CA1048279A - Process for the recovery of metallics from brass skimmings - Google Patents

Abstract

ABSTRACT

Method of recovering metals and a zinc salt by-product from brass skimmings by chemical methods by crushing to loosen and expose zinc oxide from the metallics thereof, optionally screening out the larger particles (which are mainly metallic) for direct recycling, adding the remainder to a reaction vessel together with con-centrated hydrochloric or sulfuric acid with agitation at a rate and ratio to react the zinc oxide but not the metallics (preferably accomplished by controlling such relative additions so as to maintain the reaction zone at a pH between 0.1 and 1.5), and rapidly settling and removing for recycling the metallics from said vessel and overflowing the zinc salt reaction product in solution (as a useful by-product).

Description

~4~3279 The pre~en~ invention relates to a procesi~ or treating brass-skimmings and similar waste materials fox recovery of metallics and useful by-product~ therefrom with minimal losses.
Whenever molten brass comes in contact with air tor other ox~gen containing gases), the zinc aonten~ thereof rapidly forms a ~urface layer o æinc oxide as an impurity.
Thus în the manufacture o virgin brass and in the sub~
se~uent foundry melting of the billets, a significant amount o the zinc content o the brass i9 lost as zinc oxlde which forms as a floating dross and additional signi~icant amounts of the brass alloy itself iis lost in skimmin~ off the floating impurities and fluxe~ prior to p~uring the molten brass. As a result, it has been estimated that S~ of all melted brass is lost to 3uch skimmings. The typical metallic values in such skimmlng are in excess of 85~ (primarily copper and zinc in alloyed form an~ zinc oxide).
It can ~e appreciated that s~ch losses represent a significant economic aon~ideration, which the industry since its inception has long sought to minimize by various methods for recovering the metallic content ~f such skimmings.
Two major bras~ mills alone each generate an average Qf 400 to 750 ton~ of brass skimming~ a month respectively.
Over the years several d~fferent methods have been ~`
attempted to recover these losses. While some ~f these have . .. ;. . . : . - : ., .

succeeded to some degree in reducing the lo~ses, they all still involve substantial loss of the metal content of the brass ski~nings and are additionally quite costly. There have been three principal prior art method3. Some secondary manufacturers treat the brass ski~nings pyrometalurgically, for example in a rotary furnace, tieing up the zinc oxide in fluxes, thus losing 30% to 40% of the zinc content and 10 to 20~ of the copper content and additionally charging about 1% - 3~ of the market value of the brass nugge~-s re~uxnad or the processing.
Recovery has also been achieved by mechanical means by grinding the brass skimmings, suGh as by ball milling, and separating the metals rom the impurities by hydraulic clas~ification. This re~overs up to 90% of the material in the brass skimmings present in metallic form with the washed out fines being discarded as waste constituting essentially all the zinc oxlde mlxed with other debris (including carbon and silica) and as much as 10% to 14% by weight of copper.
In a third met~od, the brass skimmings are smelted and the zinc oxide layer is blown off as waste with a 1% to 3% copper loss. Mo~t of the zinc oontent of the alloy is converted to zina oxida during this process and is also l~st during the blowing process. The brass is then cast into an an~de with khe copper content ~hereof purified and recovered separately by ~lectoly~ic deposition; see for example U.S.
Patent 1,920,820.
It i~ an object of the present in~ention to recover substantially all of the metal values of brass ski~nings ~or of similar alloys having copper and zinc as significant com-p~nents), thus realiziny typically a 10 to 20 fold reduction

