disposed in the respective component tanks and second through fourth temperature control valves 48 through 50 disposed respectively in the second through fourth steam pipes 29 through 31. Further, the temperatures of the component tanks are controlled by exchange of I heat between the portions of the reaction solution inside the component tanks and the second through fourth steam pipes immersed in the form of a coil or a planar sheet in the reaction solution.

The apparatus illustrated in Fig. 2 is provided with a synthetic solution circulation line 52 which issues from the discharge side of the pump 18 for discharge of the synthetic solution and returns into the first tank R

I

This circulation line 52 is provided therein a manual valve 51. Owing to this device, the aqueous aluminum sulfate solution can be returned to the first tank R 1 by opening the manual valve 51 when the second pH indicator-controller-recorder (PHICR) 22 (Fig. 1) serving to display the pH value of the synthetic solution displays an 23 i ::f

:X

z 44 0 0 B 0 0 O0 o o o o O 0 0 0 0o a a s o ,0 o o a a abnormal value (outside the prescribed range). Of course, the apparatus of Fig. 1 may be provided with the circulation line.

It is evident from the description given above, the following effects and advantages are derived from the present invention.

a) Since the synthesis of the aqueous aluminum sulfate solution is carried out continuously by the multi-stage tank type parallel reaction, the capacity for production is notably improved as compared with the conventional synthesis by the batchwise operation and, for a fixed volume of production, the apparatus permits a generous reduction in overall size and floor area. The apparatus, when necessary, may be installed indoors.

:Oo b) Since the adjustment of the pH values of the reaction system is effected by the use of the sulfuric alum-containing dilute aqueous sulfuric acid solution, the pH adjustment can be attained easily and the free H 2

SO

4 concentration in the first tank 2 4 is low. By controlling the' pH values of the reaction solution in the component tanks in the range of 0.8 to 2.5, therefore, the reaction tanks made of stainless steel can be safely used. The reaction tanks, accordingly, can be given necessary maintenance by proper welding, repair, and inspection. Thus, the cost of equipment is low. Since the apparatus is continuously operated, the pH value of the reaction solution can be continuously controlled without requiring the valves, pumps, stirrers, etc. to be switched from time to time. The apparatus, therefore, can be easily automated. It is also effective in improving the workabiliLy and the safety.

24 ~Vlrrmrr.rr.n*ln~~i" 'XiiT l ~L c) On the basis of the experimental data on the reaction velocity constant, the capacity for production can be increased with a minor modification of the apparatus.

d) The system of the present invention for reaction temperature control entails virtually no loss of thermal efficiency due to scaling because the heating is effected by the Suse of the heat exchange units installed independently of the reaction tanks. Further, the control of the temperature can be Seffected automatically. Owing to the ease of temperature control acoupled with the effect of continuous operation, the apparatus enjoys high energy efficiency and low power consumption and a low running cost as well.

The other advantages and effects of the present invention will be apparent from the foregoing description of the invention.

t a 25

Claims

9. 0e 99 9 *j 9 9 *9 9 9 9 -26 THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A method for continuous production of an aqueous aluminum sulfate solution from by-products occurring in the work of aluminum surface treatment, which method is characterised by the steps of feeding aluminum sludge arising in said work of aluminum surface treatment and consisting mainly of aluminum hydroxide, waste sulfuric coot \y3 acid arising during the removal of 4 ee*k in said work of surface treatment, and an aluminum sulfate-containing aqueous sulfuric acid solution arising during the recovery of sulfuric acid in said work of surface treatment to a reaction tank consisting of a plurality of4tanks, causing said fed substances to flow sequentially through a first tank through4a last tank to undergo reactions under temperature conditions in the range of 80 0 C to the boiling point of the reaction solution and, at the same time, feeding part of said aluminum sulfate-containing tocn\e.s Lo fce- aqueous sulfuric acid solution to at least one of the\ to second tank throughjthe last tank, and adjusting the pH value of the reaction solution in the last tank at a level in a prescribed range of 1.6 to 2. The method according to claim 1, wherein part of the aqueous aluminum sulfate solution emanating from the final tank is returned to the reaction tank when the pH value of the reaction solution in the final tank departs from the prescribed range. 3. The method according to claim 2, wherein part of the aqueous aluminum sulfate solution emanating from the final tank is returned to the first tank. O I; _-IL .il- -ii~ -li ri_- I) 4. The method according to any of Claims 1 through 3, wherein the pH value of the reaction solution in the first or second tank is adjusted in the range of 0.1 to The methodjaeedign to Claim 4, wherein the adjustment of the pH value of the reaction solution is effected by controlling the amount of said aluminum sulfate-containing aqueous sulfuric a acid solution fed to the first tank. 00 00 0. o S6. The method according to any of Claims 1 through wherein the control of the temperature of the reaction solution in the reaction tank is carried out by feeding steam to said 00 reaction solution. 7. The'method according to any of Claims 1 through a wherein the control of the temperature of the reaction solution in the reaction tank is carried out by exchange of heat between S said reaction solution and a steam pipe immersed in said reaction solution. 8. The method according to any of Claims 1 through 7, o"'o wherein said aluminum sulfate-containing aqueous sulfuric acid solution heated to an elevated temperature is fed to the first tank. 9. An apparatus for continuous production of an aqueous aluminum sulfate solution from by-products occurring in the work of aluminum surface treatment, which apparatus is characterized by comprising: a reaction tank consisting of first through n'thAtanks Pe-- C r aiapad---p d+Efe e ~-ntcontinuous reactions respectively of aluminum sludge arising in the work of aluminum surface treatment and 27 i if ~14 consisting mainly of aluminum hydroxide, waste sulfuric acid arising during the removal of coat in said work of surface treatment, and an aluminum sulfate-containing aqueous sulfuric acid solution arising during the recovery of sulfuric acid in said work of surface treatment, a first feed pipe as feeding said aluminum sulfate- containing aqueous sulfuric acid solution to4sai4 first tank, 5' 5an n'th feed pipe. (n for an integer of the value of at least 2) for feeding said aluminum sulfate-containing aqueous sulfuric acid solution to at least one of the second and subsequent tanks, a first pH indicator-controller-recorder disposed inside S said first or second tank, ,2D 0 a first control valve interlocked with said first pH SC-or:t C\\ indicator-controller-recorder andAd a etsei a the flow volume within said first feed pipe, an n'th indicator-controller-recorder (n for an integer of t LI the value of at least 2) disposed in at least one of said second and subsequent tanks, and an n'th control valve (n for an integer having the value of at least interlocked with said n'th indicator-controller- recorder and4ap e-d=e=eEaL the flow volume within said n'th j feed pipe. The apparatus according to Claim 9, wherein said n'th feed pipe serves to feed said aluminum sulfate-containing aqueous sulfuric acid solution to the tank immediately preceding the last tank, said n'th pH indicator-controller-recorder is disposed in the final tank, and said n'th control valve is disposed in said ©?ALI feed pipe. 28 r
11. The apparatus according to Claim 9 or Claim wherein said reaction tank is divided into said first through n'th tanks by bulkheads suspended from above so as to form an opening on the bottom part of said reaction tank and/or weir-like bulkheads raised upright so as to allow overflow of the reaction solution. B
12. The apparatus according to Claim ll,wherein a baffle S plate adapted to prevent short pass of the reaction solution is 0 0 disposed in at least one of said first through n'th tanks. o o 13. An apparatus for reaction temperature control to permit *0 continuous production of an aqueous aluminum sulfate solution from a by-products occurring in the work of aluminum surface treatment, i, which apparatus is characterized by comprising: a reaction tank consisting of first through n'th\tanks 0 S F Is -i E A kcontinuous reactions respectively of aluminum sludge arising in the work of aluminum surface treatment and consisting mainly of aluminum hydroxide, waste sulfuric acid arising during the removal of coat in said work of surface treat- ment, and an aluminum sulfate-containing aqueous sulfuric acid solution arising during the recovery of sulfuric acid in said t e. ati e-r\- work of surfaceA tzatmnt, L a first heat exchanger fsr effecting exchange of heat between the final aqueous aluminum sulfate solution emanating from said reaction tank and possessing a boiling point approximating the boiling point of the reaction solution and said alumium sulfate- containing aqueous sulfuric acid solution, 29 A 30 a first steam pipe disposed inside said first tank, a first temperature indicator-controller-recorder disposed inside said first tank, a first temperature control valve interlocked with said first temperature indicator-controller-recorder and lc d=_Lt=GOnt'd the flow volume of steam within said first steam pipe, an n'th steam pipe (n for an integer of the value of at least 2) and an n'th temperature indicator-recorder (n for an integer of the value of at least 2) disposed inside at least one of said second and subsequent tanks, and an n'th temperature control valve (n for an integer of the value of at least 2) interlocked with said n'th *ornk~el k-% 2 temperature indicator-recorder andadapted to control manually or automatically the flow volume of steam within 9t said n'th temperature indicator-recorder.
14. A method for continuous production of an aqueous aluminum sulfate solution substantially as hereinbefore described with reference to the accompanying drawings.
15. An apparatus for continuous production of an aqueous aluminum sulfate solution substantially as hereinbefore described with reference to the accompanying drawings. DATED this 24 day of January 1990 YOSHIDA KOGYO K K Patent Attorneys for the Applicant: F.B. RICE CO. o