METHOD AND EQUIPMENT FOR REMOVING ORGANIC BINDERS FROM GREEN BODIES

Abstract

Green bodies are safely, economically and efficiently debound in a dual quartz reactor by subjecting them to a steady laminar upward flow of freshly distilled solvent so that the concentration difference of soluble binder at the green body/solvent interface is at all times maximized for optimum binder extraction as per Fick's laws of diffusion. Binder extraction rate is monitored by inline spectrophotometry of the reactor overflow. Following solvent extraction, the residual insoluble binder is thermally extracted without the need to transfer the green bodies to a different vessel.

Claims

1. A method and equipment for removing organic binders from green bodies, comprising at least: a. two steel drums, herein called Boiler Sumps, each fitted with a heating jacket and equipped with level indicators, b. two quartz tank and bell jar assemblies, herein called Reactors, c. one solvent condenser, d. one vacuum pump, e. one blower capable of delivering hot air or nitrogen gas at up to 600° C., f. one flame-off burner mounted on the installation's exhaust, g. one spectrophotometer or other suitable trace organics analyzer, h. a plurality of one-, two- and three-way valves.

2. The installation as set forth in claim 1 wherein solvent in the Boiler Sumps is evaporated and the resulting vapor condensed in the condenser.

3. The installation as set forth in claim 2 wherein said condensed solvent is directed either to the Boiler Sumps or to the bottom of the Reactors by gravity flow.

4. The installation as set forth in claim 1 wherein the Reactors are loaded with green bodies.

5. The installation as set forth in claim 4 wherein said green bodies are inundated by an upward stream of condensed solvent.

6. The installation as set forth in claim 5 wherein said solvent overflowing the Reactor is directed to the Boiler Sump by gravity flow.

7. The installation as set forth in claim 6 wherein said solvent overflowing the Reactor is analyzed by spectrophotometry or other suitable trace organics analytical technique.

8. The installation as set forth in claim 7 wherein, following solvent extraction, said green bodies are vacuum dried and exposed to an upward stream of hot air, nitrogen, or a mixture of both.

9. The installation as set forth in claim 8 wherein said green bodies are thermally debound in said Reactor without having to be transferred to a different vessel.

Description

DETAILED DESCRIPTION

[0090] In what follows, the invention will be described in more detail by way of a non-binding practical example. The feedstock formulation (based on 100 g. feedstock) used in the example is:

TABLE-US-00002 weight density volume % g .Math. cm.sup.−3 cm.sup.3 Stainless steel powder 93.020 7.89 11.790 HDPE (total Organic Insoluble (OS)) 3.600 0.954 3.774 Stearin 3.281 0.840 3.906 Stearic Acid 0.099 0.940 0.104 Total Organic Soluble (OS) 3.380 0.843 4.010 Total Organic (Binder) 6.980 0.897 7.784 Total Feedstock 100 5.109 19.574

[0091] Binder extraction by Solvent Extraction (SX) relies on three simultaneous mechanisms, i.e.:

(i) Dissolution, i.e. the solubility of the wax component in the chosen solvent,
(ii) Diffusion, as a result of the random thermal motion of solute wax molecules,
(iii) Convection, i.e. the transport of solute wax molecules by solvent flow.

[0092] The effects of each of these mechanisms on the instant invention will now be reviewed in detail.

1. Dissolution

[0093] Dissolution depends on the solvent's Hildebrand solubility parameter as well as on environmental and economic considerations, e.g. temperature, flammability, pressure, ozone depletion potential (ODP) and cost.

[0094] Until the mid-1980s, CFCs, e.g. Freon 112, were in widespread use but in 1987, the Montreal Protocol banned or severely restricted their use. Consequently, chemical companies like DuPont, Wilmington, Del. and others, developed zero ODP solvents. DuPont's Vertrel MCA™, a non-flammable, proprietary azeotrope of 2,3-dihydrodecafluoropentane and trans-1,2-dichloroethylene (1,2 dichloroethene) commonly used as a solvent for waxes, resins, polymers, fats and lacquers has a Hildebrand solubility parameter of 15.2 MPa.sup.1/2 that is higher than that of the commonly used hexane (14.1 MPa.sup.1/2. This solvent has been used for the design of the equipment of the instant invention.

2. Diffusion

[0095] Fick's First Law of Diffusion states that the diffusive flux goes from regions of high concentration to regions of low concentration with a magnitude proportional to the concentration gradient.