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in lo~ses over prior processe~ and at the same time achieving thesa results at greatly reduced proces~ing co~ts.
It is a further object of the present invention to convert the zinc oxide, previously regarded and treated a~
a wa te product, so as to recover the æinc value in ~he form :~
of a usefu~ by-product.
In the specification and in the accompanying draw ings these and other objects and advantage~ will become more apparent, and additionally there are described and shown illustrative embodiments of the invention and various modifi-cations thereof are suggested, but it i5 to be understood that these are not intended to be exhaustive, but on the contrary are given for purposes of illustration in order that others skilled in the art may more fully understand the invention so that they may modlfy and adapt it in various f~rms, each as may be best suited to the conditions to a particular use, In the drawings: ~-Figure 1 is a schematic representation tpartially .
in the form of a flQw diagram) of apparatus for carrying ~ut a preferred process emb~dying the present invention;
Figure 2 is a schematic representation showing a partial modification of the apparatus depicked in ~igure l;
and Figure 3 is a flow diagram representing an alterna~
tive pr~cess emb~dyin~ the present invention. :
The objec~ and advantages o~ this invention are achieved by a strikingly simple and yet heretofore unobvious application of chemical methods by which 98 t~ 99.S~ of the :~
total metallic values can readily be reco~ered ~this alon~
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givlng huge economic value to the process) and urth~r at a proces~ing cost typically ~ignificantly less t~an prior art processes.
In its simplest form, the present invention involves crushing the brass skimmings, e.g. by hammer milling, to loo~en and expose zinc oxide (a white powder) to subsequent attack by concentrated hydrochloric acid or slightly diluted sulfuric acid under conditions which effect conversion o the zinc oxide to a solution of æinc chloride or zinc sulfate but which are insufficient to permlt any e~fective attack of the copper alloy particles ~such conditions typically involv-ing a reaction limited to less than one minute's duration and at a pH less than 3.5 and preferably ranging fxom 0~1 to 1.5). .
Prior art methods o~ treating brass skimmings con-sidered the zinc oxide as a waste product. The present invention converts the zinc of the æinc ~xide to a useful by-product and does so with an efficiency such that all ~f the metallic values arè recovered with practically no lossesc 2~ Zinc chloride is useful in ~alvanizing iron, in soldering fluxes, in medical preparations, as a catalyst, and wQod preservative, among many other uses. Zinc chloride in the past has often been produced from the 2inc skimmings fr~m galvanizing baths which typically have ammonia present in the fluxes. The present invention gives ammonia-free zinc chloride, which is highly desirable. Zinc sulfate finds uses as a preservative, in b~eaching paper, as a fungicide, and a dietary supplement.
In the broader aspects of this invent~on, the term "brass skimmlngs" is intended ~o include the s~immings rom ~L~4~
the melt from an alloy in w~lich copper and zinc ~orm signifi-cant constituents such that the dross included in said bras~
skimmings has in addition to said alloy zinc oxide as a ~ignificant component. The compo3ition of the brass skimmings will depend greatly upon not only the composition of the alloy melt, but also upon the mill practices, including the fluxes used and the skill of ~he skimmer. The higher the zinc con-tent of the alloy and the longer the melt is exposed to the atmosphere, the greater will be the amount of scum ~loating on the alloy, particularly when the zinc content of the alloy i8 in excess o 20% and when significant amounts of fluxes are added. Charcoal, coke, or some other carb~naceous material, is the u~ual cover to pravent the zinc from vapor- :
izing from the melt and also to reduce the ~ormation of zinc oxide. Other ~luxes used in an attempt to reduce the metallic content of the scum can include common salt, borax, soda ash, and even glass.
The cooled mass of brass skimmings is typically run through with metallic shot, nuggets, s~eletons, and scab~.
2~ As a preliminary step in the present inventionl this mass must be crushed at least to the extent required to loosen and expo~e most of the zinc oxide. This can be su~ficiently accompli~hed by a relatively quicX treatment by a hammer mill.
The lo~sened mass can then be passed through a number 4 mesh screen which will typically retain 20% to 50%. This retained material is 85% - 95% metal with the remainder consisting of dirt and debris from the melting pr~cess which can incIude part of the ~urnace lining, siIica, carbon, and other dirtO
This retained material is of sufficient purity and comp~sition tha~ it can be directly recycled to the meltin~ furnaces of "

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the mill or oundry Erom which it was received. It al~o ha the advantage o having ~he alloy composition of the mill from which it wa~ raceived~ If desired, thls retained material can be cleaned by a quick exposure to acid, however this is mainly for "sales appeal" ~y brightening the metal and result~
ing ln only a sligh~ up~rading of the purity.
The material passing through the screen can then be charged directly ~o a reaction vessel a~ a controlled rate. In a pilot operation, a 50~ gallon reaction vessel was continuously charged at the rat~ of 5 tons per hour. A
commercial operation will typically charge at the rate of 20 tons per hour.
; Referring to Figure 1, the reaction vessel 10 is charged by the feed device 12 receiving the minus ~4 mesh material from the screen 14 which has already been crushed by ; the hammer mill 16. Acid is added to the reaction vessel la through inlet 18 at a similarly controlled rate. If re~uired, water is added to the reaction vesseI 10 through pipe 20D
The reaction mass in the vessel 10 is continu~usly mixed and agitated b~ a~itator 22 driven by motor 24.
Depen~ing upon the by-product desired, either con-cent~ated hy~rQchl~ric acid ~r concentrated sulfuric acid ls added through inlet 18. The exothermic reaction of the aci~ with the powdered zinc oxide present in the ~rass skim-mings is very rapid and there~ore requires a reaation timeof less than a minute with no heat nor pressure being required~
The reaction time of the acid in attacking the metals present in the brass skimmings is comparatively slow, particularly since the relati~e surface area is so small due to th~
presence ~ the majori~y of the metal in the ~orm o~ nuggets, ,' 6 . , .. ., ~ :

qhot, and other relatively large particles (even after the aforesaid ~creening).
In this foregoing example of the preferred embodi-ment, the bra~s skimmings are added to the reaction vessel 10 at the rate of 5 tons per houx. ~he p~ of the reaction vessel i~ monitored by sensing device 26 whlch in turn controls valve 28 thus metering in the acld through inlet 18 so as to keep the reaa~ion mass wi~hin a ~pecific pH
range.
~pplicant has determined that if the reaction mass is kept withln the preferred pH range, then the reaction ma~s is acid enough to rapidly attaok ef~ectively all of the zinc oxide and yet is not so acid as unnecess~rily to attack the metallics present in the brass skimmings. F.or ~ :
hydrochloric acid, the desirable pH ranye is from 0.1 to little m~re than 1.0 with the optimum range being from 0.3 to 0.5. F~r sulphuric acid, the desired range is from 0.1 to l.S with the optimum range ~eing from 0.1 t~ 0.8. ~:
Although this pro~ess can be practiced out~ide these ranges, 20 it i8 generally undesirable. With too low a pH, there will : be an ex~ess of acid, requ1rins the addition of zinc duRt ~or its-equivalent) ln order to neutralize the excess acid overflowing the weir 30 from the reaction vessel 10 and to c~nvert the copper salts (resulting from acid attack of the .
~rass metal) to the de3ired zinc salts and to precipitate out metallic copperO On the other hand, with too hi~h a pH
there will be incompletely reacted zinc oxide contaminating the reaction products exiting from the w~ir 30. As the pH
of 3.5 is approached, the reaction becomes too slow for practical purposes, even if the residence time in the : 7~

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~L~4~7~
vesgel 10 i9 greatly increa~ed.
It is possible in the broader aspect~ of this inven-tion to control the reaction by monitoring other parameters or even by metered addition of pre-analyzed reactants; none of which howev~r are as simple and convenient a~ the pH control method.
The reaction product in thi~ preferred example exits from the reac~ion vessel lO by a weir overflow 300 The unreacted heavier particles, mainly consisting of solid metallics, settle to the bottom of the vessel lO and exit through valve mechani~m 32. This can be a simple diaphragm valve, a xotary valve, or even a double valve ~or batch removal of solids.
The reaction wit~in vessel lO can be subject ~o foaming. However this is normally not to~ serious a problem which can be genexally controlled, if necessary, by proper agitation or other known method~ including additive foam suppressants. :' The acid and crushe-d brass s~immings are added in a conven~i~nal manner suficiently remote ~rom the weir o~r-fl~w 30 to insure th~rough mixing and sufficient residence tim~ ~ the reactants in the r~action zone of the vessel lO
to assure that the zin~ oxide .is converted to the æinc salt before it can reach the weir overflow 30. The addition ~f water from supply pipe 20 to the reaction vessel lO control-led by valve 34 is intended mainly to keep the reaction mass sufficiently dilute to insure that the ~inc salt reaction product remains in solution. This is mainly required for zlnc sulfate. ~;
The liquid ove~flGw from the weir 30 is mainly a ' -8-~l~4~79 zinc ~alt ~olution; however, it may con~ain minor amounts of very ~ine metallic~, other contamlnants,, and perhaps some other ma~al salts, normally the copper salt. Con~e-qyently, (if necessary), this overflow can be subsequently S treated by the addition of zinc dust to convert the other more electropositive metals ~o their metallic ~orm thus creating addi~ional zinc salts. Thi~ ~tep has been illus-trated in the form of a precipitatox 36. Thereafter the liquid mass is filtered or continuously centriuged, f~r example in a separa~or 38, to remove the solid fines. These solid contaminan~ will usually contain a large mass of dirt with less than 1/2~ copper (by weight o~ the original chargs).
This copper contact is usually too small to be of concern, or even of value for reclamation.
The sepaxated solids exiting through valve mechanism 32 of reaction vessel 10 are preferably subjected to a gentle washing. In Figure 1 is illustrated an inclined screw con-veyor 40 rlsing from an overflow wash tank 42 against a countercurrent of wash water from spray device 440 I~ desir-ed, the soluble salts washed from the solid metallics can be reclaimed by combining the overflow fr~m ~he tank 42 via the ~onduit 46 (indicated by dots as being an optional alternative) to feed into conduit 48. In this case the wash water 44 ~s minimized to avoid unnecessary dilution of the ' ,~
æinc salt ~olution. Conduit 48 leads from overflow weir 30 to the precipitator 36. ~ , Depending to a large extent upon the ~uality of the brass ~kimmings received, the matexial recovered from the screw conveyor 40 will c~n~ain 80% to 95% metal, up to 8% moisture, and the remainder will ~e dirt. Further purifi~
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cation has been found to be unnecessary. The recovered material can ~e recycled dlrectly to the melting urnaceM.
Typically ~5~ to 99% or more of ~he zinc oxide i~
solubilized and separated from the metallics by conversion to a useful zinc salt by-product. The contaminant~ in the recovered metalR are signi~lcantly reduced with any remaining amounts bein~ well within acceptable limits for recharging to tha bras~ furnaces. The zinc salt ~olution by-product i ammonia-free and has less than 10 parts per million of copper. Since the zinc sa}ts result almost exclu~ively from the æinc oxide and not from the zinc in the metallic alloy, the alloy composition in the brass skimmin~s rema$ns ~ub-stantially unchanged and can usually be recycled directly, with little or no necessity for additional spel~ering.
It can be appreciated the present invention has a significant anti-pollution advantage by converting a former waste produc~ (i.e. contaminated zinc oxide) into a useful by-pr~duct, A measure ~f the effectiveness and efficiency of this invention is that the discarded cla~si~ication fines and/or flue dust fr~m the various prior art proceases for treating brass skimmings can eaonomically be processed by the present ~nvention for recovery o the metallic values an* conversion o~ the 2inc oxlde to the zinc salts.
The #4 screen after the hammer mill 16 can be omitted.
In an alternative to the proces~ illustra~ed in Figure 1, the hamm~r mill-16 and scree~ 14 can be replaced by a ball mill S0 which crushes the brass skimmings ~nto smaller particles for even more efficient and effective treatmentO After the solidified brass skimmings are run .:

.. ~ . . . . ................................. .

. . . . .. - , ~4~7~
relatively quic~ly ~hrough the ball mill and acid-treated, th0 separated ~ettled solids can be passed over a 50 to 70 me~h screen and the ~olids retained thereon will be es~en-tlally pure metal. The minus 50 to 70 me~h materials are S about 80~ metallic which can be dried and char~ed directly to a rotary furnace and even directly to an electric furnace (depending upon the u~er's know how). Altern~tively, it can be bri~uetted, advan~ageously with br~s turnings, or aiding in charging the electric ~urnaces. ~he carbon and fluxes pre~ent in the dirt is advantageous for recyclin~ and the silica and other dirt i9 probably eventually removed as fines fr~m the separator 38 a~ter several recyclings.
In a comparison ~f the present invention with the prior art practiced by the smelting and refining companies, the invention process essentially loses no copper in compari-; son to the latter's 1 to 3~ loss; ~he invention processreturns effec~tively all of the metallic zinc content of the b~ass skimmings in contrast to none xeturned-by the manufac-turers; the invention pxocess converts the zinc ~xide to a ~0 by-product in c~ntrast to treating the zinc oxide as a waste pr~du~t, and processing c~t of ~he invention procesq i9 a fraction o the smelting and reining aharges of the~manu~ac-turers. Ignorin~ the value Q~ the zinc by-pxoduct derived from the zinc oxide (wh~ch can typiaally cons~itute ~rom 15 to 25% of the weight in the brass skimmin~s), it ha~ been estimated that the lnvention pr~ce~s can typically save the mill or foundry ~150 per ton an~ ~ore (wher~ several mills generate in excess of 400 tons per mon~h of brass ~kLmmings).
Figure 2 shows a somewhat sim~lified apparatus ~ox practicing the inven~ion. The reaction ves~el 10, feed .... - . . ~ ~ . . . ~ -... . . . .. .. . .. , .. ,: . ., . - -. . ~ . - .,, : . . . . . .

12, agitator 22, etc. are es~entially th~ same; however, the valve 32 for the settled ~olid~ has been omitted altogether.
The larger solids settle directly into the lower intake portion of the screw conveyor 40a. They are elevated above the common liquid level of the vessel 10 and of the conveyor 40a against a countercurrent of the acid introduced remotely by pipe 18. Any water needed for the reaction can advantag-eously be added by a pipe 20 (al~o remotely connected to conveyor 40a) ~o as to unction additionally as a preliminary xinse upstream of the acid pipe 18. Feed pipe 52 carries ~he ~ettled material from the delivery end of conveyor 40a t~ the wash tank 42 and the receivlng end of the wash screw-conveyor ~Ob. As the s~lid~ are again elevated, they are washed ~f liguid contamlnants and any remaining fine~ in a countercurrent of water (all of which overflow weir 43).

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Claims (25)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. Method of recovering metallics and useful by-products from a solidified mass of brass-skimmings which contain zinc oxide as a significant contaminant, comprising the steps of crushing said mass to loosen and to expose the zinc oxide from the metallics thereof, adding said crushed mass to a reaction zone, adding hydrochloric or sulfuric acid to said reaction zone, mixing said mass and said acid in said reaction zone, separating out undissolved said metallic solids from said reaction zone, withdrawing from the reaction zone the solution of zinc salts resulting from reaction of the acid with the zinc oxide, the addition to the reaction zone of said acid relative to said crushed mass being at a rate and ratio to ensure that enough acid is present and at a strength to react with substantially all the zinc oxide and to react therewith rapidly enough such that the removal of said metallics from said reaction zone can be and is accomplished before essentially any dissolving of said metallics thereof by said acid has occurred.

2. A method a cording to claim 1, wherein the react-ants are intimately mixed by agitation.

3. A method according to claim 2, wherein the acid is concentrated HCl or concentrated H2SO4 and water is added if needed, sufficient to keep said zinc salts in solution in the reaction zone.

4. A method according to claim 1, wherein the larger metallic solids are separated by rapid settling, the zinc salt is separated by weir overflow and the metallic and dirt fines overflowed therewith are separately separated by mechanical means.

5. A method according to claim 4, wherein the dirt and metallic fines are separated by continuous centrifuging.

6. A method according to claim 5, wherein the dirt and metallic fines are separated by filtering.

7. A method according to claim 3, wherein the addi-tion of acid relative to said mass is controlled so as to maintain the pH of said zone between 0.1 and 1.5.

8. A method according to claim 4, wherein the addi-tion of acid relative to said mass is controlled so as to maintain the pH of said zone between 0.1 and 1.5.

9. A method according to claim 8, wherein the addi-tion of concentrated HCl relative to said mass as controlled so as to maintain the pH of said zone between 0.1 and 1Ø

10. A method according to claim 8, wherein the a di-tion of concentrated H2SO4 relative to said mass is controlled so as to maintain the pH of said zone between 0.1 and 0.8.

11. A method according to claim 4, further comprising adding zinc to the weir overflow to precipitate out metallic copper from any copper salts present.

12. A method according to claim 7, further comprising adding zinc to the zinc salt solution to precipitate out metallic copper from any copper salts present.

13. A method according to claim 4, further comprising classification washing of the metallics settled out of said zone with water to further separate them from the other reactants, reaction products, and contaminants removed from the reaction zone therewith.

14. A method according to claim 13, further comprising combining the washwater with said weir overflow, adding zinc dust to precipitate copper, and centrifuging to remove copper metal fines and other dirt.

15. A method according to claim 1, further comprising screening the crushed mass to separate the larger metallic nuggets from the fines, the separated coarser material being of a sufficient quality and purity to be recycled directly, the fines thereafter being treated by said acid.

16. A method according to claim 15, wherein said screening is through a number 4 mesh or its substantial equivalent.

17. A method according to claim 16, wherein said mass is crushed by quickly passing through a hammer mill.

18. A method according to claim 8, wherein said crush-ing is accomplished in a ball mill and the settled solid materials from said reaction zone are screened on about a 50 to 70 mesh screen retaining effectively pure metal and treating the passed solids to classification for separation of metallic fines from the other contaminants.

13. A method according to claim 4, wherein any recover-ed metallic fines are briquetted for use in recycling.

20. A method according to claim 19, wherein the fines are added to brass turnings to aid in the briquetting.

21. A method according to claim 1, wherein the addi-tion of acid relative to said mass is controlled so as to maintain the pH in said zone at about 0.1 to less than 3.5.

22. A method according to claim 4, wherein the removal from said reaction zone of said undissolved metallics is against a countercurrent of said acid being added to said reaction zone.

23. A method according to claim 22, wherein water is added upstream of said acid to rinse the removed metallics and in an amount at least sufficient to keep said zinc salts in solution in said reaction zone.

24. A method according to claim 1, wherein the solu-tion of zinc salts withdrawn from the reaction zone contains less than 1/2% copper by weight of the original crushed mass added to said reaction zone.

25. A method according to claim 4, wherein the copper value content contained in the zinc salts separated by weir overflow is less than 1/2% by weight of the original crushed mass added to said reaction zone.