[0096] In one spatial dimension:

J=−D*(δΦ/δx)

[0097] where

[0098] J is the diffusive flux in dimensions [MIL.sup.−2T.sup.−1], (e.g. mol/m.sup.2s)

[0099] D is the diffusion coefficient in dimensions [L.sup.2T.sup.−1], (e.g. m.sup.2/s)

[0100] Φ is the concentration in dimensions [ML.sup.−3], (e.g. mol/m.sup.3)

[0101] x is the position in dimensions [L], (e.g. in)

[0102] In a paper presented by Fan J. L. et al. of the State Key Laboratory for PM, Central South University, Hunan, Changsha, PRC (cf. Non-Patent Literature) the researchers state:

[0103] “At the start of debinding, the concentration difference between the specimens and the solvent is large, it is easy for the soluble component to diffuse and dissolve into the solvent from the specimens, so the debinding rate is high. With increasing time, the concentration difference between the specimens and solvent decreases, the solvent debinding enters into the dissolution control period and the concentration difference becomes the main factor to affect the debinding rate. With the decrease of concentration difference, the diffusion and dissolution rate decrease in spite of increase in the total binder weight loss.”

[0104] This research merely confirms Fick's Law of Diffusion and that the binder extraction rate will be maximized if and only if the concentration difference is maintained at a maximum which is the fundamental principle on which the instant invention is based.

3. Convection

[0105] Convective transport occurs when Organic Soluble (OS), i.e. solvated wax molecules are carried away by the solvent flow.

[0106] If θ is the volume concentration of OS molecules in the feedstock (as per feedstock formulation), we have,

dn/dx=dn/dy=dn/dz=θ

or, in one spatial dimension,

dy/dt=(1/θ)*dn/dt

where
dn/dt is the volume fraction of OS molecules being solvated per unit time, i.e. the rate at which OS molecules are being solvated and
dy/dt is the upward velocity.

[0107] The number of OS molecules being solvated is equal to the number of available OS molecule sites exposed to the solvent. This number is θ, the volume concentration of OS molecules at the green body/solvent interface.

[0108] The volume fraction of soluble matter in the feedstock (θ) is:

4.010 cm.sup.3/19.574 cm.sup.3=2.049*10.sup.−1

[0109] The soluble matter in the feedstock is stearin with properties:

[0110] molar mass, in: 891.48 g.Math.mol.sup.−1

[0111] molar volume: 891.48 g.Math.mol.sup.−1/0.84 g.Math.cm.sup.−3=1,061.29 cm.sup.3.Math.mol.sup.−1

[0112] molecular volume: 1,061.29 cm.sup.3.Math.mol.sup.−1/6.022*10.sup.23 mol.sup.−1 or 1.762*10.sup.−21 cm.sup.3

[0113] molecular diameter, a (based on the hard sphere model):

a=6*1.762*10.sup.−21 cm.sup.3/π).sup.1/3=1.5*10.sup.−7 cm

[0114] The Diffusion Coefficient is given by:

D=SQRT(k.sup.3/π.sup.3m)*(T.sup.3/2/Pa.sup.2)

[0115] where k is Boltzmann's constant [0116] T is the absolute temperature (20° C.+273.15) [0117] P is the pressure (1 atm)

[0118] yielding D=2.15*10.sup.−15 cm.sup.2 s.sup.−1

[0119] Consequently, a 1.5*0.sup.−7 cm thick film of solvent covering a 1 cm×1 cm surface of green body (i.e. 1.5*10.sup.−7 cm.sup.3 of solvent) will generate 2.049*10.sup.−1×1.5*10.sup.−7 cm.sup.3=3.07*10.sup.−8 cm.sup.3 of solvated matter per cm.sup.2 of green body surface.

[0120] This solvated matter must be carried away by the solvent stream as fast as practical in order to maintain the maximum concentration gradient in the spent solvent and thereby the highest dissolution rate.

[0121] The solvent upward velocity or upflow (mm/s) is the variable controlling the rate at which solute molecules are being carried away. Empirically it has been determined that an upward velocity of about 10 mm/min (1.67*10.sup.−1 mm/s) is adequate.

[0122] In the example used to illustrate the invention, the green parts are processed in a Ø220 mm×400 mm (15 lit) Quartz Reactor Tank. Thus at an upward velocity of 10 mm/min, it will take 40 min (to fill an empty Reactor Tank, substantially less for a loaded one. This corresponds to a solvent flowrate of 15 lit/0.66 h or 22.52 lph which defines the necessary condensation capacity of the solvent condenser(s).

CONCLUSION, RAMIFICATIONS AND SCOPE

[0123] In conclusion, the major advantage of this invention resides in the ability to safely, economically and efficiently remove organic binders from green bodies.

[0124] Although the invention has been described with respect to specific preferred embodiments thereof, many variations and modifications will immediately become apparent to those skilled in the art. It is therefore the intention that the claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications